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The Liar Paradox:
A Consistent and Semantically Closed Solution
Ryan Edward Young
A thesis submitted for the degree of Doctor of Philosophy
of The Australian National University
This thesis develops a new approach to the formal denition of a truth predicate that allows a
consistent, semantically closed denition within classical logic. The approach is built on an analysis
of structural properties of languages that make Liar Sentences and the paradoxical argument
possible. By focusing on these conditions, standard formal denitions of semantics are shown to
impose systematic limitations on the denition of formal truth predicates.
The alternative approach to the formal denition of truth is developed by analysing our intuitive
procedure for evaluating the truth value of sentences like P is true. It is observed that the standard
procedure breaks down in the case of the Liar Paradox as a side eect of the patterns of naming
or reference necessary to the denition of Truth as a predicate. This means there are two ways in
which a sentence like P is true can be not true, which requires that the T-Schema be modied
for such sentences.
By modifying the T-Schema, and taking seriously the eects of the patterns of naming/ reference
on truth values, the new approach to the denition of truth is developed. Formal truth denitions
within classical logic are constructed that provide an explicit and adequate truth denition for their
own language, every sentence within the languages has a truth value, and there is no Strengthened
Liar Paradox.
This approach to solving the Liar Paradox can be easily applied to a very wide
range of languages, including natural languages.
The work contained in this Thesis is my own and, unless otherwise attributed, is not the result of any
joint eort or research.
Firstly, I would like to thank Peter Roeper for his
time, eort, patience and attention to detail. His willingness to pause and
take time to consider and think through all of my ideas, even the worst ones,
has been greatly appreciated. While our discussions tended to go round in
circles, he succeeded in bringing order and rigour to my work which I would
have struggled to achieve otherwise.
I am very grateful to Ingolf Max, and the other academics at the Institut für
Logik und Wissenschaftstheorie at the University of Leipzig, for their time and
support during my stay there. I learnt a lot and my ideas matured signicantly
in the stimulating environment they have created.
I am also very grateful to Brian Garrett, Daniel Stoljar, Margaret Brown and
many others for their ability at various times to put up with me and make
sure that I successfully got to the end of this thesis.
Finally, I have to thank Rebecca, Gwyllym and Sebastian for being there, not
complaining and distracting me when I needed it. Their inspiration and ability
to keep me grounded has contributed immeasurably to the quality of the work.
Acknowledgement.
1 Understanding the Liar Paradox
Why Languages? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grammar-Only Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Logical Languages
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What is the Liar Paradox? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Solutions to the Liar Paradox
What is a Solution to the Liar Paradox? . . . . . . . . . . . . . . . . . . . . . . . .
Cases of the Liar Paradox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solutions in the Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 On the Structure of Formal Languages
Recursive Model Theoretic Semantics
The Denition of Semantic Model
. . . . . . . . . . . . . . . . . . . . . . . . . . .
The Conceptual Structure of Model Theory
Sentences as Truth Bearers
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Outline of a New Approach
Truth in Formal Languages
Evaluability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Application to Formal Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 First Formal Denition
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Classical Sentential Logic with Truth . . . . . . . . . . . . . . . . . . . . . . . . . .
Paradoxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Second Formal Denition
Is this necessary? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Truth-bearers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preliminaries for a Formal Denition . . . . . . . . . . . . . . . . . . . . . . . . . .
Classical Sentential Logic with Truth (2) . . . . . . . . . . . . . . . . . . . . . . . .
7 Application to Natural Languages
Understanding the Formal Denition . . . . . . . . . . . . . . . . . . . . . . . . . .
Application to Natural Language . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Philosophical Consequences
The Liar Paradox is one of the oldest and has been one of the most intractable paradoxes in
the Western philosophical tradition.
It dates back at least to the time of Aristotle, as its use
1 However, despite
is attributed to Eubilides, one of Aristotle's contemporaries and adversaries.
constant philosophical and logical attention since this time, and in stark contrast to other ancient
paradoxes such as Zeno's Paradoxes of Motion, no consensus has been reached over how to respond
to the Liar Paradox. Moreover, this lack of consensus has become more pronounced in recent times,
despite the fact that more specic attention has been paid to the paradox since the middle of the
twentieth century than in any previous period. The lack of consensus is not, however, due to any
failure by philosophers and logicians to devise solutions since the problem we face is far from a
lack of solutions; rather, we have an overabundance of conicting ones.
These problems are on the surface surprising, since one of the dening features of the Liar
Paradox is the simplicity with which it can be presented. A typical presentation of it only requires
a couple of sentences, and it does not depend on any specialised or technical concepts. Moreover,
an ordinary presentation is sucient to allow one to grasp the basic mechanism by which the
paradox occurs and have an idea of what is required in order to solve the paradox.
To see this, we will consider the following sentence:
This sentence is not true.
This sentence (1) is simple and easy to understand. However, if we make the obvious assumption
that the `This sentence' in the sentence refers to the sentence itself, we run into the following
If sentence (1) is true, then it follows that what it says must be true. What it says
is that it itself is not true.
Therefore, if (1) is true, it follows that (1) is not true.
However, if we assume the other alternative, that sentence (1) is not true, then what
sentence (1) says is in fact true, from which it follows that (1) is true. Therefore, if (1)
is not true, then (1) must be true.
1 Roy Sorenson. A Brief History of Paradox. Oxford: Oxford University Press, 2003, Ch 7, pp83-99
2 Michael Glanzberg. The Liar in Context. In: Philosophical Studies 103 (2001), pp. 217251, p. 217
In the space of a few lines, we have established that if the sentence (1) is either true or not true,
then it must also be the other. Thus it must be either both true and not true; or neither true nor
not true. Since neither of these alternatives seems plausible, we have a paradox.
The consequences of this paradox are, if we take the conclusion established by the argument
seriously, profound.
¬P ∧ ¬¬P .
For if we accept either of these conclusions, we have accepted the truth of
In the rst case it is of the form
P ∧ ¬P
and in the second case it is of the
This is unacceptable on a standard understanding of reasoning. Moreover, on
traditional accounts of reasoning, if there is a true contradiction, everything is provable.
means, that if the above argument is valid in English (or any fragment of English or any other
language), then every sentence within English is provably true. It would therefore be impossible to
use English to reason or discuss reasoning since every sentence of English can be shown to be true.
If this conclusion is correct, it not only renders pointless vast amounts of writing and discussions,
but on a more personal note, means that there is no point completing this thesis, since everything
written in it can be proven to be both trivially true and trivially not true.
Stated in this form, the conclusion of the reasoning in the Liar Paradox cannot be correct.
Something must have gone wrong in the argument, and our task is to identify the problem.
major project of this thesis is to provide a stable and coherent diagnosis of the problem, however
as the vast literature on the subject demonstrates, this is not an easy task.
The Liar Paradox is not simple
As is clear from the exposition above, the Liar Paradox is simple to present, and the mechanism
by which the Paradox arises seems easy to understand.
We have a situation where we have a
grammatically correct sentence that claims of itself that it is not true. This, however, contradicts
the standard truth conditions for declarative sentences, that a declarative sentence is true if what
it claims is true. This internal contradiction, by means of our intuitive assumptions about truth,
produces the type of argument which is typical of the Liar Paradox: on the assumption that the
relevant sentence is true, we can directly derive its untruth; and on the assumption of its untruth,
we can derive its truth. This simplicity in presenting the Liar Paradox and understanding its basic
mechanism, however, does not imply that the Liar is a simple paradox. Instead it tends to hide
the complexity of the phenomena that surround it.
A quick survey of the range of literature on the Paradox neatly illustrates the complexity of
the relevant phenomena. For not only do few authors agree on the correct solution to the Paradox,
but not many more authors agree on how exactly to identify the problem posed by the Paradox.
For example, some locate the problem in traditional assumptions about logic,
3 others locate the
example, see Graham Priest. Truth and Contradiction. In: The Philosophical Quarterly 50.200 (2000),
pp. 305319; Graham Priest. The Logic of Paradox. In: The Journal of Philosophical Logic 8 (1979), pp. 219241;
Hartry Field. Semantic Paradoxes and Vagueness Paradoxes. In: Liars and Heaps: New Essays on Paradox. Ed. by
problem in our understanding of truth bearers,
4 others locate the problem in the role of context5
or the limited applicability of ordinary assertions , while yet others attribute the diculty to a
faulty understanding of the pragmatics of language
7 or even take the Liar Paradox at least in some
8 There are strong intuitions behind each
sense as evidence of the ultimate indenability of truth.
alternative, and the range of dierent alternative diagnoses illustrates the complexity of the issues
that surround the Liar Paradox.
This complexity can be most clearly seen in the range of dierent examples of the Liar Paradox
that have been uncovered by modern research. Historically, the examples of the Liar Paradox that
were advanced usually involved somewhat articial, contrived sentences. These were examples such
as This sentence is false; or I am now lying. Modern research, however, has shown that the Liar
Paradox arises far more commonly than these rather articial types of examples. The simplicity of
the mechanism that generates the Liar Paradox ensures that it often arises where we would least
like it to arise. There are two issues that have arisen in modern research which are particularly
The rst was most forcefully demonstrated by Kripke in his seminal paper Outline of a Theory
9 although the type of example that Kripke uses pre-dates Kripke's work in the litera-
of Truth,
10 Kripke considered the ordinary assertion Most of Nixon's assertions about Watergate are
false.
11 While this is normally an unproblematic assertion, Kripke described a situation in which
it becomes paradoxical, namely when Nixon's assertions are evenly balanced between truth and
falsity except for the one assertion that the person who uttered the above statement is telling the
As can be easily checked, these two assertions create a two sentence version of the Liar
Paradox. The lesson that Kripke drew from this was that many, probably most, of our ordinary
assertions about truth and falsity are liable, if the empirical facts are extremely unfavorable, to
exhibit paradoxical features.
12 That means that, unlike the impression generated by the types
of examples that were traditionally used, there is nothing inherently problematic in the sentences
themselves that are used to generate the Liar Paradox.
The second issue in relation to the ubiquitousness of the Liar Paradox that has arisen in modern
research can be summarised under the label of the Revenge Problem. Modern researchers routinely
J. C. Beall. Oxford: Clarendon Press, 2003, pp. 262310
example, see Keith Simmons. Reference and Paradox. In: Liars and Heaps: New Essays on Paradox.
Ed. by J. C. Beall. Oxford: Clarendon Press, 2003, pp. 230252; Alan Weir. Token Relativism and the Liar. In:
Analysis 60.2 (2000), pp. 156170
5 See
Glanzberg, The Liar in Context, see n. 2; Michael Glanzberg. A Contextual-Hierarchical Approach to
Truth and the Liar Paradox. In: Journal of Philosophical Logic 33 (2004), pp. 2788
Jon Barwise and John Etchemendy. The Liar: An Essay on Truth and Circularity. New York: Oxford
University Press, 1987
A. P. Martinich. A Pragmatic Solution to the Liar Paradox. In: Philosophical Studies 43 (1983), pp. 6367
example, see Charles S. Chihara. Priest, the Liar, and Gödel. In: Journal of Philosophical Logic 13 (1984),
pp. 117124
9 Saul Kripke. Outline of a Theory of Truth. In: Journal of Philosophy 72.6 (1975), pp. 690716
10 For example, see L. Jonathon Cohen. Can the Logic of Indirect Discourse be Formalised? In: The
Symbolic Logic 22.3 (1957), pp. 225232
11 Kripke,
see n. 9, p. 691
use the Liar Paradox itself to critique proposed solutions to the Paradox. The standard strategy
works as follows: any proposed solution must draw a line between truth and untruth somewhere;
once this line has been identied, one can usually construct a version of the Liar Paradox which
operates on this line (whether or not it is expressed in terms of truth). Thus one can almost always
provide a counterexample to a proposed solution to the Liar Paradox by reformulating the Paradox
in a way that uses the distinctions that the solution proposes. In this sense, the Liar Paradox takes
revenge on the proposed solution, and we have the Revenge Problem. The ease with which this
can normally be done demonstrates the power of the Liar Paradox and has lead to the suggestion
that ultimately the Liar Paradox is not solvable.
Paradoxical vs Non-Paradoxical
While it will be shown that such a pessimistic conclusion is not warranted, it captures the scope of
the challenges presented by the Liar Paradox. Any meaningful discussion of, or solution to, the Liar
Paradox must draw a clear distinction between the paradoxical cases and the non-paradoxical cases,
and any proposed solution must satisfactorily deal with all paradoxical cases without incorrectly
aecting any non-paradoxical cases. Both Kripke's argument and the Revenge Problem, however,
demonstrate that this is not an easy distinction is to make accurately.
Kripke's argument undermines a lot of work and discussion of the Liar Paradox, since these
are often characterised by discussions of Liar Sentences. While there are sentences which unquestionably trigger the Paradox, Kripke showed that it would be fruitless to look for an intrinsic
criterion that will enable us to sieve out ... those sentences which lead to paradox.
13 That is,
there is nothing in Liar Sentences as sentences which can distinguish them from non-paradoxical
cases. The clearest example of this are cases like Kripke's where the same sentence can be either
paradoxical or non-paradoxical depending on the context. This means that any attempt to draw
the distinction between the paradoxical and the non-paradoxical purely at the level of sentences
cannot work. This raises the signicant question of how we are to draw this distinction.
The Revenge Problem, on the other hand, raises the question of whether it is in fact possible
to draw a solid distinction between the paradoxical and the non-paradoxical. As mentioned, the
problem turns on the fact that once a proposed solution has distinguished between truth and
untruth in any sense, the paradox can be usually formulated using this distinction.
any proposed solution must draw the line somewhere between truth and untruth, and in doing
this must develop some distinction between the paradoxical and the non-paradoxical. When the
Revenge Problem bites, the line between truth and untruth is shown to be inadequate, and therefore
the distinction that was made between paradoxical and non-paradoxical cannot be correct. If the
Revenge Problem cannot be avoided, then it will also be impossible to make a solid distinction
between the paradoxical and the non-paradoxical.
This process can be neatly illustrated by the simple solution to the Liar Paradox which proposes
the introduction of a category of sentences which are neither true nor false as a solution to the
Liar Paradox. This is commonly done on consideration of paradoxical sentences of the form This
sentence is false. However, as is well known, any simple version of this solution fails to account for
any example which replaces false with not true. That is, the solution proposes the category of
`neither true nor false', which means that the line between truth and untruth shifts to being between
`true' and `neither true nor false'. As soon as we reformulate the Liar Paradox in terms of this
distinction (This sentence is not true ), the Liar Paradox arises again. The analysis of the paradox
which supports this proposed solution failed to correctly draw the line between the paradoxical
and the non-paradoxical, and therefore the solution failed.
While this is a simple example, the
frequency with which the Revenge Problem causes exactly the same problems suggests that it may
in fact be impossible to denitively draw a line between the paradoxical and the non-paradoxical.
It will be demonstrated in this thesis that this suggestion can remain as a suggestion, as it
is in fact possible to make a sharp distinction between the paradoxical and the non-paradoxical.
However, a sharp distinction is only possible if we take Kripke's point outlined above very seriously.
If we want to draw a distinction between the paradoxical and the non-paradoxical, we
need to be clear about what sort of thing we are considering as paradoxical or not.
For exam-
ple, it is often assumed, either explicitly or implicitly, that in the case of the Liar Paradox it is
the sentences that are paradoxical. If this is the case, then the distinction between the paradoxical and the non-paradoxical has to be a distinction between the paradoxical sentences and the
non-paradoxical sentences.
Kripke showed, however, that this is not possible as many perfectly
satisfactory sentences can be paradoxical in unusual situations.
The rst challenge in drawing
the required distinction is to get clear on what sort of thing we need to consider in drawing the
distinction.
What sort
of thing
is paradoxical?
The problem identied by Kripke is that it would be fruitless to look for an intrinsic criterion
that will enable us to sieve out ...
those sentences which lead to paradox.
14 That is, there is
no guarantee that the sentences which are (potentially) paradoxical will share any common set
of properties on which we can draw a coherent distinction between the paradoxical and the nonparadoxical and therefore on which we could base a satisfactory solution. Kripke's conclusion is
based on the observation that any sentence which includes a truth ascription could be paradoxical
in appropriate circumstances.
There is therefore nothing dierent about paradoxical sentence
when compared to non-paradoxical sentences, since many sentences can be both paradoxical and
not paradoxical in dierent contexts.
Given the role that context and external situations play in Kripkean examples, some role for
context has to be included in the sort of thing we want to investigate. The obvious solution to
this problem would therefore be to investigate situations or states of aairs, in conjunction with
sentences, as the sort of thing on which we could draw a distinction between the paradoxical and
the non-paradoxical.
While it is plausible that there may be a clear distinction between paradoxical and nonparadoxical situations, any strategy of this sort runs into problems when we consider the innitely
large range of possible combinations of sentences and situations. If we consider only a sentence
such as That is not true within English, there are innitely many possible referents of this sentence (including itself ) embedded in innitely many possible situations. An innite subset of these
referents will in turn refer to some other sentence, or set of sentences, and in each of these there will
be again innitely possible referents of the sentences in this innite subset. The fact that some of
these referents to the sentence That is not true will, in the appropriate context, be paradoxical,
is purely due to the combinatorics of the situation. However, identifying exactly which of these
possible combinations of sentence and context are paradoxical and which are not is, at the very
least, not an easy task. When we in turn consider the huge variety of sentences that we can start
with, it should be clear that identifying a rule-governed method of producing all possible types of
situation/sentence combinations which are paradoxical is a mammoth task, which may or may not
be solvable. At the very least it is a major research project in its own right. Without such a rulegoverned method, it is impossible to be sure that we are drawing the line between the paradoxical
and the non-paradoxical correctly.
Moreover, even if we can draw a line between the paradoxical and the non-paradoxical on
this basis, it may not be very useful in terms of grounding a solution to the Liar Paradox. The
distinction between the paradoxical and the non-paradoxical will, using sentence/context sets, be
largely drawn through an understanding of the combinatorics of the situation. However, we cannot
change the combinatorics that lead to the Liar Paradox without a fundamental reworking of the
way we use languages such as English. Requiring this as part of a solution to the Liar Paradox is
neither likely to be plausible nor to gain widespread acceptance.
Given that there are problems with investigating sentences and sentence/contexts combinations,
we could consider the common philosophical reex to look at dierent possible truth-bearers.
To take a couple of examples, we might consider looking at propositions, or sentence-tokens.
Importantly, each of these requires some sort of context sensitivity in their denition.
is often necessary to determine the proposition expressed by a sentence; and a sentence-token,
as a token, necessarily belongs to a particular context. However, while each of these have been
advocated as solutions to the Liar Paradox, each of these is faced with essentially similar problems.
It has been argued that taking propositions as primary truth bearers can solve the problem
since Liar Sentences either do not express a proposition,
15 or express a false proposition.16 These
solutions naturally face the standard problems that all accounts of propositions face. However, independently of these problems, turning to propositions as part of an attempt to better understand
the Liar Paradox produces the same problems as sentences. For whether we consider the sentence
itself, or the proposition expressed by Most of Nixon's assertions about Watergate are false
nothing intrinsic can be found which separates paradoxical from non-paradoxical use of that sentence. The proposition expressed will be the same whether it is embedded within a paradoxical
context or not. Considering propositions does not help us draw any line between paradoxical cases
and non-paradoxical cases.
The alternative move to take sentence tokens as the basic truth bearer does not help either.
would be entirely consistent on this account that one assertion of Most of Nixon's assertions about
Watergate are false is true, and while another is false because it is paradoxical. However, this is
not enough since there is nothing in the sentence tokens themselves which dierentiate paradoxical
tokens from non-paradoxical tokens. We still need something about the context, but that opens
up the problems outlined above in the case of sentences. Thus considering sentence tokens does
not in itself add any more information to an attempt to dierentiate between the paradoxical and
Thus focusing our investigation on dierent possible truth bears does not help us distinguish
the paradoxical from the non-paradoxical since there is nothing in the truth bearers themselves
which reect the dierence between the paradoxical Kripkean cases from non-paradoxical cases.
Adding the relevant context directly into our considerations, however, adds enormous complexity,
which brings in questions about the decidability of the problem. Moreover, a distinction on these
grounds would be based on a consideration of the combinatorics of language reference, which is
not something that can be plausibly altered for natural languages.
Looking at the alternatives to taking sentences as the focus of the investigation into the Liar
Paradox examined here, any alternative truth-bearer to sentences runs into the same problems as
Kripke showed existed for sentences, since it is not possible to distinguish between paradoxical and
non-paradoxical cases without referring to external context. However, including external context
in the basic unit of investigation poses problems of a dierent sort, the immense complexity that
arises on this approach undermines attempts at progress. It is very dicult to be sure that the
15 See
Laurence Goldstein. A Unied Solution to Some Paradoxes. In: Proceedings of the Aristotelian Society
100 (2000), pp. 5374
16 See E. Mills. A Simple Solution
17 Kripke, see n. 9, p. 691
18 For one example, see Alan Weir.
to the Liar. In: Philosophical Studies 89 (1998), pp. 197212
Token Relativism and the Liar. In: Analysis 60.2 (2000), pp. 156170
line between the paradoxical and the non-paradoxical has been drawn correctly.
Each of these alternatives focus our investigations on actual paradoxical cases. However, the
structure of the paradoxical argument in the Liar Paradox suggests that this is not necessary. The
Liar Paradox argument always begins with a hypothetical assumption: Suppose this particular
sentence is true, .....
of If that particular sentence is not true, ...... This means that the Liar
Paradox only requires the potential existence of a paradoxical case to arise. So long as, for example,
we can construct an appropriate example of the Liar Paradox in English, then the consequences
posed by the Paradox above need to be faced. We cannot, for example, deal with the Liar Paradox
by banning the use of paradoxical sentences or statements. The Liar Paradox bites if these sentences
are merely constructable.
Given that the Liar Paradox turns on the possibility of certain types of sentences or situations,
this suggests that the focus our investigations into the Liar Paradox should be on the conditions
which make these sentences and the Liar Paradox possible, rather than actual cases of the Paradox.
This would mean that our focus should be on the linguistic structures that make paradoxical
sentences possible, and the reasoning which generates the Paradox. That is, if we are to take this
suggestion seriously, we should be investigating languages, rather than sentences or propositions
or situations.
This idea that the focus of investigations into the Liar Paradox should be focused on languages
is supported by the brief account of the Paradox given above. As stated above, the most serious
consequence of taking the Liar Paradox seriously is that our ability to use languages to express
truth is called into question.
This consequence is at the level of languages, and moreover any
serious attempt to solve the Liar Paradox therefore either seeks to revolutionise our understanding
of languages, or constructs a new formal language to demonstrate why it is not a problem. Since
languages are central to solving the Liar Paradox, it is reasonable that they should become the focus
of our attempts to understand and explain the paradox. That is, we should focus on understanding
the conditions set up by languages which make the Liar Paradox possible.
This approach is further supported by the simple observation that in general there is a much
clearer distinction between languages which are aected by the Liar Paradox and those which are
not. To take slightly extreme examples, English is obviously aected in some sense, while Classical
Sentential Logic is not. The fact that the Liar Paradox does not aect Classical Sentential Logic
is not a deep property of that language, it simply does not possess suciently rich vocabulary.
However this is a property that does not depend in any way on context.
Similarly, the fact
that English is aected is also context independent. No matter how the world is, the linguistic
structures in English allow the construction of liar sentences, and once these are constructable we
can run the Liar Paradox argument. This existence of a context independent distinction between
the paradoxical and the non-paradoxical allows us to dene the Liar Paradox much more easily
and eectively.
Each of these reasons strongly suggests that languages should be the primary focus in investigations into the Liar Paradox.
They dene the conditions which make the Paradox possible,
they are the main focus in developing solutions and there seems to be a clear distinction between
languages that are aected by the Liar Paradox and those which are not aected.
How are Languages aected by the Liar Paradox?
While we can pick a couple of examples to show that there should be a clear distinction between
languages which are aected by the Paradox and those that are not, this distinction cannot be
made without a clear idea of what it actually means for a language to be aected by the Liar
Paradox. For there are signicant dierences between the ways that languages can be aected by
the Liar Paradox.
The rst dierence is that some languages may be necessarily aected while others are only
contingently aected. Any language in which it is possible to construct examples like This sentence
is not true case above, will be necessarily aected. No matter how the world is, no matter what
facts are true, the language will be aected by the Liar Paradox. However, there will be languages
where the problematic sentences are ones like Kripke's examples, where they are paradoxical only if
certain facts are true. Kripke's example is only paradoxical if Nixon's assertions about Watergate
are perfectly balanced between true and false except for the one that refers to the speaker of the
Languages where examples like this are the only paradoxical cases are contingently
aected by the Liar Paradox.
The focus of this thesis will be on languages which are necessarily aected for a couple of reasons.
Firstly, the languages we are most interested in are necessarily aected. Secondly, constructing a
language which is only contingently aected is a non-trivial task, since the linguistic mechanisms
which allow the paradox in contingent cases almost always allow necessary cases. Finding clear
rules which disallow necessary cases but allow contingent cases is dicult, and will not help us
understand or solve the paradox.
Another dierence in the way that languages are aected by the Liar Paradox can be illustrated
by a couple of sketched cases. Suppose we take the language formed by adding a suitable mechanism
for referring to sentences (such as a Gödel Numbering) to a Classical logic and by adding the TSchema as an axiom schema. So long as there is at least one paradoxical sentence (the Diagonal
Lemma is taken to demonstrate that this will be the case) a contradiction will be derivable in the
language, and everything will be provable. This means that the language is trivialised - everything
is provable and therefore one can no longer assert anything meaningful in it. If we consider English,
on the other hand, it is aected by the Liar Paradox in the sense that we can construct sentences
which, under certain assumptions, trigger the Paradox and hence from which we can derive a
contradiction. We will discuss what these assumptions might be later, but the important thing to
note is that the Paradox only arises if certain non-trivial assumptions are made.
In the rst case, the Liar Paradox arises purely from the denition of the language, and therefore
the language is trivialised, and therefore useless.
If we wish to x the situation, it is necessary
to change part of the denition of the language and therefore dene a new language. Two formal
languages with diering denitions and diering sets of theorems are necessarily dierent languages.
In the second case, the paradox does not necessitate that we give up on English and dene a new
language to use instead. The normal response to the Liar Paradox is to question the assumptions
that are necessary for the paradox, as these are not considered to be essential to the denition of
English. In the rst case the Liar Paradox renders the language useless, and in the second case
the Liar Paradox calls into question assumptions about the language.
In order to understand this dierence more clearly, it is necessary to examine a key dierence
between natural languages such as English, and formal languages like Classical Predicate Logic.
Formal languages are normally dened so that the key principles of reasoning or logic
to that language are a part of the explicit denition of the language.
19 that apply
Thus, for example, the
dierence between the languages of Classical Predicate Logic and that of Intuitionistic Predicate
Logic is not in the vocabulary or the grammatical rules on what constitutes a valid sentence (or
well formed formula), but in the principles of reasoning (i.e. the axioms and /or rules of inference)
dened within those languages.
The particular rules of reasoning, whether dened as inference
rules or axioms, are an explicit part of the denition of normal formal languages.
Natural languages, on the other hand, do not include an explicit denition of the principles of
reasoning that apply to the language. If we come across a string of words such as The dog that
the ball, we can condemn it as ungrammatical and therefore not an English sentence.
other had, if we come across an invalid argument, say Not every dog chases balls, therefore, no
cat chases balls, this is perfectly valid English. However, it is condemned on grounds of reason or
logic, not on the grounds that it is not valid English. This suggests that, unlike the case of normal
formal languages, principles of reasoning are not intrinsic to the denition of natural languages as
one can presumably alter the principles of reasoning without altering the language.
However, it might be commented that natural languages do not have an explicit denitions of
anything, so it means little to say that there is no explicit denition of the relevant principles of
reasoning in a natural language. Formal languages are explicitly dened and therefore reasoning
is explicitly dened; natural languages are not explicitly dened and therefore reasoning is not
explicitly dened.
While this is strictly true, it does not mean that we cannot draw a relevant
distinction between the grammatical rules of English and the principles of reasoning that apply to
English, which does not exist for formal languages. It is very rare to have a substantial argument
19 This
is primarily meant to include purely logical reasoning.
connectives and quantiers.
That is, the reasoning which governs logical
about whether a particular sentence is grammatically sound, but it is far more common to have a
discussion about whether a particular piece of reasoning is actually valid. If a grammatical rule is
changed, it is a change to the English language. We accept that a language can change over time
due to new grammatical constructions.
However, if an accepted principle of reasoning changes,
we take it as a change to our understanding of the language.
It seems absurd to argue that a
philosopher changes English when they present a new philosophy of language. There are however
no rules of reasoning that we can change in the case of formal languages without changing the
language.
One nal challenge to the validity of this distinction is that it is only a surface distinction,
and that when one considers the semantics of the relevant languages the distinction disappears.
The dierence between Classical Logic and Intuitionistic Logic does not only lie in the dierence
in the principles of reasoning, but also in the dierent semantic interpretation that the symbols
in the dierent languages have. The dierent semantics for the dierent languages, it might be
argued, determines the dierent principles of reasoning. Furthermore, in languages like English,
the semantics of logical words like If determine the correct principles of reasoning for English,
and therefore there is no dierence between formal languages and natural languages like English.
While there may be a correct or true semantics for English, the lack of consensus on this
matter means that if there is, we do not know what it is. The meaning of logical words such as If certainly xes some rules of inference. It seems highly unlikely that we could accept that someone
understands the meaning of If if they deny Modus Ponens. However, unlike the case with formal
languages, the meaning of If does not fully determine the correct rules of reasoning about it.
To borrow the terminology of Natural Deduction, while Modus Ponens denes the appropriate
Elimination rule of If , the correct Introduction rule or rules are not always clear. The debates
over the nature of conditionals are a good illustration of this. A similar story can be told for many
logical words and important concepts in English. The standard meanings of these words partially
dene the relevant reasoning (in a strict logical sense), but they are not completely dened. This
is not the case for formal languages.
Importantly, this dierence between standard formal languages and natural languages corresponds to the dierence in the way that the Liar Paradox aects languages as identied above. If
the principles of reasoning are an essential part of the denition of the language, then if a contradiction arises then the language typically becomes useless. If the principles of reasoning are,
at least partly, a matter of interpretation or assumption, then the Liar Paradox called these into
question, rather than making the language useless.
Grammar-Only vs Logical Languages
This distinction that has been identied between formal and natural languages is so useful in
understanding the Liar Paradox, that we will adopt explicit terminology for the dierent types of
language. We will label languages which explicitly include the logical principles of reasoning for
that language, as Logical Languages and languages which do not explicitly include principles of
reasoning as Grammar-Only Languages. It should be noted that the sense of Grammar that is
being invoked here is that there are rules governing what are and what are not valid sentences.
These rules may rely on semantic information such as meanings of words, as they often do in
natural languages. Thus a Grammar-Only language is one which has a set of rules about what are
valid sentences, but does not explicitly include in its denition complete information about correct
logical reasoning about or from sentences. As argued above, natural languages, such as English,
count as Grammar-Only languages on this denition.
Although the distinction has been drawn between formal and natural languages, not all formal
languages are necessarily logical languages, and not all natural languages are necessarily GrammarOnly Languages. Furthermore, it is often useful to treat a Logical language as having a GrammarOnly part, and a Logical part. Thus, for a formal language, the Grammar-Only part is the set
of syntactic rules that determine what are allowable sentences, or well formed formulas.
Logical part are the axioms or rules of inference which determine which sentences are provable
and which are not. This means, for example, that Classical and Intuitionistic logics have identical
Grammar-Only parts, and only dier in the Logical part.
Importantly, this distinction between Logical and Grammar-Only Languages corresponds to
dierent ways that a language can be aected by the Liar Paradox. A Logical Language is aected
in the sense that it may or may not be trivialised by the Liar Paradox. That is, the Liar Paradox
may render a Logical Language trivial (and useless) since everything is provable in that language.
Since the principles of reasoning for a (fully) Logical Language are completely dened, it makes
sense to ask whether or not the Liar Paradox trivialises the language, and almost always there will
be a denitive answer. For Grammar-Only Languages, however, it does not make any sense to ask
whether such a language is trivialised by the Liar Paradox. Such languages are only dened by a
set of grammatical rules, and grammatical rules do not generate sets of provable sentences. Thus
it does not make sense to say that the Liar Paradox means that everything is provable in English.
Thus Grammar-Only Languages are not aected by the Liar Paradox in the sense that the Liar
Paradox can trivialise them.
However, there is a real sense in which Grammar-Only Languages are aected by the Liar
Languages such as English are aected by the Liar Paradox since sentences can be
constructed in such languages that, under the assumption of certain principles of reasoning about
the language and terms within the language, lead to the argument typical of the Liar Paradox,
and hence a contradiction. Thus Grammar-Only Languages are aected by the Liar Paradox in
the sense that sentences (or something equivalent) are constructable in the language which, under
the relevant assumptions, trigger the Liar Paradox.
This distinction between Grammar-Only and Logical Languages is useful for understanding the
Liar Paradox as it helps clarify the dierent senses in which one can say that a language can be
aected by the Liar Paradox. Languages can aected by the Liar Paradox in the sense that the
language is trivialised by the Paradox, or in the sense that sentences (or similar) are constructable
within the language which trigger the Paradoxical reasoning under appropriate assumptions. Languages which are trivialised by the Liar Paradox must obviously be also aected in the second
sense, and the appropriate assumptions must be reected in the principles of reasoning dened
within the language.
Grammar-Only Languages can only be aected in the second way; while
Logical Languages can be aected in both ways.
This distinction between Grammar-Only and Logical Languages, and the corresponding distinction
between the dierent ways that a language can be aected by the Liar Paradox helps clarify the
problem posed by the Liar Paradox, and hence helps identify strategies for solving the problem.
As phrased before, the problem posed by the Liar Paradox is that if the paradoxical conclusion is
correct, any statement in English can be proven and therefore is true. That is, to use the term
adopted in the previous section, English would be trivialised - everything would be provable in it
and therefore it would be useless.
However, it was argued in the previous section that the Liar Paradox does not trivialise a
Grammar-Only Language such as English, since English does not include explicit rules of inference
as part of its denition. Rather, the Liar Paradox aects Grammar-Only Languages in the sense
that under certain assumptions about logical reasoning and truth, the Paradoxical reasoning arises.
The Liar Paradox only trivialises Logical Languages.
We can however take English plus the intuitively correct principles of reasoning and truth
accepted in the argument presented above to be one linguistic system, say
terminology adopted here,
English*
above is therefore a proof that
is a Logical Language.
English* .
The paradoxical argument given
is trivialised by the Liar Paradox. We cannot accept the
assumptions inherent in the argument without modication, since that leads to an inconsistent
logico-linguistic system in which everything is provable.
20 That is, our intuitive understanding of
English, Truth and the way that languages represent Truth is inconsistent.
Put in this way, the challenge of identifying the incorrect assumptions in the paradoxical argument becomes the challenge of nding a set of assumptions about truth and reasoning which
20 Assuming
a suciently classical logic.
satisfy two conditions. Firstly, they capture the correct concepts and correspond to our intuitions.
Secondly, that the Logical Language corresponding to the linguistic system formed by adding these
to English (say
English+ )
is not trivialised (i.e.
aected in the more substantial sense) by the
The appropriateness of this way of framing the problem can be seen in the numerous attempts
in the literature to dene a formal solution to the Liar Paradox. The main components of a formal
solution are a formal language and a truth denition for that language, and the aim is to develop
such a solution which is not aected by the Liar Paradox. The aim is that a correct formal solution
would demonstrate the logical structure for
English+ , the correct linguistic system which includes
English and the correct assumptions about reasoning and truth. That is, these formal solutions
are designed to embody the correct assumptions, and to demonstrate the mechanism by which the
Paradox is avoided is we adopt the correct understanding.
Unfortunately, as commented before, there exist too many solutions rather than no good ones.
There exist many formal solutions to the Liar Paradox in which the formal language is not trivialised. That means philosophically that there are many candidates for
aected by the Liar Paradox in the sense that
which are not
is not trivialised. This should mean that
the challenge that we face is choosing between these competing candidates, and developing some
criteria by which we can choose between them. However, while all of these formal solutions provide
a Logical Language which is not aected by the Liar Paradox, it does not mean that they can all
function as candidates for
English+ .
In a philosophical follow-up to his formal denition of truth, Alfred Tarski oered a powerful
argument that, in the terminology adopted here, any Logical Language which satises certain
plausible assumptions is necessarily inconsistent.
The key assumptions in his argument were
articulated by Tarski as follows:
(I)-We have implicitly assumed that the language in which the antinomy is constructed
contains, in addition to its expressions, also the names of these expressions, as well as
semantic terms such as the term "true " referring to sentences of this language; me have
also assumed that all sentences which determine the adequate usage of this term can be
asserted in the language. A language with these properties will be called "semantically
closed."
(II) We have assumed that in this language the ordinary laws of logic hold.
There is a third principle that is crucial to his argument, which Tarski considered that he had
established as correct, what is now known as the T-Schema. For Tarski this is expressed as: X is
the clearest account see Alfred Tarski. The Semantic Conception of Truth: and the Foundations of Seman-
tics. In: Philosophy and Phenomenological Research 4.3 (1944), pp. 341376
22 Tarski,
The Semantic Conception of Truth, see n. 21, p. 348
true if, and only if, p.
23 Tarski uses X as a name for the sentence p. What is important to note in
this context is that the rst is an assumption about the Grammar-Only part of a Logical Language,
the second is an assumption about correct reasoning and the third is an explicit assumption about
Truth. To translate his point into our terminology, Tarski argued that any Logical Language that
includes a plausible truth denition (i.e. one that satises the T-Schema), that satises the ordinary
laws of logic and whose Grammar-Only part is semantically closed, is necessarily trivialised by the
English+ ,
whatever the correct logical and alethic assumptions are, is a Logical Language
whose Grammar-Only part (English) is semantically closed. For this reason, Tarski thought that
English was necessarily inconsistent and needed to be replaced by a scientic, logical language for
24 This conclusion of Tarki's has justiably been rejected by modern
correct discussion of truth.
philosophers and logicians, but it reveals an important aspect to the challenge of providing a formal
solution which is a plausible candidate for
The Grammar-Only part of
semantically closed, and therefore the Grammar-Only part of any formal language which provides
a formal solution must be semantically closed.
capture the logical structure of
Otherwise, the formal solution cannot correctly
That is, if we want a formal solution which correctly
deals with the Liar Paradox as it poses a philosophical challenge for natural languages as we use
them, that solution must be semantically closed.
However the vast majority of (and arguably all) formal solutions to the Liar Paradox are not
semantically closed. This means that these solutions cannot capture the correct assumptions about
reasoning and truth which allow us to reason satisfactorily in English in the ways we habitually
do. They can only be accepted as a valid solution if we change the way we use English, if they
can be accepted at all. The aim of this thesis is to provide a new formal solution which allows
a semantically closed Grammar-Only part to a Logical Language and therefore is a plausible
candidate for
Summary of rest of the Thesis
The development of a new formal solution must be grounded in an accurate analysis of the Paradox.
It was argued above that investigating languages oers a clear method for dierentiating between
the paradoxical and the non-paradoxical, and this dierentiation will be the focus of investigations
in the rst chapter.
Given that two dierent ways that languages can be aected by the Liar
Paradox, it follows that there are two dierent distinctions that need to be drawn and we can
divide this investigation into two projects. The rst distinction is the line between Grammar-Only
languages that are aected and those that are not. That is, we need to identify which properties a
23 Tarski, The Semantic Conception of Truth, see n. 21, p. 344
24 Alfred Tarski. The Concept of Truth in Formalized Languages.
Oxford University Press, 1956, pp. 152278, p. 165
In: Logic, Semantics and Metamathematics.
Grammar-Only Language must possess for Liar Sentences to be constructable within that language.
The rst project therefore is to develop an account of the grammatical conditions necessary for
Liar Sentences to be constructable in a (Grammar-Only) Language.
The second distinction is between those Logical Languages which are trivialised by the Liar
Paradox and those which are not. We need to uncover which principles a Logical Language must
satisfy for the Liar Paradox to trivialise that language. As noted before, every Logical Language includes a Grammar-Only part, some rules about which sentences are grammatically valid. In formal
languages, these are normally purely syntactic rules. The Grammar-Only part of a Logical Language must therefore possess the properties necessary for Liar Sentences to be constructable within
that language, which will be identied in the rst project.
This means that the second project
is to provide an account of the principles of reasoning necessary for the Paradoxical reasoning to
produce a contradiction and hence trivialise the relevant (Logical) language. These two projects
will be tackled in the rst chapter, with the aim of providing a comprehensive account of which
languages are aected by the Liar Paradox (in both senses) and therefore a deeper understanding
of what the Liar Paradox is.
The second chapter will examine various formal solutions that have been advanced. Tarski's
work will be a particular focus, since Tarski more than anyone else is responsible for dening the
parameters of the modern debates about the Liar Paradox. Moreover, the various formal solutions
which have been advanced since Tarski can be understood as attempts to construct non-trivial
Logical Languages which are as close to being semantically closed as possible. It will be argued
that many existing formal solutions suer from a systematic limitation. They cannot, within the
logical language, assign certain sentences about Liar Sentences the correct truth status, according
to the internal properties of the solutions themselves.
The aim of the third chapter is to track down the cause of this systematic limitation. The fact
that there is a common limitation within existing solutions suggests either that it an unsolvable
problem, or that there is a systematic aw in the strategies that are currently being used.
is important here to note the dierence between what is here dened as a logical language and
a formal language.
A logical language is a language which includes an explicit denition of its
principles of reasoning.
A formal language is a language in which a mathematical or symbolic
logic is dened. Given that the formal solutions that are advanced are constructed within modern
formal languages, it is vital to question whether there are assumptions within modern formal logic
which are incompatible with a satisfactory solution to the Liar Paradox. It will be argued that
two key semantic assumptions about formal languages, namely that truth values are completely
dened with respect to a semantic model and that sentence types are the primary truth bearers,
are incompatible with a satisfactory philosophical solution to the Liar Paradox.
The second half of the thesis will turn to developing a more satisfactory solution to the Liar
Paradox on the basis of the analysis in the third chapter. The fourth chapter will develop a blueprint
for a more satisfactory formal truth denition by focusing on our understanding of how truth is
dened in a language, and particularly the way that we evaluate the truth value of sentences which
include the truth predicate.
With this focus, the interaction between predicates like truth and
the reference structures necessarily for the denition of a truth predicate become very signicant,
and it is shown that care is needed in the denition of a truth predicate. This blueprint will be
developed entirely independently of any particular formal logic, and will therefore be applicable to
any potential formal logic. The blueprint is shown to be successful in solving the Liar Paradox in
a very appealing way in the fth chapter, where it is implemented in a Classical Sentential Logic.
This denition is provably consistent, which is a remarkable result.
The sixth chapter builds on and improves the blueprint from the fourth and fth chapters by
exploring the questions about the appropriate truth bearers that arose in the analysis in the third
chapter.
It is argued treating name/sentence pairs as truth bearers in formal languages allows
us to improve the denition in the fth chapter to a consistent, semantically closed formal truth
denition.
Once again, this is dened in a Classical Sentential Logic, and arguably provides a
philosophically satisfactory formal truth denition.
The seventh chapter concludes the thesis by summarising the key features of the approach
adopted in chapters four to six, and discusses its applicability to understanding truth in natural
languages.
It is argued that the key problem the Liar Paradox produces, that we cannot be
sure our use of natural languages is consistent or coherent, is solved as this approach provides
an understanding of truth which is consistent, applicable in natural languages and semantically
closed.
Understanding the Liar Paradox
The key point that was made in the Introduction is that the most important type of thing to
investigate when considering the Liar Paradox is languages. There is a clear distinction between
paradoxical and non-paradoxical languages, and languages set up the conditions which make the
paradox possible. Moreover, the key philosophical problem posed by the Liar Paradox is a problem
about whether our use of natural languages is consistent and therefore whether we can actually use
them in the way that we assume we do. This change of focus from sentences or cases to languages
means it is necessary to rethink our understanding of the Liar Paradox.
As argued in the Introduction, drawing a distinction between the paradoxical and the nonparadoxical is a key task in understanding a paradox. Given that languages set up the conditions
which make the Liar Paradox possible, we will look at the conditions under which languages are
aected by the paradox.
Languages which are aected by the Liar Paradox will satisfy these
conditions. Therefore identifying these conditions will provide a set of criteria which distinguish
languages which are aected by the Liar Paradox from those which are not. This investigation will
proceed from the two dierent types of language identied in the Introduction, as Grammar-Only
and Logical Languages are aected dierently by the Liar Paradox.
1.1 Why Languages?
The Introduction introduced the idea that languages are central and essential to any understanding
of the Liar Paradox, but it is important to make this idea and the reasons for it clear. The concept of
language at play is a very broad one, which encompasses, at a minimum, natural languages, formal
languages and potential formalisations of intuitive reasoning. As a paradoxical argument, the Liar
Paradox obviously depends on principles about language and certain principles of reasoning to get
o the ground. Also crucial to the our investigation is what we will refer to as the logico-linguistic
structure of a language, that is the collection of logical and linguistic (in a broad sense) principles
CHAPTER 1.
which hold when we consider the language in question. However, this logico-linguistic structure
depends on the language in question, and in any case Logical Languages incorporate much of it into
their explicit denition. Languages therefore will be taken to be our primary focus of investigation.
The importance of languages in investigating the Paradox can be seen rstly in the fact that
the Liar Paradox calls into question our ordinary use of natural languages. The grammatical rules
of natural languages allow us to build and use sentences in particular ways which are inconsistent
on our intuitive understanding of the logico-linguistic structure of natural languages. Moreover, in
the literature, the standard approach to presenting a seriously worked through solution to the Liar
Paradox is to provide a formal truth denition in a formal language. The key point to note about
these solutions is that invariably it is the language which is modied more heavily to deal with the
Paradox, rather than the truth denition. That is, the problem posed by the Liar Paradox, and
our approach to its solution turn on languages, which is good evidence that language should be
central to any investigation.
More importantly however is the fact that it is a language which establishes the conditions which
make the Liar Paradox possible. When the grammar of a language satises certain conditions, it
will follow that a liar sentence will be constructable; and when the logico-linguistic structure of that
language satises further conditions, then that sentence will give rise to a contradiction. Without
the relevant conditions being satised at the language level, the Paradox will not arise.
This is particularly important since one of the important features of the Liar Paradox is that
it arises so long as a relevant example is constructable within a language. The structure of the
paradoxical argument it requires only the assumption that a sentence is true (or not true) means that the paradoxical reasoning will arise even if a problematic sentence is never actually
used or constructed. The important fact is therefore whether problem cases are constructable, not
whether they exist. Thus the key factor in determining whether the Liar Paradox arises is whether
the language allows the relevant cases to be constructable.
The properties of the language are
central to the Liar Paradox, and we will therefore be focusing the investigation on languages.
1.1.1 The Focus on Languages
The goal of focusing on languages in understanding the Liar Paradox is to identify the conditions a
language must satisfy for it to be aected by the Liar Paradox. This will help identify precisely what
it is that leads to the Paradox arising, and therefore clarify the possible approaches to resolving
the paradox. There were, however, two dierent types of languages identied in the Introduction
which were aected by the Liar Paradox in dierent ways.
These two types of languages will
therefore be investigated separately and two separate sets of criteria will be devised.
The rst will identify when a Grammar-Only Language is aected by the Liar Paradox, in the
sense that a contradiction arises under certain assumptions about reasoning and truth. The second
will identify when a Logical Language is aected, in the sense that it is trivialised. These criteria,
if successful, will accurately distinguish between all the languages which are aected by the Liar
Paradox, and all languages that are not; and it is a distinction that cannot be aected by the
Revenge Problem. It will therefore provide a solid conceptual basis on which a solution to the Liar
Paradox can be built.
An in depth analysis of the role of languages in the Liar Paradox may nevertheless seem to be
unnecessary, since it does not seem dicult to identify what the Liar Paradox is. There are certain
sentences which are true if, and only if, they are not true; and therefore give rise to a contradiction.
These sentences are typically formed by setting up a situation where the sentence ultimately says
of itself that it is not true, possibly via other sentences. While this is correct, analysing the Liar
Paradox in this way does not provide any insight into why these sentences arise, or the conditions
under which they cause a paradox. Without a clear understanding of these, we cannot have truly
understood the Paradox and do not have a strong basis for formulating a resolution.
This however does not imply that paradoxical sentences and situations are not important,
and are not to be investigated.
There is no Liar Paradox if there are no paradoxical sentences
However, while they will be investigated, the focus of the investigation must be
dierent. Rather then trying to understand what it is which makes these cases paradoxical, the
aim will be to identify the necessary linguistic and logical conditions which make these cases
possible. Paradoxical cases arise only if certain linguistic and logical structures are available, and
these structures are only available in languages that are aected by the Liar Paradox. The focus
of investigation into paradoxical sentences and situations will therefore be on the minimum set of
linguistic and logical conditions which could make the particular cases possible. The aim is to use
dierent cases to build a set of necessary conditions which all languages which are aected by the
Liar Paradox must satisfy.
1.1.2 Separating the Liar Paradox from other Paradoxes
Before this can be done, there remains one factor which needs to be claried. It may seem like
a minor point, but we need to be able to identify what it is that make the Liar Paradox the
Liar Paradox, as opposed to any other Paradox. Trivially, any genuine paradox will lead to an
inconsistency and a trivialised logical language. If we are to precisely dierentiate languages which
are aected by the Liar Paradox from languages which are not aected, it is important to have a
clear idea of what the Liar Paradox is, as dierent logical languages will be aected by dierent
paradoxes. Without a clear distinction between the Liar Paradox and other paradoxes, it is not
possible to ensure that any developed distinctions relate to the Liar Paradox, and not to paradoxes
in a more general sense.
It should be noted that we would hope that any solution that is developed on this basis could
solve other paradoxes, particularly closely related paradoxes like Curry's Paradox. However, the
aim is to develop a solution to the Liar Paradox, and for this it is important to precisely understand
the Liar Paradox. If we discover, on this basis, that there are other paradoxes that are caused by the
same assumptions or mechanisms, then this discovery should be made from a precise understanding
of each of the paradoxes separately, not by associating the paradoxes as similar from the beginning
of the investigation.
The most obvious feature that distinguishes the Liar Paradox from some other paradoxes is
that, as the Liar, it has something to do with truth. Moreover, simple Liar Sentences appear to
be distinguished by the fact that the content of the sentence is not consistent with the sentence
being either true or not true. In the most common case, what the sentence says (This sentence is
not true) directly contradicts the conditions under which the sentence, as a declarative sentence,
could be true (or not). That is, for the sentence as a declarative sentence to be true, what it says
must hold. However, the semantic content of the sentence (what it says about what is the case)
means that this cannot be the case. The semantic content of the sentence is inconsistent with the
standard conditions under which the sentence would be true.
The concept of the semantic content of a sentence being adopted here is to be understood as
what the sentence says is the case, or similarly, as what a competent user of the language would
understand about the way things are by understanding the sentence. Under dierent philosophies
of language, it could be equivalent to concepts such as the meaning of the sentence, or the truth
conditions. Here we are trying to analyse the situation without recourse to any specic philosophy
of language, and so will not decide between any of the alternatives.
The key fact is that the
semantic content of a sentence is the information we use to test whether the sentence is true or
not. Intuitively, if the semantic content (what the sentence says) holds, then the sentence is true
and vice versa.
It is important to note that the contradiction which arises in the Liar argument is a `direct'
contradiction. That is, the semantic content contradicts the relevant conditions themselves. It is
not the case that the combination of what the sentences says and some truth conditions together
imply a contradiction, but that they contradict each other. This fact is reected in the argument
structure that is distinctive of the Liar Paradox: If we assume that a particular sentence is true,
then it follows that that sentence is not true; similarly, if we assume that the sentence is not true;
then it follows that it is true.
The entailment in the case of the Liar Paradox by which each
conclusion follows is very strict. We do not need any auxiliary hypotheses or information beyond
what the relevant sentences say (the semantic content) and the conditions on the relevant sentences
(as declarative sentences) being true. Moreover, the contradictory conclusion is not reached via
the derivation of any other inconsistency, but follows directly from these relevant facts. This direct
contradiction also means that the arguments are symmetrical when one assumes truth or one
assumes untruth.
This fact that no auxiliary hypotheses or information beyond the semantic content of the sentences and the conditions on the relevant sentences being true are needed in deriving the contradiction holds for multiple sentence versions of the Liar Paradox. In the case of a multiple sentence
version of the Liar Paradox, if we assume that any one of the relevant sentences is true (or not
true), then simply from what the various sentences say and the relevant conditions on the sentences as declarative sentences being true, it follows that the initial sentence is not true (or true).
Importantly, every sentence in the derivation will either be one of the sentences or its negation, a
statement which expresses the semantic content of a sentence, or a statement of the conditions on
a declarative sentence being true. No other type of sentence is necessary in the derivation of the
contradiction, and the rst contradiction that can be derived contradicts the initial sentence.
This strict characterisation of the Liar Paradox allows us to dierentiate the Liar Paradox from
both Curry's and Yablo's Paradoxes. In the case of Yablo's Paradox, the argument structure is
not symmetrical for the assumption of truth versus the assumption of untruth. If we assume that
a relevant sentence in a Yablo series is not true, then it is not possible to prove that that sentence
must be true without rst proving a separate contradiction. It follows that Yablo's Paradox is not
a case of the Liar Paradox, which is assumed by the fact that we use dierent names for the two
dierent types of paradox.
In the case of Curry's Paradox, the classic exposition turns on the fact that from a particular
type of sentence, it is possible to prove any sentence in the language.
The fact that we could
prove that the sentence itself is not true is not essential to the paradox in the way that it is in
the denition of the Liar above. It does not mean that there are not cases where Curry's Paradox
and the Liar Paradox coincide. There are obvious examples where a sentence can be both a Curry
Sentence and a Liar Sentence, but the two paradoxes are distinct as paradoxes.
Nevertheless, the fact that it is possible for certain examples to count as both Liar Sentences
and Curry Sentences illustrates the two paradoxes are closely related. It turns out, moreover, that
the solution advanced later deals equally well with both of these Paradoxes. However, this does
not imply that they are the same paradox. In particular, it will be shown below that there are
languages which can be aected by one of these paradoxes and not the other. However, the central
mechanism which gives rise to the paradox is identical for both paradoxes and so they can share
a common solution.
However, since the aim of this chapter and thesis is to deal with the Liar
Paradox, only languages which are aected by the Liar Paradox will be explicitly dealt with.
1.2 Grammar-Only Languages
As mentioned above, the investigation into a set of criteria which dierentiate languages which are
aected by the Liar Paradox from those which are not will be broken into two parts. The rst is an
investigation into the set of necessary and sucient criteria for a Grammar-Only Language to be
aected by the Liar Paradox. That is, the aim is to develop a set of criteria which identies those
languages for which, under the assumption of certain sets of principles of reasoning, a contradiction
is derivable in exactly the pattern described. As we are working with Grammar-Only languages,
these principles of reasoning are not part of the denition of the language, but are rather part of
either an assumed understanding of the language, or of a logical language of which the GrammarOnly language is a part.
Since it is not possible to survey all possible Grammar-Only Languages, the development of
these criteria cannot proceed through a systematic survey of all aected Grammar-Only Languages.
We will instead begin with an initial set of criteria which are clearly sucient to produce the Liar
Paradox. That is any language which has these criteria will be aected by the Liar Paradox. Each
member of this set will be challenged individually in order to determine what is actually necessary
to produce the Liar Paradox. In this way we will produce criteria which are both necessary and
sucient. We will begin with an intuitive set of criteria that is developed from the example of the
Liar Paradox given in the Introduction.
The example of the Liar Paradox given above was triggered by the following sentence:
Since we are concentrating on Grammar-Only Languages, we want a set of criteria which need to
be satised if this sentence is to be grammatically acceptable. With this in mind, it seems clear
that (1) is only grammatically acceptable in a language which has the following properties:
It has a truth predicate.
Sentences can refer to themselves.
Truth can be correctly predicated of sentences.
It has a negation.
There are no restrictions on combining the negation, the truth predicate and a reference to
other sentences within the scope of the truth predicate within a sentence.
If any of these ve conditions is missing, then sentence (1) is potentially ungrammatical, so the set
of conditions seems reasonable. Furthermore, if any language satises this set of ve conditions, it
will be aected by the Liar Paradox, since an equivalent example to (1) can be constructed. Hence
these ve are sucient conditions for a language to be aected by the Liar Paradox. However, it
can be shown that as written none of them are strictly necessary conditions.
1.2.1 Truth Predicate
The rst condition in the set above is that the Language includes a truth predicate. This condition
is in fact implicitly assumed within much of the debate about the Liar Paradox. The Liar Paradox
is normally considered to be a paradox about truth, and the formal focus is on correctly dening
a formal truth predicate. However, it is not necessary that a language contains a specic truth
predicate. For example, consider the following sentence:
What this sentence says is not the case.
Although there is no mention of truth within this sentence, the paradoxical reasoning arises. If
what sentence 2) says is true, then as 2) says that what 2) says is not the case, what 2) says
is not the case, and hence 2) is not true.
Similarly, if what 2) says is not true, then as that is
what 2) says, what 2) says is actually the case. This reasoning is identical to the standard Liar
reasoning, even though there is no truth predicate in the sentence (and the truth predicate can
also be removed from the argument). That a language possesses a truth predicate is therefore not
a necessary condition on that language being aected by the Liar Paradox.
This does not, however, mean that the Liar Paradox has nothing to do with the concept of
In the case of the sentence 2), the obvious comment is that although the sentence does
not use a Truth Predicate, it still makes implicit use of the concept of truth, or at least a very
similar concept. To claim that what some sentence says is the case is equivalent to saying that
that sentence is true. The conditions on either of these assertion being correct are in at least all
reasonable cases identical. Furthermore, the circumlocution what sentence X says is the case is
one way in which the concept of truth can be claried. Thus, although the sentence 2) does not
explicitly use a truth predicate, it contains an expression which expresses the concept of truth.
It is not dicult to see that this could be classed as a necessary condition on cases of the
Liar Paradox, and therefore languages which are aected by the Paradox. The structure of the
paradoxical argument turns on a direct conict between the standard conditions on a sentence
being true, and the content or meaning of that sentence.
This conict is only possible if the
content of a sentence can expresses something about the truth or untruth of a relevant sentence.
Without this linguistic ability, the necessary conict cannot arise and there can be no Paradox.
Given that the ability to express something about the truth or untruth of a sentence requires the
ability to express the concept of truth within the relevant language, it is clear that this ability is a
necessary condition on the Liar Paradox aecting a particular language. That is, it appears that
only Grammar-Only languages which can express a concept equivalent to truth are aected by the
1.2.2 Sentences
While it seems necessary that a Grammar-Only language can express a concept equivalent to truth
in order that the Liar Paradox can arise, it is clear that this is not enough. The second condition
listed above, that Truth can be correctly predicated of sentences, is also required for otherwise
the contradiction cannot arise.
The Liar Paradox turns on the ability of a sentence to identify
some sentence (possibly itself ) as true, when other conditions rule that it should be not true. If
truth cannot be correctly predicated of sentences, then this ability does not exist in the language,
and therefore the Liar Paradox cannot arise.
However, it does not follow that, as stated, this condition is a necessary condition. Something
along these lines is necessary, but the statement of the condition that Truth can be correctly
predicated of sentences makes assumptions that are not required in general. In particular, the
assumption of sentences is not necessary, and not simply the assumption that sentences are truthbearers.
The inherent assumption that a language needs to contain sentences (or equivalently
complex grammatical constructions) is not necessary for the Liar Paradox. The Liar Paradox can
aect very simple languages, which do not contain any of the grammatical complexity of ordinary
For example, consider a very simple language which includes two gestures. Thumbs up corresponds to saying that what has been asserted is correct. Thumbs down means the opposite,
that what is asserted is incorrect. One could further imagine, if necessary that these signs are part
of say a Caveman language which has certain sounds or symbols for things like I am hungry;
or Dangerous animal coming. Such a language would be extremely simple, yet the Thumbs up
and Thumbs down symbols would have a very useful role is conrming or denying information.
However, one can further easily consider the situation where Person A gives the Thumbs up
sign to Person B; while Person B is giving a Thumbs down to Person A. If what A asserts
is true, then it means that what B asserts is true; which means that what A asserts is not true.
Similarly, if what A asserts is not true; then B is correct in giving the Thumbs down sign; but
that means that what A asserts is actually true. We have a classic Liar Paradox.
This means rstly that grammatical complexity is not necessary for the Liar Paradox to arise,
since it can arise in such a simple language. We cannot therefore assume any particular complexity
or type of language as necessary for a solution to the Liar Paradox. Any set of symbols, which
has a potential interpretation or which means something, can potentially be aected by the Liar
Paradox. Given that this is a very minimal account of what a language is (a set of symbols with
a potential interpretation); there are no restrictions on what type of languages can be aected by
Secondly, this shows that many of the debates about the correct truth bearer are not always
relevant to understanding the Liar Paradox.
It is not at all clear how we would classify the
problematic gestures in the Paradoxical example. They are not sentences or sentence tokens on
any normal denition, and while they might assert a statement or express a proposition it is not
clear at all how to understand these things. One might also want to say that the relevant signs
or gestures are true or not, as the case may be. In any case, something ought to be true or not
true in this situation, and this is all that is necessary for the Paradox to arise: (at least many
of ) the symbols which make up the language must be capable of either being true or not true; or
expressing something which is true or not.
This is nothing remarkable, since it follows directly
from the denition of a language as a set of symbols with an interpretation.
Furthermore, it is important to note that however we interpret the correct truth bearers, it does
not aect whether a Grammar-Only Language is aected by the Liar Paradox. The denition of a
Grammar-Only language being aected by the Liar Paradox is that it produces the Paradox under
some understanding of the language. For any choice of truth bearer, there will be a corresponding
understanding (or a corresponding set of logical assumptions) which gives rise to the Paradox.
While debates about the correct truth bearer may be relevant to the correct understanding of a
language, they do not aect whether a Grammar-Only language is aected by the Liar Paradox.
As a matter of convenience and terminology, sentences will be habitually referred to as the
normal truth bearers in this thesis. This does not reect any philosophical conviction, but rather
reects both the necessity of having some term to use and the formal interests of the investigation.
Formal languages are typically dened so that sentences are the truth-bearers.
However, it is
almost always possible to substitute sentence for any other truth bearer and when the appropriate
modications are made, the arguments will be still valid.
1.2.3 Negation
If we assume that a necessary condition on a language being aected (in any sense) by the Liar
Paradox is that it is possible to express the concept of truth within that language, it is obvious that
this condition is not sucient. The concept of truth, by itself, cannot generate the contradiction
within the Liar Paradox. It can only generate the non-contradictory puzzles surrounding the Truth
- Teller, that is cases such as This sentence is true. In these cases, no contradiction can be derived,
but there is no apparent way to derive the truth or falsity of the relevant sentence.
Obviously, if we have a way of expressing the concept of truth within a language, it is also
necessary to have a negation within that language.
The Liar Paradox turns on the fact that it
necessarily follows from the content of a sentence that that sentence is not true.
Without the
negation, the Paradox cannot be formed. It is not necessary to assume anything substantial about
the behaviour of the negation within the language.
Almost any negation is sucient for these
However, if we want to identify the truly necessary conditions, the separate existence of a
negation and a way of expressing the concept of truth is not required. So long as there is a method
of expressing the concept of untruth, or of not being true, and this can be predicated of other
`sentences' then it is possible to produce a paradoxical example. Thus for example, it is possible
that a language could have the ability to express the concept of Falsity without being able to
directly express the concept of Truth. The Liar Paradox would still arise in this language. While
the types of formal and natural languages that are commonly dealt with all include a method of
expressing Truth and a negation, this is not necessary for the Paradox to arise.
It might seem as though this point is unimportant, since all of the languages that we are
interested in include a negation and a way of expressing Truth. However it is important to the
way that we understand the paradox, and what provides a satisfactory solution. A solution, for
example, which relies on a dierent denition of negation cannot be a general solution, since it
cannot deal with languages which do not include a negation, yet are still aected by the Liar
The rst necessary condition on Grammar Only languages being aected by the Liar Paradox
is that the language includes a method of expressing untruth. Normally we would expect this to
be via a method of expressing truth and a negation, but this is not strictly necessary.
1.2.4 Self-Reference
It has been often pointed out that self-reference, although included in the list of sucient conditions
above, is not necessary for the Liar Paradox. The most obvious examples are the multiple sentence
versions of the Liar Paradox, where a set of sentences all refer to each other and the end result is
a reference loop which is paradoxical. A simple example is the following pair of sentences:
The following sentence is true.
The previous sentence is not true.
Assuming any truth value for these sentences leads to contradiction, in the same way that sentence
(1) leads to a contradiction. This means that the argument has exactly the same structure and
therefore, by the characterisation above, we have a case of the Liar Paradox. This case, however,
does not include any self-reference, so the possibility of self-reference is not a necessary condition
for the Liar Paradox to aect a language.
This type of case, and any similar ones, suggest that cases which rely on self-referentiality
are only special cases of a broader phenomenon involving extended circular reference loops. The
paradox occurs because there is a set of sentences which each refer to some other sentence(s) in
the set and the combination means that there is a closed loop of reference. If we follow the path
of reference we eventually get back to where we started.
It was argued above that the distinguishing feature of the Liar Paradox is that a contradiction
can be derived only using the sentences themselves, the semantic content of the sentences and the
conditions on the sentences as declarative sentences being true; and that the rst contradiction
derived contradicts the initial assumption.
We can see here that the condition on sentence (3)
being true as a declarative sentence is that sentence (4) is true (since this is the semantic content
of (3) ). However, if sentence (4) is true, then by what (4) says, it must be the case that (3) is not
true, from which it follows that (4) is also not true. Thus a contradiction arises without having
to appeal to any other sentences or broader information, and the equivalent argument holds if we
assume that (4) is not true.
This type of argument structure is only possible if we can argument back to a conclusion about
the sentence we started with. Given that the steps allowed in the argument are only the sentences,
their semantic content and the conditions on them being true as declarative sentences, the only
available method for progressing from one sentence to another is by a sentence referring to another.
In this case the semantic content (what it says) will identify a further sentence. The fact that we
get back to where we started means that there is some chain of reference that leads back to the
original sentence. This is only possible if circular reference loops are possible within the language.
Thus, it seems necessary for the Liar Paradox to arise that a language allows circular loops of
reference - chains of reference where we get back to where we started. This condition is, however,
hard to test as it is not immediately apparent from the denition of a language whether it will
allow complicated structures such as circular loops of reference. However, it is easy to test rstly
whether the language allows sentences to refer to other sentences; and secondly whether there are
any restrictions on which sentences can refer to other sentences. If the language allows sentences
to refer to other sentences and there are no restrictions on this, then the combinatorics of reference
mean that (unless the languages has a very limited range of sentences) circular reference loops will
almost certainly arise. The key condition that determines whether the Liar Paradox is possible is
whether circular loops of reference arise. If sentences are allowed to refer to other sentences in the
language, then circular loops of reference will arise unless the reference is restricted in a way to
prevent this occurring.
1.2.5 Reference
More needs to be said about exactly what is meant by reference. In the examples above, reference
has been assumed to work on a naming model - we refer to a sentence by inserting a name of that
sentence in another sentence. This is a simple type of reference which is not representative of all
possible types of reference, or even of all examples of the Liar Paradox.
Perhaps the most interesting example is the liar sentence advanced by Quine. In this case there
is no explicit reference, or naming, of the sentence itself, although the sentence appears paradoxical.
Quine's example is the following:
yields a falsehood when preceded by its own quotation yields a falsehood when preceded by its own quotation.
2 then this sentence is
Assuming that `falsehood' is to be interpreted as meaning a false sentence,
a correct sentence of English and it is paradoxical. The paradoxical reasoning which characterises
the Liar Paradox arises in the following way. If the predicate yields a falsehood when preceded
by its own quotation is true when preceded by its own quotation, then E is true. However, E says
that this predicate yields a falsehood when preceded by its own quotation, so what it says is false,
which contradicts the assumption that it is true.
Similarly, if yields a falsehood when preceded by its own quotation actually yields a falsehood
when preceded by its own quotation, then what the above sentence says is true.
As the above
sentence is the result of preceding yields a falsehood when preceded by its own quotation by its
own quotation, then the predicate cannot yield a falsehood. Again we have a contradiction.
This sentence, however, does not appear to refer to any other sentences, at least in the same
way that the sentences above referred to other sentences. However, if we assume as we must that
`falsehood' means a `false sentence', the E provides a description of a sentence which it is talking
about. This sentence is the sentence that is produced when yields a falsehood when preceded by
its own quotation is preceded by its own quotation.
therefore referred to is the sentence E itself.
But this sentence which is described and
Hence, because of the meaning of the sentence, E
refers to itself.
Thus the sentence refers to itself, only via the meaning of the sentence rather than through
an explicit naming of a sentence. However, this weaker sense of reference is sucient to give rise
to the Paradox.
In fact, if we consider the paradoxical argument, any means of reference from
one sentence to another will suce. Thus reference in the list of criteria has to be taken in the
broadest possible sentence. So long as the reference allows truth (or untruth) to be predicated of
the sentence being referred to, the paradox can arise.
1.2.6 Restrictions
This nal item on the initial list is There are no restrictions on combining the negation, the truth
predicate and a reference within a sentence. As with the previous items, this item is not strictly
necessary. However, this is not due to substance of the item but rather to its exact wording. It has
1 This
is sometimes referred to as Quine's Paradox, see W. V. O. Quine. The Ways of Paradox. In: The Ways
of Paradox and Other Essays. Cambridge, Mass.: Harvard University Press, 1966, pp. 121
2 Appending
a predicate to a quotation yields a sentence, so we must interpret falsehood as a false sentence for
this sentence to be grammatically correct.
been shown that negation, a truth predicate and sentences are not strictly necessary, so as worded
this item cannot be necessary.
However, the principle holds in general.
So long as a language
includes restrictions on the way that i) its method of expressing untruth of entities in the language
and ii) its way of referring to these entities can interact, it is possible that the Liar Paradox will
not arise.
To make up an example, it is possible that a language allows truth to be predicated of other
sentences, but the negation of truth cannot be predicated of other sentences.
In this case the
Liar Paradox would not arise, even though the language satises the other necessary conditions.
Similarly, there might be a language which allows sentences to refer to other sentences, but not
in the scope of the truth predicate. The Liar Paradox is only in general possible if there are no
restrictions on the way that the other necessary conditions interact so that unrestricted reference
within the scope of the (un)truth predicate is possible.
1.2.7 Summary
In summary, we have developed a set of four necessary conditions for Grammar-Only Language to
be aected by the Liar Paradox. If any language satises all of these conditions, then it will be
aected by the Liar Paradox. That is, under appropriate assumptions about reasoning, truth and
possibly about matters of fact, a contradiction arises. However, any language that does not satisfy
all of these will not be aected by the Liar Paradox.
The list of conditions is as follows:
1. The language has a way of expressing the concept of untruth.
2. This concept can be predicated of/applied to the types of things which are used to express
facts/information.
3. The types of things which are used to express facts can refer, by whatever means, to other
things of this type.
4. There are no restrictions on the way that the mechanisms in the rst three points can be
combined. That is, there are no restrictions on the application of the concept of untruth to
the types of things which express facts within the language.
The phrasing of these necessary conditions is deliberately broad, so as to not to limit these conditions to any types of languages. As argued, we cannot make any assumptions about the complexity
of the languages involved, or about the specic mechanisms involved. Thus, as mentioned, although
the rst condition will normally be constructed using a negation and a truth predicate, languages
without this can still be paradoxical.
3 The
key point is that this condition allows circular reference loops to arise.
It is interesting to point out that if we conduct the same types of analysis for Curry's Paradox,
then conditions 2), 3) and 4) will remain essentially the same, but the rst condition will have
to change. In the case of Curry's Paradox, it is necessary that there is a way of expressing the
concept of truth, and that the language contains a conditional (or equivalent).
Obviously, English and other natural languages satisfy these four conditions. Moreover, there
is no way that these conditions can be restricted in a natural language to prevent the Liar Paradox
without signicantly altering the language in question.
1.3 Logical Languages
The second major part of this chapter is to look at the necessary conditions for Logical Languages
to be aected by the Liar Paradox.
Given that any Logical language can be separated into a
Grammar-Only part and a Logical part, the rst thing to note is that the Grammar-Only part of
a Logical language has to be satisfy the conditions listed above if the Logical language is aected
by the Liar Paradox. If the Grammar-Only part is not aected, then no potentially paradoxical
sentences can be constructed and therefore the Logical language cannot be aected. What we are
interested in in this section are therefore the conditions that the Logical Part needs to satisfy in
order that the Logical Language is aected.
In the Introduction, a Logical Language was dened to be aected by the Liar Paradox only
if it is trivialised, that is, only if everything is provable in the language. The reason for adopting
this criteria is that a number of important solutions to the Liar Paradox have been advanced
that accept the validity of some contradictions. It follows that we cannot use inconsistency as the
mark of an aected Logical Language, since that would rule out these solutions as valid without
even considering them. However, the traditional reason for ruling out inconsistencies is that it is
assumed that an inconsistency precludes anything in the language from being true, in other words,
an inconsistency would render the language useless as nothing identiably true can be asserted in
The obvious alternative therefore is to adopt triviality in the sense that everything is provable,
since a language in which this is the case is completely useless. One cannot use it to say anything
as everything is equally true and false, and everything entails everything else. Nothing identiably
true is assertible in such a language. There may be other alternatives where there is a language
in which not everything is provable, but it is equally useless. For example, it may be possible to
have a language in which the negation of every sentence is provable, but some sentences are not
provable.
This would be equally useless, if such a language exists.
However, as these are only
possibilities, they will be ignored.
Adopting this denition of a Logical Language being aected gives rise immediately to the rst
condition on a Logical Language being aected by the Liar Paradox. A Logical Language is only
aected if the ex contradictio quodlibet principle holds for the logical part of the language. That
is, only Logical Languages in which a contradiction implies everything are aected in the full sense
being adopted here.
Secondly, while it is possible that a Grammar-Only language which does not have a negation or
a concept that directly expresses truth can be aected by the Liar Paradox in the relevant sense,
this does not hold for Logical Languages. The pattern of argument that is distinctive of the Liar
Paradox begins with the assumption that a certain sentence is true (or not true) and concludes that
it is not true (or true). This argument can only be expressed in a language which includes both
a negation and the concept of truth. These comprise a second condition on a Logical Language
being aected by the Liar Paradox.
However, simply possessing a way of representing the concept of truth within the language is
not sucient to generate the paradoxical reasoning. A key assumption in the argument is that a
particular sentence is true. If this assumption does not make sense, then the argument falls over.
Thus it must be the case that the concept of truth applies to the grammatical sentences in the
language in the natural way: they are either true or not true. While strictly speaking, the paradox
can arise if only one paradoxical sentence must be either true or not true, this would be a very
arbitrary language. We will adopt the simpler condition that the sentences in the language are
either true or not true, and importantly, the method of expressing the concept of true captures
these accurately.
It was argued above that a distinctive feature of the Liar Paradox is that the relevant derivations
proceed without needing any auxiliary hypotheses or information beyond the meanings and truth
conditions of the relevant sentences. We see this clearly in the typical argument that follows from
a sentence such as:
Sentence (1) is not true.
For this sentence, the argument can be broken down as follows:
If sentence (1) is true, then it follows that what it says must be true. (Condition on
Truth as a Declarative Sentence) What it says is that it, itself is not true. (Semantic
Content) Therefore, if (1) is true, it follows that (1) is not true. (Derivation) However,
suppose we assume the other alternative, that sentence (1) is not true.
Then what
sentence (1) says is in fact true (Semantic Content), from which it follows that (1) is
true (Condition on Truth as a Declarative Sentence). Therefore, if (1) is not true, then
(1) must be true. (Derivation)
Without going into the details, this pattern can be seen in any other paradoxical argument that
arises from the Liar Paradox. However, this fact that no auxiliary information is required imposes
a clear, and intuitive, restraint on the truth conditions for sentences. If no auxiliary information
is allowed, then the truth conditions on a sentence cannot rely on anything beyond the semantic
content of that sentence. In other words, a sentence is true if, and only if, its semantic content
holds. This is another way of expressing the T-Schema: `P' is true i P. Thus, for the Liar Paradox
to bite, it must be the case that the T-Schema holds for the language. It is not necessary that the
T-Schema holds within the language, but only that from the sentence P is true, it is possible to
derive P, and vice versa. This imposes a fourth necessary condition on a Logical Language being
aected by the Liar Paradox, the T-Schema holds for that language.
Once we have established that a language allows the derivation from P is true to P and vice
versa (and the contrapositives) for all sentences, and that the language includes a negation and
the concept of truth, the standard pattern of derivation will arise for relevant cases.
That is,
from the assumption of the truth of a sentence (or equivalent) it will follow that the sentence is
not true; and vice versa. However, this is not sucient to establish that the logical language is
aected in the strong sense, as this pattern by itself does not give rise to a contradiction.
require sucient reasoning ability to turn this pattern into an explicit contradiction, so that ex
contradictio quodlibet applies. The required reasoning is very minimal, and is not unique. Various
combinations of principles of reasoning or normal assumptions will be sucient. We will provide
only one example using a Natural Deduction framework to illustrate. However, there are a range
of dierent possible combinations of principles that give an equivalent result.
The situation is that we have established the following two patterns of argument:
T rpP q
¬T rpP q
For example, if we assume further that the relevant language includes Reduction Ad Absurdem
∧-Introduction
(using Natural Deduction), then we get the following:
T rpP q ∧ ¬T rpP q
RAA ln. 1,3
We have derived a contradiction from the inclusion of two generally uncontroversial premises.
In general, any combination of principles which allow the relevant patterns of derivation to be
combined to produce a contradiction are sucient to mean that a relevant Logical Language is
We have identied ve conditions that a Logical Language must satisfy if it is to be aected in
the strong sense by the Liar Paradox:
1. ex contradictio quodlibet holds.
2. The Logical Language includes the concept of truth and a negation.
3. Sentences in the language are either true or not true (and the concept of truth in the language
captures this).
4. The T-Schema holds for the language.
5. The language includes sucient reasoning to derive a contradiction from the relevant pattern.
It follows that in any Logical Language whose Grammar-Only part satises the conditions in
the previous section, and whose Logical part satises these, everything is provable.
any language for which one of these does not hold is not trivialised in the same sense. We have,
therefore, a set of conditions which dierentiate languages which are aected from languages which
are not aected.
1.4 What is the Liar Paradox?
We have identied a set of nine conditions which are jointly necessary and sucient to distinguish
languages which are aected by the Liar Paradox from languages which are not. As argued, this
means that any Logical Languages (or logico-linguistic understandings of languages) which satisfy
these nine conditions for both the Grammar-Only and Logical Languages above are necessarily
trivialised.
The Liar Paradox can simply be taken to express this fact, that languages which
satisfy these conditions are trivialised, and therefore useless; and that languages that do not satisfy
these nine conditions are not necessarily useless. In this sense, the Liar Paradox is an interesting
observation about dierent types of languages.
This interesting observation has signicant consequences for our understanding of natural languages and the concept of truth in natural languages. As argued above, natural languages satisfy
the four conditions on Grammar-Only languages being aected by the Liar Paradox. Given that
any understanding of truth and reasoning in a natural language can be represented as a Logical
Language, it follows that any such understanding that satises the second set of conditions will be
4 Strictly
speaking, for the paradox to bite it is only necessary that one paradoxical sentence is true or not true,
but this is highly articial.
5 Similarly,
it is only necessary that the T-Schema to hold for one paradoxical sentence for the paradox to bite,
but this is again highly articial.
inconsistent and useless. However, as pointed out previously, an intuitive understanding satises
these principles.
The problem that the Liar Paradox produces is therefore that a correct understanding of truth
and reasoning must abandon at least one of these conditions, however it is not at all clear which
one can be given up. The next chapter will examine the approach that various existing solutions
take with respect to these criteria.
Solutions to the Liar Paradox
The previous chapter developed a precise understanding of the Liar Paradox, by identifying conditions under which it is possible.
It was argued that there is a list of nine conditions which
distinguish languages which are aected by the Liar Paradox from those languages which are not.
Four of these conditions relate to the vocabulary and grammatical construction of the language
and hence are conditions on Grammar-Only languages; the other ve are conditions on Logical
languages as they relate to the concepts and reasoning present within the language. Any logical
language which satises all nine conditions is trivialised by the Liar Paradox as everything will be
provable in that language.
As noted before, we can understand the Liar Paradox as the fact that any language which
satises all nine conditions is trivial, and therefore cannot be used for anything we would want to
use a language for. The formal and philosophical problems that the Liar Paradox poses are that
natural languages, combined with intuitive reasoning, and some formal languages, satisfy all nine
conditions. The signicant problems that this paradox poses requires a solution, and this chapter
will focus on solutions to the Liar Paradox, both the question of what makes a good solution and
some already existing solutions.
The understanding of the Liar Paradox developed in the previous chapter makes the possible
approaches to solving the Liar Paradox clear, but it does not oer anything that would help identify
the correct approach. If a language does not satisfy one of the nine conditions, that language will
not be aected by the Liar Paradox, and therefore will not be trivialised. In this fairly formal sense,
solving the paradox is relatively easy: one simply needs to provide a language, or understanding
of a language, which does not satisfy one of the conditions. While this would be a solution in the
formal sense, it may not be a solution in the sense of resolving the philosophical problems that
arise, and which have prompted investigation into the Liar Paradox in the rst place.
The philosophical issue is that the nine conditions are not all equal philosophically, while it
may be possible that each could be got around, the cost of doing so varies. For example, one could
decide that the solution to the Liar Paradox could be got by removing talk of untruth from a
This would prevent the Paradox, but at the cost of making the language unsuitable
for many purposes, especially philosophical discussion.
The diculty is that the Liar Paradox
itself does not provide any reliable guide to a satisfactory philosophical solution. The Paradox is
simply a fact about the consequences of certain conditions on languages, and while these conditions
demonstrate the areas that need to be considered for a solution, they do not provide any guide to
the solution in themselves.
Nevertheless, the philosophical problem itself does provide some guide to the scope of possible solutions.
The philosophical problem is that natural languages satisfy the four conditions
on Grammar-Only languages being aected by the Liar Paradox; and our intuitive understanding of reasoning and truth satisfy the ve conditions on a Logical Language being aected.
the combination of natural language and intuitive reasoning about it is trivialised and therefore
unusable.
If a proposed solution relies one of the four conditions on Grammar-Only languages, then to
adopt it we must alter the way we use natural languages and their grammatical rules to take this
into account. Such a solution would however be unsatisfactory for at least two reasons. Firstly,
it would seem heavy handed to mandate dierent grammatical rules for natural languages on
account of a logical paradox. This is not how natural languages are used or develop and is unlikely
to be heeded in general. Secondly, we use and reason with sentences that require the oending
conditions regularly and consistently, without having to be careful about how we use them. Either
we are mistaken about our own use, or it is possible to use natural languages consistently and
the Liar Paradox does not actually aect the logical structure behind natural languages.
should obviously only concede the rst of these options if there is absolutely no other option, as it
calls into question a vast amount of human endeavour and thinking. A satisfactory philosophical
solution should leave the rst four conditions alone and nd a solution in one of the remaining ve
conditions on logical languages.
We will therefore be looking for a solution which provides a logical language which satises all
four conditions on a Grammar-Only language being aected, and does not satisfy at least one of the
conditions on a logical language being aected. However, we can be even more specic, as natural
languages denitely satisfy one of the conditions on a logical language. The relevant condition is
that the Logical Language includes the concept of truth and a negation, which natural languages
satisfy. This leaves four conditions in play in the search for a satisfactory philosophical solution,
as one can provide a case that each of the other conditions can be given up, although the strength
of the cases varies.
The fact that cases can be made for all of these conditions demonstrates that it is not possible
to provide a denitive unchallengeable solution to the Liar Paradox, as the solution must draw on
factors outside the paradox. We could only claim that there was such a solution if we had complete,
universally accepted philosophy of language.
We cannot claim to have or to be proposing such
a complete solution. Nevertheless, we can see the Liar Paradox as a useful tool in developing a
broader philosophy of language, as it can be used to rule out paradoxical philosophical positions.
The focus of this work is however elsewhere.
We will be concentrating on developing a formal
logical language that provides a philosophically satisfactory formal truth denition.
2.0.1 What conditions are in play?
The most basic condition on a solution being philosophically satisfactory, as discussed, is that a
logical language which embodies the solution satises the conditions which any natural language
must satisfy out of the nine conditions identied in the previous chapter. For reference, these nine
conditions are:
combined.
The rst four of these conditions, which the conditions on Grammar-Only languages being aected
by the Liar Paradox, are all satised by natural languages, as they are conditions on the way that
sentences can be constructed and the vocabulary of the language. So any satisfactory solutions
must satisfy all of these.
As argued above, any reasonable understanding of natural languages
must also satisfy condition 6. Truth and negation exist in natural languages, and therefore any
understanding of natural languages must include these.
This leaves four conditions which are in play in terms of a satisfactory solution to the Liar
Paradox, namely:
ex contradictio quodlibet holds.
Sentences in the language are either true or not true (and the concept of truth in the language
The T-Schema holds for the language.
The language includes sucient reasoning to derive a contradiction from the relevant pattern.
There are strong arguments that each of these hold, yet it is also possible to form arguments
against each of these.
For the moment, we will not look in depth at the reasons for holding or
abandoning any of these, as this will be done in the context of solutions to the Liar Paradox in the
literature. Before we look at these, however, it is important to clarify what exactly is meant by a
solution to the Liar Paradox.
2.1 What is a Solution to the Liar Paradox?
Within the literature on the Liar Paradox, there is not only disagreement about the correct
solution to the paradox, but there seems to be a great diversity of opinion as to what constitutes a
solution. At one end, some authors oer a diagnosis of the incorrect assumption and demonstrate
how the paradoxical reasoning in a natural language context is blocked for some examples when
these are corrected.
At the other end, others simply present a technical denition of a formal
language in which there is a truth predicate and the paradox does not arise.
These are very
dierent projects, and neither captures fully what an adequate solution to the Liar Paradox is.
The rst approach recognises that the problem that needs to be solved is primarily a question
about the coherence of our use of reasoning and natural languages. Solving this requires a diagnosis
of the problematic assumptions so that we can learn to reason correctly with natural languages
without fear of incoherence. However, simply showing that the normal paradoxical reasoning does
not arise in a small number of cases does not demonstrate that there is no incoherence within
natural language and reasoning. It may have been simply moved to a dierent type of argument,
or arise with a dierent type of case. Correcting the problematic assumption also requires making
claims about the philosophy of language more broadly which are not often explored in this context.
Simply providing a formal solution, however, does not guarantee any solution of the overriding
philosophical issues.
One can construct many consistent formal systems which include a truth
predicate, but the important question is whether these formal systems are appropriate as a way of
understanding the logical structure of natural languages. To use the point made above, the formal
systems must satisfy all of the relevant conditions that natural languages satisfy, and the mechanism
that prevents the paradox in the formal system must be plausible in a natural language context.
However a full formal solution allows us to decide whether the paradox has been systematically
prevented, as we have techniques we can investigate the consistency of the system with.
So a comprehensive solution must combine elements of both of these approaches, oering both
a natural language justication, or at least motivation, for the problems identied plus a formal
denition that can show that the result truly is not aected by the paradox. The investigation of
dierent solutions below will focus on solutions oered in the literature which are comprehensive
in this sense. However, before examining these, it is worth looking further at the cases that need
to be discussed. While the previous chapter discussed some more complicated examples than the
most simple cases, there are further cases which are relevant to the discussion.
2.2 Cases of the Liar Paradox
We will continue to treat the following as the stereotypical case of the Liar Paradox:
In the previous chapter, the following cases were also introduced:
However, there are many more, less articial, examples of Liar Sentences which need to be taken
into account. For example, the following case is also paradoxical:
I am currently lying.
Policeman (in court): Most of what the defendant says in the witness stand will not be
Defendant (says only this): What the policeman said was true.
Each of these cases introduce further complexities into the analysis of the Liar Paradox, which
will become clear in the discussion of the dierent solutions. However, one remark can be made
from a survey of the cases here. We can see from the range of sentences in the dierent cases here
that there are no common grammatical properties shared by all of these sentences. This further
strengthens the point made above that we cannot look for a solution amongst the grammatical
conditions. It is not possible to nd any grammatical properties that are unique to paradoxical
sentences. Any satisfactory solution must look at the four logical conditions identied above.
example is adapted from one given in L. Jonathon Cohen. Can the Logic of Indirect Discourse be For-
malised? In: The Journal of Symbolic Logic 22.3 (1957), pp. 225232
2.3 Solutions in the Literature
This thesis will briey examine seven dierent solutions in the literature to understand how they
deal with the challenge of the Liar Paradox.
The solutions chosen for investigation are a small
selection of the dierent approaches in the literature, but they all have a well-worked through
formal account to accompany the philosophical discussion. The analyses of the dierent solutions
rely on the framework that has been developed in the previous chapter and the Introduction, as
this will help highlight the exact nature of the solution. In particular, we will focus on which of the
nine conditions is not met in order for the solutions to work. Of particular interest are common
elements to the dierent approaches and similar problems that arise across multiple solutions.
2.3.1 Alfred Tarski
Alfred Tarski published the rst formal truth denition in his seminal paper Der Wahrheitsbegri
in den formalisierten Sprachen
2 In doing so, Tarski established the modern for of this area of
research and has profoundly aected the scope of subsequent research into formal truth predicates
and the Liar Paradox. Tarski's solution is therefore the natural place to start any investigation
into existing solutions to the Liar Paradox in the modern literature.
The clearest indication of the importance of Tarski's work is that he introduced two ideas which
are now central a lot of work in modern logic and philosophy. The rst idea is the concept of a
formal meta-language in which concepts can be dened that cannot be dened within the normal
(object) language. The second idea is the T-Schema, namely the schema that 'P' is true if, and
only if, P. Both of these concepts play a crucial role in Tarski's solution to the Liar Paradox and
are indispensable elements of modern philosophy and logic. However, before turning to Tarski's
formal solution, it is worth looking at the philosophical motivation and justication.
Central to Tarski's approach is his conviction that
In [colloquial] language it seems to be impossible to dene the notion of truth or even
to use this notion in a consistent manner and in agreement with the laws of logic.
This disagrees profoundly with most modern approaches, including the approach to the Liar Paradox that has been oered in this thesis. Admittedly, in line with Tarski it has been argued in this
thesis that, if correct, the Liar Paradox demonstrates that our use of the concept of truth in natural
languages is necessarily inconsistent. However, while Tarski seemed to take this as evidence of the
aws in natural languages, we have taken it as evidence of aws in our understanding of natural
languages. Tarski's response to the problems he saw with natural languages was to formulate a
precise, logical, scientic language in which the concept of truth can be consistently and accurately
2 Alfred
Tarski. Der Wahrheitsbegri in den formalisierten Sprachen. In: Alfred Tarski: Collected Papers. Vol. 2.
Birhäuser, 1986. The English translation was published in Alfred Tarski. The Concept of Truth in Formalized
Languages. In: Logic, Semantics and Metamathematics. Oxford University Press, 1956, pp. 152278. All quotes are
taken from the English translation.
3 Tarski,
The Concept of Truth in Formalized Languages, see n. 24, p. 153
4 The idea was to dene a replacement for natural languages. The standard modern approach
is to reform our understanding of truth and/or reasoning in natural languages to legitimise our use
of natural language, particularly as it seems impossible for us to ever completely replace natural
language talk of truth with a formal language.
Tarski's argument for the inconsistency of natural languages begins with a justication of the
T-Schema as a principle which any truth predicate must satisfy. Natural languages, when treated
as a Logical Language, automatically satisfy conditions 1, 2, 3, 4 and 6, and classical logic, which
Tarski accepts, satises conditions 5 and 9. As Tarski is a classical logician he seems to accept the
principle that every sentence is either true or false, and that a truth predicate should be able to
report this, which means he accepts condition 7. Given that the last principle is the T-schema, it
is unsurprising that Tarski saw natural languages as necessarily inconsistent, as combined with his
account of reasoning and truth all nine conditions are satised and hence natural languages are
necessarily inconsistent.
Tarski's specic argument for the inconsistency of natural languages turns on whether it is
possible to provide a rule based method of naming natural language sentences that allows the
T-schema to be satised. As it seems to be impossible, Tarski takes it as evidence that natural
languages are inconsistent.
5 This shows that Tarski did not consider any of his logical assumptions
to be contestable, and thus was focusing eectively on the conditions on Grammar-Only languages.
It also highlights a distinction between natural languages and formal languages which becomes
relevant later in the thesis.
Where a formal language includes names as a means to refer to
sentences, it does so in a precise, regimented way which normally pairs one sentence with one
Natural languages are, on the other hand, very liberal with names and techniques for
referring to other sentences. There are a wide variety of ways we can refer to a sentence, including
demonstratives, denite descriptions and quotation marks, and it is possible to dene new ways
within natural languages. This large dierence raises the question of whether an accurate account
of natural languages can be tted into the standard naming structure of a formal language.
There is another simple argument for natural languages being inconsistent that Tarski does
not use, but which is useful to highlight another relevant distinction between formal and natural
It is obviously the case that the way that people use natural languages is often in-
consistent. People often contradict themselves and others with their assertions. However, this is
a dierent type of inconsistency to the type the Liar Paradox presents. While people often use
natural languages inconsistently, it is still possible that they can be used consistently, if we are
careful with what we assert. If correct, the problem posed by the Liar Paradox demonstrates that
it is not in fact possible to use natural languages consistently.
This distinction between how natural languages are actually used and how it is possible to
4 Tarski, The Concept of Truth in Formalized Languages, see n. 24, p. 165
5 See Tarski, The Concept of Truth in Formalized Languages, see n. 24, pp.
use them does not exist for formal languages. The rules governing a formal language only allow
provable sentences to be stated or asserted within the language. This means that if a person spoke
a formal language correctly, they could only assert true sentences.
It is not possible to assert
a falsehood in a formal language without breaking a rule of that language.
Natural languages
do not have a similar distinction between legitimate and illegitimate assertions as part of the
rules of the language. For example, there is no distinction between Snow is white and Snow is
black as legitimate English sentences. While there are undoubtedly rules about what is and is
not assertible when we use natural languages, these rules are provided by things like context and
cultural expectations, not by the language itself.
To put it dierently, one can (and many do)
assert lies in a natural language without violating any rules of the language.
Tarski's approach to the Liar Paradox was to see problems with the construction of natural
languages, and therefore seek to replace them, at least for talk about truth. This is however not
a realistic option, as it is not possible to transcend our use of natural languages.
suppose we wanted to justify moving to a particular formal language, the justication for this must
occur in a natural language otherwise we could not understand the point of changing. However,
if natural languages are necessarily inconsistent, then we cannot be sure that our justication is
valid, and therefore that we are justied in moving to the formal language. While it is argued that
a more valuable approach to resolving the problems with natural languages is to try to ascertain
whether it is ever possible to use natural languages consistently, it is important look at the way
Tarski constructs a formal truth denition, both due to its inuence and that it highlights common
The Formal Solution
Given that Tarski rejects the suitability of natural languages for a truth denition, it is unsurprising
that he focuses his solution on the rst four conditions in our list, the conditions on a GrammarOnly language being aected by the Liar Paradox.
The key structural component to Tarski's
solution that allows him to avoid paradox is his distinction between an object language and a
metalanguage. For Tarski, we must always distinguish clearly between the language about which
we speak and the language in which we speak.
6 While we would normally talk about objects
in the object language, we can only talk about an object language in a metalanguage.
formal denition, Tarski only allows the denition of a truth predicate for sentences in the object
language within the metalanguage. Thus, for a sentence
in the object language,
P is true is only
dened in the metalanguage. When we look at the nine conditions above, it follows that the object
language does not satisfy conditions 1, 2 and 6. That is, the object language does not contain the
concepts of truth or untruth and therefore these concepts cannot be predicated of things within
the language.
This prevents the paradox from occurring, since the type of circular reference which is central
to the paradox cannot arise, as the truth predicate is only dened in the metalanguage to refer
to sentences in the object language.
Importantly, the truth predicate in the metalanguage is
restricted so that it only applies to sentences in the object language, not to other sentences in the
metalanguage.
This means that the metalanguage does not satisfy condition 4, since the truth
predicate cannot apply to sentences of the metalanguage - there is a restriction on the what the
truth predicate can be predicated of.
If we were to want a truth predicate for sentences in the
metalanguage, we would have to treat the metalanguage as an object language and move to a
meta-metalanguage. However, Tarski does not consider this as it is not necessary for his project
of replacing natural languages with a precise formal language for truth. The ability to deal with
sentences such as ` P is true is true' is relatively unimportant as these types of sentences are not
normally necessary in a formal scientic language.
While the distinction between object language and metalanguage provides a method for preventing the Liar Paradox, it does not provide any denition of truth.
There are two other key
components to Tarski's formal denition which deal with the actual denition of a truth predicate.
The rst is the T-Schema, and the second is his recursive denition of the truth predicate.
The T-Schema, that `P' is true i P, has been widely adopted as a denition of truth, however
Tarski uses it in a very particular way.
He introduces the T-Schema as the key point in his
Convention T which provides the necessary conditions which any satisfactory truth denition must
7 That is, for Tarski, the T-Schema provides a test which must be true of any satisfactory
satisfy.
denition of truth. If it does not follow from a truth denition that say `Snow is white is true
i snow is white', then for Tarski that truth denition cannot be correct. The T-Schema does not
dene anything in Tarski's denition, and he takes care to show that his truth denition satises
the T-Schema.
Tarski further provides a formal denition for a particular mathematical theory.
This has
advantages of style in that Tarski can assume that his formal theory includes a large amount
of mathematical reasoning which is rarely included within modern formal languages due to the
complexity of some of the concepts. Most relevantly, this approach allows Tarski to use mathematical concepts to oer a recursive denition of the truth predicate within the metalanguage. Thus
Tarski's actual denition of the truth predicate is by recursion over satisability. The details are
unimportant to this discussion, however they are very familiar to any modern logician, as Tarski
uses the same technique as is now used for dening truth values in a model.
7 Tarski, The Concept of Truth in Formalized Languages, see n. 24, pp.
8 See Tarski, The Concept of Truth in Formalized Languages, see n. 24,
Tarski's formal denition is simple and formally elegant, and can be easily shown to be consistent.
However, it has well-known problems as a philosophical account. The most famous problem is the
9 and Saul Kripke10 . The critique turns on
critique oered separately by both L. Jonathon Cohen
situations that arise in natural languages where perfectly ordinary sentences can turn out to be
paradoxical. These contingently paradoxical cases normally involve two ordinary assertions which,
in the circumstances, turn out to set up a two sentence version of the Liar paradox. The example
give above is a classic example of this phenomenon:
The problem is that this type of situation cannot even be set up in a Tarskian situation as both
sentences would need to be in a metalanguage with respect to the other. As an account of truth
in natural languages, the Tarskian approach is limited as it cannot even formulate this legitimate
pattern of natural language reference.
This point can perhaps be made even more clearly if we consider a non-paradoxical case which
is nevertheless cannot be formulated in a Tarskian system.
A simple case is a case where two
friends each assert of the other that The vast majority of what he says is true. Both assertions
can be true, yet each sentence is in the scope of the quantication of the other.
In a Tarskian
system each must be in a metalanguage with reference to the other, which is not possible. These
problems make it clear a Tarskian system cannot provide a satisfactory philosophical analysis of
natural languages.
This fact, however, would not surprise Tarski, as he did not believe that a
satisfactory philosophical analysis of natural languages is possible. His aim was to replace natural
language talk of truth, rather than to explain it.
This critique of the Tarskian approach provides a neat practical example of the general point
that has been made several times in our analysis of the Liar paradox. Natural languages satisfy
all of the conditions on a Grammar-Only language, while the Tarskian approach does not satisfy
these conditions. The general point that has been made is that this means the Tarskian solution
cannot provide an account of truth and reasoning in natural languages, and the critique above
demonstrates one reason why this is general point holds. While Tarski oers a very elegant formal
solution, it does not do anything to help resolve the primary philosophical problem that has been
identied, as it cannot be applied as an account of truth and reasoning in natural languages.
9 Cohen, Can the Logic of Indirect Discourse be Formalised?,
11 This example is adapted from one given in Cohen, Can the
n. 10
see n. 10, p. 226
Logic of Indirect Discourse be Formalised?, see
2.3.2 Kripke
The paper by Saul Kripke, Outline of a Theory of Truth
12 , contains a formal truth denition
which explicitly aims to x the some of the limitations inherent in the Tarskian approach.
particular, Kripke aimed to avoid the obvious limitation in Tarski's system, from a natural language
perspective, that a truth predicate cannot be dened that applies to its own language.
distinction between object language and metalanguage in Tarski's approach explicitly prevented a
truth predicate applying in its own language. Kripke's denition is explicitly an attempt to provide
a denition of a truth predicate that does not have this limitation.
Kripke bases his critique of Tarski, and his analysis of the problem posed by the Liar paradox
on the cases of contingent liar sentences, such as the example above. The key lesson that Kripke
drew from these examples was that it would be fruitless to look for an intrinsic criterion that will
enable us to sieve out as meaningless, or ill-formed those sentences which lead to paradox.
It follows that if there is no intrinsic criterion, nothing in the sentences themselves, then there
must be some external criterion we can use. Kripke identies an external criterion in the following
way:
If ... sentences themselves involve the notion of truth, their truth value in turn must
be ascertained by looking at other sentences, and so on.
If ultimately this process
terminates in sentences not mentioning the concept of truth, so that the truth value
of the original statement can be ascertained, we call the original sentence grounded ;
otherwise, ungrounded.
This external criterion of groundedness gave Kripke an intuitively compelling way of separating
paradoxical from unparadoxical sentences, and it lent itself very easily to a formal recursive denition.
In a brief summary, Kripke's formal approach was to extend the recursive denition of truth
values of sentences to include the truth predicate.
Kripke's denition starts with a base set of
sentences closed under the standard recursive denitions of connectives that is taken to include
all true sentences which do not include a truth predicate (the facts). The truth predicate is then
dened recursively on top of this set by forming a hierarchy of new sets which include P is true
for every sentence P in the previous set. If this recursive denition is continued transnitely, then
there are guaranteed to be xed points where the sentences in the set stabilises. Kripke took the
xed points to be complete sets of true sentences.
This formal denition captures the concept of groundedness in a natural way as only grounded
sentences will ever be included in the set hierarchy.
Any sentence in the hierarchy will either
12 Saul Kripke. Outline of a Theory of Truth. In: Journal of Philosophy 72.6 (1975), pp. 690716
14 Kripke, see n. 9, p. 693-4. This concept was previously introduced in Hans G. Herzberger. Paradoxes of Grounding in Semantics. In: The Journal of Philosophy 67.6 (1970), pp. 145167
be in the base or refer to a sentence (or sentences) at a lower level in the hierarchy. This lower
sentence (or sentences) will in turn either be in the base or refer to another lower sentence (or
sentences).
For all sentences in the hierarchy if this process is continued, they will eventually
only refer to sentences in the base set of facts. In other words, every sentence in the hierarchy is
grounded. Moreover, paradoxical sentences will never be included in the hierarchy as, as argued in
the previous chapter, they only occur when a sentence belongs to a circular reference chain. The
recursive denition of the hierarchy guarantees that no circular reference chains will be included.
It follows that paradoxical sentences are never true, but are never false either. In this way Kripke
formulates a consistent denition of truth in which the truth predicate can apply to sentences
within its own language.
Kripke therefore oers a formal denition which is a natural extension of existing techniques
in logic, and addresses one of the main concerns with Tarski's work, however there are a number
of limitations to it.
One signicant limitation was noticed by Kripke himself, as he noted that
There are assertions that we can make about the object language which we cannot make in the
object language.
15 The example Kripke gave was the sentence `The Liar Sentence is not true'.
This sentence is intuitively true since the Liar Sentence is never included in the set hierarchy
of true sentences.
That is, it is a consequence of Kripke's solution that Liar Sentences are not
true, but while a sentence which expresses this would be grammatically acceptable in the object
language, it has no truth value. This means that the object language in Kripke's solution does
not satisfy condition 7, since there are sentences which cannot be said to be either true or not
true. Interestingly, Kripke's solution also does not satisfy condition 8, since the T-Schema is not
in general valid in the object language. It is valid for all grounded sentences, but the T-Schema
for ungrounded sentences does not have a truth value.
While there is some intuitive support for the idea that a paradoxical sentence might not be the
sort of thing that is truth-apt, the exact shape of the problem for Kripke is more serious. One of
the key theses in Kripke's solution is that Liar Sentences are not true since they are not grounded.
However, for any sentence
P , if P
for any given Liar Sentence,
is not grounded then P is not true is also not grounded. Hence
L, the sentence L is not true
is not grounded and therefore not true.
Thus it is not possible to assert a key thesis of Kripke's truth denition within the object language,
as it is not true in the object language. As Kripke notes, to assert this key thesis it is necessary to
move to a meta-language, and thus the ghost of the Tarskian hierarchy remains.
Ironically, Kripke's core critique of Tarski's solution was that invoking a meta/object language
distinction cannot work when we are analysing natural languages.
Since Kripke's solution ulti-
mately relies on a metalanguage to assert the relevant theses, it follows that Kripke's own solution
has a similar natural limitation in its application to natural languages: it cannot deal with dis-
cussion of the Liar Paradox, since key theses of his solution are not true without resorting to a
There is another aspect to Kripke's proposed solution which limits its usefulness as a philosophical account. Kripke's approach in using his recursive denition is to construct a semantic model of
a consistent truth denition within a language. The existence of a sound model demonstrates that
it is possible to have a consistent truth denition within a formal language. However, Kripke does
not himself oer any denition within the object language, whether by means of axioms or rules
of inference. The limitations with this are clear when we consider natural languages. Kripke, if
correct, has eectively shown that it is possible to have a consistent truth predicate within natural
languages, however, does not give any clue as to how this truth predicate can be dened or even
understood within a natural language.
Thus Kripke provides the denition of a semantic model for a consistent truth denition in
which the truth sentence can apply to sentences in its own language. As such it is an improvement
on Tarski as a philosophical analysis of natural languages, in particular as it does not impose
any restrictions on the construction of sentences in the language. His denition does not satisfy
conditions 7 and 8, and has the signicant limitation that key results of the denition are not true
within the relevant language.
This limits its applicability as a philosophical account of natural
languages, as there is no metalanguage that transcends natural languages.
2.3.3 Dialetheism
Dialetheism, the position that accepts the existence of true contradictions, is an increasingly popular response to the Liar Paradox. It has been most vigorously been advanced by Graham Priest,
17 Essentially, this approach argues that
who is also one of the original proponents of the position.
all of the premises in the paradoxical argument are correct, and we should therefore accept the
conclusion a contradiction as correct. That is, for the dialetheist, any Liar Sentence is both
true and false.
Judged against the nine conditions outlined previously, dialetheism is a very neat solution, as
it simply requires rejecting condition 5. That is, it is simply necessary to develop a logic in which a
contradiction does not prove everything, and the T-Schema can be used as the denition of truth.
While to a classically trained philosopher or logician this seems bizarre, there are well worked
through formal logics with the required properties, including a sound model for the logic.
most famous of these logics is Priest's
LP ,
presented in his paper The Logic of Paradox.
18 Thus
essentially the dialetheist oers a formally simple solution in which all of our ordinary intuitions
17 See,
among others Graham Priest. Logic of Paradox Revisited. In: Journal of Philosophical Logic 13 (1984),
pp. 15379; Graham Priest. In Contradiction: A Study of the Transconsistent. Dordrecht: Martinus Nijho Publishers, 1987; Graham Priest. What is So Bad about Contradictions? In: The Journal of Philosophy 95.8 (1998),
pp. 410426; Graham Priest. Truth and Contradiction. In: The Philosophical Quarterly 50.200 (2000), pp. 305319
18 Graham
Priest. The Logic of Paradox. In: The Journal of Philosophical Logic 8 (1979), pp. 219241
about truth hold, only that we must give up consistency as a prerequisite for truth.
Aside from the simplicity of the formal denition the dialetheist provides, one of the primary
arguments in favour of dialetheism is the claim that it allows semantic closure. That is, all sentences
within the logic have a truth value, and the assertion that these sentences have the particular truth
value is a valid sentence within the language. This is a highly desirable property that we would
want to hold for natural languages, if we want to be able to use natural languages in the way we
normally do, particular for philosophical discussions.
The dialetheist position therefore has a number of signicant advantages, which obviously must
be weighed against the cost of accepting true contradictions. While there are various considerations
about whether this is a reasonable approach to take, they are beyond the scope of this thesis. There
is however a more specic weakness in Priest's approach in particular that has interesting parallels
with a limitation in Kripke's solution.
One of the key theses of the dialetheist approach is that Liar Sentences are both true and
false. However, if we go through the semantics of Priest's
we can see that not only are Liar
Sentences both true and false, but sentences such as The Liar Sentence is both true and false
19 While Priest does not see this as a problem, it is a weakness in
are also both true and false.
the dialetheist approach. Liar Sentences are obviously problematic sentences which, according to
Priest, arise at the limits of reason.
20 However, asserting that The Liar Sentence is both true and
false is a substantive philosophical claim that dialetheism relies on. If this philosophical claim is
not valid, then dialetheism cannot be correct. However, according to the dialetheist, this sentence
has exactly the same truth status as obviously problematic Liar Sentences. Within dialetheism, a
sentence asserting the key philosophical position of the theory is no more and no less successful as
a claim about reality than a Liar Sentence.
This problem becomes more acute when we recognise some of the consequences of this position.
If someone attempts to disprove dialetheism by proving that the claim The Liar Sentence is both
true and false is false or even not true, the dialetheist can simply accept this as proving half of
the thesis that this sentence is both true and false. That is, the core thesis in dialetheism is not
falsiable to the dialetheist it cannot be disproven. Thus either the fact that the Liar Sentence is
both true and false is a necessary logical truth, or it is a problematic in the same way that scientic
theories which are not falsiable are problematic.
The interesting thing about this weakness in the dialetheist position is that it mirrors the
limitation in Kripke's solution. In both cases, sentences about a Liar Sentence have the same truth
value as the Liar Sentence within the relevant language, even if we would interpret the sentence
about the Liar Sentence as making a substantive philosophical claim.
19 For example, see Priest, The Logic of Paradox,
20 This is the theme ofGraham Priest. Beyond the
see n. 3, pp. 238-9
Limits of Thought. Cambridge: Cambridge University Press,
2.3.4 Barwise & Etchemendy:
Barwise and Etchemendy, in their work The Liar: An Essay on Circularity and Truth
21 , introduce
two ideas from the philosophy of language which they use to construct a coherent solution to the
Liar Paradox. The rst idea is the distinction between the assertion of a negation and a denial,
while the second is the idea of situation semantics.
While the rst idea assists in their precise
analyses of liar sentences, it is the second idea that plays the crucial role in preventing their
solution becoming inconsistent and therefore incoherent.
This analysis will focus on the second
idea.
In Barwise and Etchemendy's approach, sentences are not taken to be true universally, but
rather true in particular circumstances or situations. The broad outlines of this idea are familiar,
however Barwise and Etchemendy implement this idea in a very precise way using a non-standard
set theory developed by Peter Aczel. For Barwise and Etchemendy, the fact that a sentence is only
true in a particular situation means that the truth of that sentence within that situation can only
be asserted from outside that specic situation. We have to move to a broader situation of which
the original is a part to be able to assert that the sentence is true in the original situation.
By imposing this requirement, Barwise and Etchemendy's approach prevents the paradox from
biting since the conclusions in the argument relate to truth in a dierent situation to truth in the
original sentence. While this hierarchical approach is reminiscent of Tarski's solution, it operates
in a signicantly dierent way since they do not impose grammatical restrictions on the valid
sentences. Instead, the approach eectively uses a hierarchical approach to separate dierent components of the paradoxical argument to dierent levels in the hierarchy so the complete argument
cannot run. This allows the truth predicate in a language to report the truth of other sentences
in that language, if we take the truth predicate as indexed over situations. In this way it avoids
many of the problems that Tarski faced.
On the list of nine conditions, Barwise and Etchemendy's approach seems to fail two and
possibly more conditions. It fails condition 4, since there are restrictions on the valid combinations
of sentences with the truth predicate as we need to index truth to situations. It also fails condition
7 since sentences in the language are not simply either true or not true as their truth is relative
to particular situations. Many sentences do not have any truth value in many situations, as they
are entirely irrelevant to the given situation. There is also an open question as to whether there
is a univocal truth predicate in Barwise and Etchemendy's approach, or whether there are a large
number of indexed truth predicates. If the second option is correct, then condition 6 also fails.
However, there is a big limitation with this approach since it is never possible to move to a
universal situation. If we move to a universal situation, there is no broader situation to move to
21 Jon Barwise and John Etchemendy. The
Press, 1987
Liar: An Essay on Truth and Circularity. New York: Oxford University
which means as the quarantining eect cannot occur and the Liar Paradox will arise again. However, the impossibility of moving to a universal situation in Barwise and Etchemendy's approach
means that universal statements cannot be completely universal - they must be always interpreted
as about a particular situation. This is in conict with many of our deepest philosophical habits,
and calls into question the coherence of any philosophical discussion of the Liar Paradox. If there
is no universal situation, any conclusion we draw about the Liar Paradox cannot apply universally
but only to the situation relevant to the discussion. This is presumably as broad as it is possible to
make, but it cannot be universal. This means that, if correct, Barwise and Etchemendy's solution
cannot hold universally but only within some sort of limited situation.
It seems dicult to accept that an approach which intends to completely resolve the Liar
Paradox cannot allow that its core theses hold universally.
While it does not necessarily follow
that the theses are false in a broader situation than the relevant one, one can never know, as
any claim about the Liar Paradox in all situations is not legitimate. Interestingly, this problem
falls into a similar category to the problems with Kripke's and the dialetheist's approaches.
each case, key philosophical claims by the relevant theory about the Liar Paradox or liar sentences
cannot have the intuitively correct truth status within the theory.
2.3.5 Gupta & Belnap:
22 oer a carefully worked through
Gupta and Belnap, in their book The Revision Theory of Truth,
solution which is based on a theory of denitions and draws signicant inspiration from Kripke's
work. Central to this approach is an argument that circular denitions, such as in the case of the
Liar Paradox, are often legitimate denitions even if they run into trouble in cases like the Liar
Paradox. While the details of Gupta and Belnap's approach are beyond what can be covered here,
their key strategy for avoiding the paradox is worth reviewing.
The key ideas in their approach are neatly summarised as follows:
We recognize the Liar to be pathological, but we do not want to say that it is neither
true nor false. The sentences `the Liar is not true' and `the Liar is not false', whether
viewed as belonging to the object language or to some metalanguage are pathological
in exactly the same way as the Liar is pathological. To correctly describe the semantic
status of the Liar we need to appeal to notions such as categoricalness.
In terms of the conditions about, Gupta and Belnap thoroughly reject Condition 7, since they argue
that there are sentences which are neither true nor not true, but are pathological. More could be
said about what pathological means, but the basic idea is that something goes so wrong in the
22 Anil Gupta and Nuel Belnap.
23 Gupta and Belnap, see n. 22,
pathological.
The Revision Theory of Truth. The MIT Press, 1993
p. 255. Categorical sentences for Gupta and Belnap are sentences which are not
denition of Liar Sentences that it is no longer for these sentences to be considered as truth-apt,
they cannot have any truth status at all, not even a negative truth status.
There are a number of comments that can be made about an approach of this sort, including the
fact that it seems to be depriving negation of its intuitive meaning. One would normally interpret
the set of not true sentences as being the complement of the true sentences, however Gupta and
Belnap posit a third set of pathological sentences.
However, with regards to the philosophical
strengths of this approach, there are two key aspects that are worth focusing on.
Firstly, Gupta and Belnap, in line with a number of more recent authors,
24 appeal to a concept
other than truth to help make the necessary distinctions to prevent the Paradox. For Gupta and
Belnap, the key concept is categoricalness since it is only categorical sentences which are truthapt. However, as Gupta and Belnap admit, it is possible to use the concept of categoricalness to
reformulate a strengthened liar paradox, which they cannot avoid.
25 This strengthened paradox
does not mean, however, that their formal approach is inconsistent, as they rely explicitly on a
distinction between an object language and a metalanguage.
While in Tarski's work the concept of truth was dened in the metalanguage for sentences
within the object language, in Gupta and Belnap's approach the concept of truth is dened within
the object language for sentences within the object language. However, the key concept in their
approach of categoricalness is not dened within the object language but only within the metalanguage. The fact that this concept is dened in the metalanguage for sentences within the object
language means that the strengthened paradox is avoided in exactly the same way that Tarski
avoids the original paradox: the paradoxical cases cannot be formulated.
However, while the object language includes a consistent denition of truth for its own language,
there are obvious limitations in apply this approach as a model for understanding reasoning and
truth in natural languages. On Gupta and Belnap's approach, the coherence of the concept of truth
in the object language depends crucially on a concept that cannot be formulated within the object
language. If we apply this to natural languages, then it should follow that the coherence of the
concept of truth in a natural language depends crucially on a concept that cannot be formulated
within that language. However, Gupta and Belnap presented their arguments clearly in a natural
language and dened the concept of categoricalness clearly in a natural language. This presumably
means that categoricalness cannot be the relevant concept, if we wish to apply this approach to
In fact, if we wish to apply this general style of approach to natural languages, we are left with
a signicant problem. This style of approach depends there existing some sort of concept that is
not denable within the natural language but which underlies our concept of truth. However, it
24 Such as Vann McGee inVann McGee. Truth,
Vagueness and Paradox: an essay on the logic of truth. Indianapolis:
Hackett Pub. Co, 1990; and Hartry Field, see below.
25 Gupta
and Belnap, see n. 22, pp. 255-6
is not clear that we can grasp a concept that cannot be formulated in a natural language, as we
denitely cannot articulate it. If we cannot articulate or even grasp the concept, there is a big
question as to how we can then know that this concept guarantees the coherence of our concept of
truth. Yet again, like the previous solutions that have been investigated, these approaches rely on
philosophical theses which cannot be true within the relevant theory or language.
2.3.6 Michael Glanzberg
The key idea to Michael Glanzberg's solution to the Liar Paradox is that the propositions that
sentences which include the truth predicate express are context dependent, and that the relevant
context changes through the paradoxical argument.
26 This means that when a liar sentence and
its negation are both proven within a paradoxical argument, it is not the case that two contradictory propositions are expressed by the two sentences. There is therefore no contradiction and no
Glanzberg bases his analysis of the way that the context changes on an argument that what
we assert something of the form `s is true', what we are saying is something like `the proposition
expressed by sentence
s is true',
or more informally, `what she said when he said s is true'.
means that there is an implicit quantication and the sentence actually has the form: there is a
such that
expresses the proposition
is true. Glanzberg argues
for a context change that results from the domain of quantication for the existential quantier
in the sentence changing as consequence of the argument that normally generates from the Liar
28 Al-
Glanzberg acknowledges the similarities in his approach to Barwise and Etchemendy's.
though his approach does not suer the same limitation that Barwise and Etchemendy's does,
there is a related and smaller limitation.
The key fact about the context shift that Glanzberg
relies on is that it occurs during the paradoxical argument. Thus while the Liar Sentence may not
express a proposition at the start of the argument, and therefore cannot be true; by the end of the
argument the domain of truth conditions has expanded so that there is a proposition to express,
and what it expresses it true for the previous context.
Glanzberg's analysis, as for any solution to the Liar Paradox, is presumably to be taken as a
universal conclusion about how to solve the Liar Paradox. If we have a universal conclusion about
all instances of the Paradox, however, we know something about the truth status of any Liar
Sentence without needing to go through the argument. However, if Glanzberg's analysis is correct,
it is dicult to see how this is the case. To show this we will consider the following example:
is not true
26 For example, see Michael Glanzberg. The Liar in Context. In: Philosophical Studies 103 (2001), pp. 217251
27 Glanzberg, A Contextual-Hierarchical Approach to Truth and the Liar Paradox, see n. 5, p. 32
28 See Glanzberg, A Contextual-Hierarchical Approach to Truth and the Liar Paradox, see n. 5, pp. 38-40
We can immediately recognise
approach, then we know that
as a Liar Sentence. If we take the nal conclusion of Glanzberg's
will express a true proposition in the appropriate context. How-
ever, as we have not gone through the relevant reasoning,
is therefore not true.
reasoning,
does not express a proposition and
Thus it seems that we know that if we were to go through the relevant
would express a true proposition, but because we are too lazy to, it does not express
a proposition at all. The truth value of
therefore seems to depend on whether we have actually
performed some act, which is counterintuitive given that the sentence
does not refer to or de-
pend on any matter of fact. This leaves open the possibility that two perfectly rational agents can
disagree about the truth value of
and be both rationally justied in their belief.
If we look at this approach against the nine conditions above, Glanzberg's approach does not
satisfy condition 6, since there is not a single concept of truth included in the language.
relevant concept depends on the relevant context.
Glanzberg's approach also mirrors Kripke's
approach in that he oers a complete semantics for his language, without attempting to provide
a denition of truth internal to the language. In Glanzberg's denition, this has the result that
key concepts are not denable within the language, and in particular, his key conclusion that the
Liar Sentence expresses a proposition in some contexts and not others is not denable within the
relevant language.
2.3.7 Hartry Field:
Hartry Field has recently oered a very detailed and impressively worked through solution to the
Liar Paradox that is based on a generalisation of fuzzy logic semantics.
Field develops a new
logic which does not satisfy the Law of Excluded Middle for his solution, as he argues that there
are certain cases where the Law of Excluded Middle does not hold, including the Liar Paradox
and the Sorites Paradox.
Despite the signicant interesting and original logical work in the
detail of his solution, there are core philosophical ideas in Field's approach are similar to other
approaches, including Gupta and Belnap's. For example, where Gupta and Belnap rely the concept
of categoricalness to avoid the paradox, Field's solution turns on a concept of `determinateness':
in which one can say of certain sentences like the Liar that they are neither determinately true nor determinately not true.
And saying this isn't just saying that their
truth value is unknowable; it has real import about non-factuality: to say that a sentence
A isn't determinately true or determinately untrue commits one to not
the corresponding instance of excluded middle
A ∨ ¬A30
Like Gupta and Belnap, Field's solution rejects condition 7 above, as there are sentences which
are neither true nor not true. Field claries this rejection by using the concept of determinately
29 Hartry
Field. Semantic Paradoxes and Vagueness Paradoxes. In: Liars and Heaps: New Essays on Paradox.
Ed. by J. C. Beall. Oxford: Clarendon Press, 2003, pp. 262310pp. 262-3
Field. Saving Truth from Paradox. Oxford University Press, 2008, p. 325
true, which gives a plausible reason as to why negation is not the complement in this case. There
is nothing that can be determinately said about such sentences, and therefore we cannot say of
the relevant properties whether they do or do not hold. Moreover, the concept of determinateness
is expressible in the object language in Field's approach, which makes his approach plausibly
applicable to natural languages.
Aside from the cost of abandoning the law of excluded middle in certain cases, there is a
particular weakness within Field's approach. Field eectively avoids the ordinary Liar Paradox by
moving from a concept of truth to a concept of determinate truth. This means that a strengthened
paradox can be formulated in terms of determine truth. Field resolves this in turn by moving to
a concept of determinately determinate truth, then to determinately determinately determinate
31 While these continual moves work formally, they have obvious intuitive problems
truth and so on.
in an application to natural languages as it seems alien to rely on an innite sequence of predicates
like this.
A further problem is that it is necessary to treat all of these as distinct but related
concepts.
If we seek to unify them into a single concept, the Liar Paradox bites again and the
system is inconsistent. A particular problem is that we have the expressive resources in a natural
language to unify the sequence of determinately trues in to a single concept, and it we do so the
paradox bites again.
Field does not take this to be a signicant problem as his project operates in a similar way to
Kripke's. He provides a formal model for a denition of truth that can consistently apply to its
own language. He does not oer an explicit denition that might be appropriate within a natural
As noted before, this approach has a number of limitations in terms of providing a
philosophical solution to the Liar Paradox. Firstly, it oers limited philosophical insight, if we are
not provided with a means by which we can dene and work with a truth predicate within a natural
language. Secondly, approaches which adopt this method often depend on certain things not being
denable within an object language, which are denable within a natural language.
While this
may provide a nice formal model, it limits the application of these approaches to natural languages,
which is where the problem arises and is most acute.
The examination of the various solutions in the literature has revealed a number of similarities
across the solutions. In terms of the nine conditions above, a range of conditions were relevant
to dierent solutions.
The most common condition to not hold was, interestingly, the seventh
condition, that sentences are either true or not true, and that this can be expressed within the
relevant language. Unfortunately, this condition is at the top of any list of philosophical desiderata,
as we would like a language in which we can express the truth or untruth of all sentences in the
31 Field,
Semantic Paradoxes and Vagueness Paradoxes, see n. 3, p. 299
Even more interesting is the fact that, even in solutions which satised condition 7, there are
almost universally sentences that arise for which we cannot assert an intuitively correct truth
value.
For Kripke, it is sentences such as The Liar Sentence is not true; for the Dialetheist,
there are questions over sentences like The Liar Sentence is both true and false; for Barwise and
Etchemendy, it is sentences about any universal assertion; while both Gupta and Belnap, and Field
have sentences which are not true in an intuitive sense, but which are categorised as something
Also signicant is that all of these solutions rely heavily on a model theoretic approach to
the denition of truth in which there is a clear distinction between the model against which the
semantic properties are dened and the language.
In most cases, including Kripke, Gupta and
Belnap, Glanzberg and Field, the truth denition was demonstrated to work in a semantic model
without providing a clear denition for the model. There is an element of irony in this, as all of
these dierent solutions are seeking to avoid the problems in Tarski's formal truth denition, while
relying crucially on formal semantic techniques which were pioneered by Tarski and, in the case
of the recursive semantic denition used in model theory, introduced by Tarski in his formal truth
The existence of these common limitations within existing solutions to the Liar Paradox is
striking, particularly as they occur across very dierent solutions. This suggests that there is some
systematic limitation being reached, either in what can be expressed or in the techniques being
used. The next chapter will examine this issue and diagnose reasons for these common limitations.
On the Structure of Formal
As argued in the Chapter 1, the Liar Paradox consists in the fact that any Grammar-Only language
which satises the given set of four criteria is necessarily aected by the paradox. Furthermore,
any Logical language (or any natural language in combination with relevant assumptions about
truth and reasoning) in which a further set of ve properties hold is necessarily inconsistent and
trivialised. A successful solution to the Liar Paradox must give up at least one of these conditions,
and the range of solutions presented in Chapter 2 makes it clear that it is not obvious which
condition should go.
One of the most challenging aspects of evaluating dierent solutions to the Liar Paradox is that
the paradox itself does not dierentiate between the various conditions.
If any of the oending
conditions does not hold in a logical language, the Paradox does not arise and is therefore dealt
with. There is nothing about the Liar Paradox itself that can decide between given solutions.
This means rstly that the justication for a correct solution, and the basis of the criticisms of
various solutions must come from somewhere else, not simply the paradox. Thus, for example, a
signicant criticism of the Barwise-Etchemendy solution is not that it does not resolve the Paradox,
but that it does not allow universal assertions and these are not something that we want to give
Similarly, the dialetheist solution is criticised for its acceptance of true contradictions, and
Tarski's solution is criticised for the inability to represent truth in its own language, not the ability
to prevent the Paradox. However, there are no explicit generally accepted criteria in the literature
against which we can evaluate proposed solution to the Liar Paradox against. Each author tends
to dene the criteria in a way that naturally privileges their solution.
ON THE STRUCTURE OF FORMAL LANGUAGES
3.0.1 The Philosophical Problem
The distinction between Grammar-Only and Logical languages, however, oers a clear analysis of
the philosophical problem caused by the Liar Paradox, and from that a plausible set of criteria
arises that a solution must satisfy to solve the philosophical problem. Importantly, these criteria
have support within the literature. In the rst chapter, it was argued that any Grammar-Only
language which satises four criteria is necessarily aected by the Liar Paradox. That is, there is
some understanding of reasoning and truth in that language which leads to a contradiction and
triviality. Importantly, however, all of these four criteria are satised by natural languages. The
philosophical problem is that our intuitive concept of truth and reasoning satises the further ve
conditions on a Logical Language being aected and therefore trivialised. This calls into question
our ability to use natural languages to express a concept of truth.
A satisfactory philosophical solution to the Liar Paradox must therefore accept the four conditions on a Grammar-Only language being aected, and modify the Logical conditions. Otherwise
the solution requires that we seriously modify our use of natural languages in order to talk coherently of truth within natural languages, which requires that we accept that previous use of
natural languages is incoherent, at least in the context of discussions about truth. We therefore
want to provide a formal language with a truth denition which satises these four conditions on
Grammar-Only languages. Otherwise, the solution cannot resolve the philosophical problem.
It was shown, however, in the second chapter, that some solutions to the Liar Paradox work by
giving up on one or more of the criteria on Grammar-Only languages. Tarski's solution, in which
it is not possible to assert the truth of any sentence in the same language, is the most obvious
example to this. Importantly, however, much of the research into the Liar Paradox since Tarski has
been a conscious attempt to get around the restrictions that Tarski's solution imposed. The most
important restriction was that, under Tarski's solution, the facts about truth for a language could
only be asserted within another (meta-) language. This seems to directly contradict everything
that we do and want to do with natural languages, particularly in discussion of philosophy and
linguistics. There is no obvious meta-language to appeal to and if we cannot express certain truths
in our normal languages then it is not clear if these truth can ever be expressed. As discussed in
the second chapter, there has been a general philosophical aim to nd a solution which matches
natural languages more closely, and which implies that we do not have to give up on or change
3.0.2 Semantic Closure
This aim obviously matches the goal here, which is to nd a solution in which all the criteria for a
Grammar-Only language to be aected by the Paradox are satised, since natural languages satisfy
these, but which nevertheless oers an account of reasoning and truth which is not trivialised.
However, it is not good enough that we have an internally coherent account of reasoning and
truth, since it is possible to have such an account which leaves out crucial information or facts.
Many of the solutions discussed in the second chapter satisfy all the criteria for a Grammar-Only
language, in the sense that all of the relevant sentences are grammatical.
However, there are
grammatical sentences which do not get the correct truth value, according to the truth denition
being adopted. For example, Kripke oered a highly coherent and consistent truth denition which
could not allow a central truth of that denition, namely that the Liar Sentence is not true, to
be expressed and to be true within the language of the denition. Similarly, Gupta and Belnap's
solution depends on the fact the the Liar Sentence is not categorical, yet this claim is not even
grammatical in the relevant language. The internal coherence was bought at the cost of being able
to express all the relevant truths within the language.
This failure has been expressed within parts of the literature as the failure of languages to be
semantically closed. A language is semantically closed in this sense if all of the truths about the
semantic properties of the language are assertable within the language. Given that the focus of this
thesis is on formal languages where the most important semantic information is the truth value
of sentences, we will generally restrict the concept of semantic closure to the ability of a language
to correctly assert the truth value of every sentence within that language. However, in solutions
like Gupta and Belnap's or Field's, concepts such as categoricalness or determinately truth are
also crucial semantic concepts and should be expressible with the correct truth value within the
language. The most important case to consider is normally whether for a Liar Sentence
truth predicate
T r,
T r(l), ¬T r(l)
T r(l) ∧ T r(¬l)
is provable and therefore true within
the formal language. The point is that, any truth denition should imply that a Liar Sentence is
either true, not true (which includes false and neither true nor false) or both true and false, and
an important condition on a language being semantically closed is that this fact can be asserted
for all sentences with a truth value within the language.
We have identied two crucial criteria against which an ideal truth denition will be measured. It
must allow the denition of a logical language which is semantically closed in this sense and whose
grammar-only part satises the four criteria listed in Chapter 1.
As is clear from Chapter 2, this ideal has not been achieved by currently proposed formal
solutions. What is most interesting in this are the similarities in the way that the dierent solutions
fail. While solutions are capable of dealing with Liar Sentences, they very regularly cannot then
allow sentences about those Liar Sentences to have the desired truth value. That is, conclusions
which are central to articulating the solution cannot be assigned the correct truth value within the
relevant language. Given the ubiquity of this problem, it seems either to be an essential property
of the Liar Paradox, or a systematic error in the way that the Liar Paradox is being examined.
As argued in the Introduction, the Liar Paradox should be treated as a paradox that involves
languages, rather than only sentences or cases. If there is a systematic error, then this should be
one that involves the way that languages, and particularly formal languages, are being treated.
It is therefore necessary to examine the structural properties of modern formal languages to see
whether they introduce any systematic problems of the sort indicated. Naturally, the structural
properties that are most relevant are the assumptions which govern the semantic structure of
formal languages.
That means, in the modern context, it is vital to examine the consequences
of assuming that Recursive Model Theory denes the semantics of formal languages with formal
truth denitions and the Liar Paradox. An investigation into the consequences of assuming Model
Theory is the purpose of this chapter.
The strategy to be adopted is to isolate the key structural assumptions when a Model Theoretic
approach is adopted for semantics, and then to investigate the interaction of these assumptions
with formal truth denitions and the Liar Paradox. This will be done by assuming that an arbitrary
language which includes a formal truth denition satises each of the assumptions, and observing
the consequences of liar, and related, sentences. It will be shown that this approach is very fruitful
since the structural assumptions within Model Theory are incompatible with a philosophically
ideal, semantically closed, formal truth denition.
3.1 Recursive Model Theoretic Semantics
In general terms, modern formal semantics is characterised by an approach which denes the
semantic properties of sentences within a language recursively, and with respect to a model of that
language. This type of approach, and the mathematics that underlies it, falls under the title of
Model Theoretic Semantics. Within the scope of Model Theoretic Semantics, however, there are
two key assumptions that are separable and have dierent consequences when we examine them
with respect to formal truth denitions.
The two relevant assumptions are i) the concept of a semantic model, i.e. that there is some
formal structure (the model) against which all of the semantic properties of sentences in a language
can be dened; and ii) the assumption that these semantic properties can be dened recursively,
that is, they can be dened initially for the most basic sentence components and then recursively for
more complicated sentences based on the grammatical structure of the sentences. While these are
normally two parts to the one model theoretic denition, the two assumptions will be investigated
separately here since each assumption has dierent consequences for formal truth denitions.
The concept of a semantic model, as something against which the semantic properties (particularly the truth values) of all sentences in a language is dened, is of obvious interest for our
understanding of truth within a formal language. On this method of dening semantic properties,
a sentence in the language is true if it is true with respect to the model. It follows that the act
of dening the truth value of sentences in a language against a model assumes that the relevant
concept of truth for sentences is truth in the model.
For this reason, we will investigate this
concept rst and look at whether dening semantic properties in this way has any eect on the
successful solution of the Liar Paradox.
It will be argued that no semantically closed language
which includes a truth predicate can have a Model which denes the semantic properties of all the
sentences of that language. That is, no semantically closed formal language is sound in the normal
model theoretic sense. Recursivity will not be assumed in the analysis and so this conclusion is
independent of the assumption that the semantic denition for the language is recursive.
This fundamentally important conclusion can be interpreted in two dierent ways. Firstly, if
one assumes the necessity of the standard model theoretic approach to formal semantics, then it
provides a clear proof of Tarski's conclusion that a semantically closed language is impossible.
On the other hand, if alternative approaches to dening the semantics of formal languages are
possible, then this conclusion shows that the denition of formal semantics needs to be changed,
if we wish to achieve semantic closure.
This conclusion is not only supported by a formal argument, but is supported by a conceptual
analysis of logical structure of model theory. It will be argued that the structure of model theory is
not consistent with some minimal conditions on the concept of truth. Thus the inability of formal
denitions to achieve semantic closure is not an accident but is a necessary consequence of the
conceptual structure of standard model theory. Any truth predicate that reects the concept of
truth within a model cannot allow sentences such as Liar Sentences to be completely semantically
evaluated against the model since the structure of the system of reference prevents such sentences
from being interpreted within the model.
The assumption of Recursivity will be investigated separately from this analysis of the concept
of a model. It will also be shown that assuming a recursive denition of semantics is problematic
in the context of a satisfactory solution to the Liar Paradox.
There are phenomena which are
essentially involved in the paradox which cannot be accommodated within a recursive semantics.
It will be argued, however, that this is not inherently due to the assumption of Recursivity, but due
to the assumption that sentences (or sentence types) are truth bearers and that identical sentences
have identical truth values.
In order to unify the following discussion, we will universally assume that we have a formal
which includes a truth predicate
we will let upper case letters, such as
LT ,
and we will use the
T r and a system of naming.
As a matter of notation,
P , Q and R, be metatheoretic symbols that stand for sentences
notation as a name forming device on sentences in
LT .
Tarski. The Semantic Conception of Truth: and the Foundations of Semantics. In: Philosophy and
Phenomenological Research 4.3 (1944), pp. 341376pp. 348-9
pP q
is a symbol for a name of the sentence
However, in order to discuss the Liar Paradox,
we need to have actual cases to discuss. Actual cases necessarily involve some particular sentence
within the language, and we will therefore assume that
with the name
a; ¬T r(b)
contains the following sentences:
b; ¬T r(b) ∧ ¬T r(b)
In terms of notation, we are using lower case letters (i.e.
Strictly speaking, these names are dened within
T r(b)
T r(a)
a, b, c, d ) as names of sentences.
but to avoid the confusion of assigning the
same sentence two names, we will use the same lower case letter as the metatheoretic name of
the sentence. Where it is important to distinguish between the letter as a name and the letter as
standing for the sentence, we will be careful to clarify.
3.2 The Denition of Semantic Model
As noted above, the orthodox way to approach the denition of semantics for formal languages
is to use Model Theory.
The key idea of this is that as sentences within a formal language do
not have any intrinsic semantic properties, all of the semantic properties of sentences within a
formal language are dened with respect to an external, precisely dened structure, a Model. This
means, among other things, that concept of truth for sentences in a formal language only makes
sense when it is the concept of truth in the Model. Given that formal languages include a proof
structure, a Model can only be used to dene the semantic properties of a formal language, if every
provable sentence in the language is dened to be true with respect to the Model.
This idea is simple and intuitively appealing as it seems to capture something important about
formal languages. Formal languages are dened by allowing certain types of strings of symbols as
legitimate, and allowing certain types of derivation patterns to be carried out on strings of symbols.
The meanings of dierent symbols, and at times derivation rules, are not xed by the denition
of the language. Instead they are abstract symbols that can be used in dierent contexts to mean
dierent things. This method of denition, which is very similar to the abstraction that occurs
in mathematics, is one of the strengths of formal logic. There is no ambiguity in the reasoning
allowed, yet the formal language can be used to describe any system which has the same structure.
Thus, for example, modal logics are often used to study knowledge, since knowledge operators
share certain structural similarities to necessity operators, even though intuitively the meanings
are very dierent.
The concept of a semantic model turns this intuition into a precise formal denition.
semantic properties of sentences in a language not dened as part of the denition of the language,
but are dened with respect to something external which can be seen to supply meanings and truth
values to symbols. This is structurally similar to the way that the meaning and truth of natural
language sentences are dened with reference to the real world, and the real world is obviously
external to any natural language.
If we follow this angle, it does not make sense to ask about the semantic properties of the strings
of symbols, since they depend on the context of application. However, when we are considering
truth values, and meanings of certain logical connectives, the situation is not as straightforward.
Within a formal language, certain strings of symbols are always provable from the derivation rules,
and therefore must always be true. Otherwise the notion of provability does not make sense. The
question then arises as to whether the truth of these strings of symbols is a fact about the formal
language, and is therefore not dened against something external. There is signicant debate about
these issues, including whether logical consequence is a model theoretic or proof theoretic notion.
As we are focusing on Model Theoretic semantics, we only need to consider the solution to
this question adopted within model theory. A model theoretic denition treats the truth values of
all sentences in the same way: they are dened with respect to a model. The provable sentences,
however, also dictate which formal structures are allowable models, for only structures in which
the provable sentences are true can reasonably supply a denition of the semantic properties of
a language. This means that formal structure can only be a model for a formal language if the
structure and language have enough structural similarity.
The key point is that the semantic properties, or at least the truth values, of all sentences
within a language are dened with respect to a model in Model Theoretic semantics. Truth for a
formal language is normally dened, and thought of, as `truth in a Model.'
As a point of practise, there are normally multiple, non-identical models of any formal language.
This does not mean, however, that there are multiple concepts of truth, or that the concept of truth
is badly dened. The requirement that all provable sentences are true in the model means that
all models will agree on at least the provable sentences, so there will always be a set of sentences
which are true in all models. This means that often the most useful concept of truth to work with
is true in all models. In other cases, where it is clear what sort of structure a language is meant to
describe, the concept of truth that is adopted is true in the intended model. Thus, for example,
the intended model for Peano Arithmetic in various formal language is number theory on the set
of natural numbers, even though there are other non-isomorphic models.
While each of these approaches are not identical, they do not represent dierent concepts of
truth and the same considerations arise. The analysis in this chapter applies equally, whether we
are considering truth in a model, all models or an intended model.
3.2.1 The Denition of a Model
There are problems that arise when a formal truth predicate is dened within a model theoretic
semantics. These problems arise from the combination of any standard denition of a model with
the intuitively correct properties for a formal truth predicate within model theoretic semantics.
It is important to note in what follows that we are not assuming anything about the recursivity
of the model theoretic denitions. While model theory is almost universally dened recursively,
recursivity and the concept of model theory are separable assumptions, which are being analysed
separately in this chapter.
Some of the denitions may seem odd at rst, but this is because
recursivity is not assumed.
To see the problems, we must begin with a denition of a model.
Given that we are only
investigating the core concept of a model, we need only a very generic denition. The following is
a standard denition:
be a formal language,
is a model of
denote consequence in
some mathematical
if, and only if, the following condition holds, for any sentence
`P
M |= P
is only a model of
from no assumptions) holds in
if every theorem in
(i.e. every sentence in
that is provable
While this denition captures the key idea, it leaves an important philosophical aspect of the
denition implicit.
of the model
is dened as a sentence of the language, and is therefore not an element
To be precise, it does not make sense to write
to mean that
The relationship between the language and the model must be set up by means of some
interpretation or valuation function which associates sentences in the language with elements of
the model. For example, in Classical Logic, there are valuation functions which interpret sentences
in the language as truth values in the model. This function is normally treated as a part of the
denition of the model, and is left out of the supplied denition since including it would make the
notation harder to follow, and it does not aect the argument in any way.
The denition also only considers one particular type of model, where we have a model of the
language itself. Often it is more useful to look at models of a theory within the language. In this
context a theory is simply a consistent set of sentences within the language. The denition of a
model of a theory in a language is essentially identical to the above denition:
language and
T `P
be some mathematical structure.
following condition holds, for any sentence
L, T
be a theory in that
if, and only if, the
MT |= P
The original denition is a special case of this when we have an empty theory.
It was noted above that there are occasions when true in an intended model or true in all
models are more useful concepts that simply true in a model. If we are working with truth in an
intended model, then the above denitions will hold (otherwise the intended model could not be
a model). Truth in all models, which is often represented formally as
|= P
as opposed to
M |= P ,
is a more complicated notion. The key dierence is that it is normally assumed that a Model is
complete in the sense that every sentence in the language is assigned a truth value by the Model.
That is, in a classical model, for any sentence
only one of
MT |= ¬P
holds.
When we are considering truth in all models, it is not possible that every sentence has an identical
truth value in every model, so there will be many (contingent) sentences which do not have a truth
value with respect to the set of all models. Nevertheless, the above denition in the form of If
|= P since If
M |= P must hold for every model.
These denitions, plus the requirement that a model is complete, provide a general characterisation of semantic models. However, the key factors in an examination of formal truth denitions
are the properties a satisfactory truth predicate must have in a model theoretic context.
3.2.2 The Truth Predicate within Model Theory
The aim of this investigation is to evaluate the consequences of assuming a Model Theoretic
denition of semantics for formal truth denitions. The important factors that therefore need to
be established are the semantic properties of any truth predicate if a model theoretic denition is
adopted. In order investigate this, we will assume that we have our xed language
xed theory
a truth predicate
and has a valid model of the theory
MT .
includes a
The theory may
be a null theory, but we will allow there to be a theory as it is a more general case. This means
that the principle given in the denition above, namely that If
MT |= P is assumed
to hold. We will motivate a number of dierent principles which must hold if the truth predicate
is to capture, even extensionally, the same concept as being true in a model.
The fundamental idea of the model theoretic approach to semantics is that the semantic properties, such as truth, of sentences within a language are measured against the models of that
language. Hence, a sentence
that the interpretation of
is true on the assumption of the theory
within the model
MT ,
just in those cases
is true in the all the models that
is true.
This suggests that the following principle should be satised by a formal truth predicate:
Condition 1:
T ` T rpP q
That is, for any satisfactory truth predicate, `P is true' should be provable from
cases when
is true in the model
in exactly those
Assuming this, however, would imply that everything that
is true is provable in the language (on the assumption of the relevant theory). This is in general
simply not the case, since we are only rarely interested in theories that assign a truth value to all
sentences. This condition needs therefore to be weakened, and we use one half of the biconditional
in our rst condition on a satisfactory truth predicate:
Satisfactory Truth Principle 1: If
This obviously captures the idea of a truth predicate in a model theoretic context. If it can be
proven that a sentence is true, then that sentence must hold in the model.
dierently, we can consider a counterexample. If there is some sentence
MT 2v Q,
Q,
To put this point
T ` T rpqq
then the truth predicate cannot be satisfactorily dened - for it identies as true
a sentence which is not true in the relevant model. However, this suggests that we need to add
a further criteria on the satisfactory denition of the truth predicate, for we also do not want
sentences to be identied as not true, which are true in the model. That is, our second condition
on a satisfactory truth predicate is:
T ` ¬T rpP q
MT 2 P 2
These two denitions must be satised for any formal predicate which can reasonably be interpreted
as truth in a model theoretic setting. Thus, for any formal truth denition, if these two principles
are satised, then the denition satises a minimal criterion on being a reasonable truth denition.
However, considering the denition of a truth predicate within a model theoretic framework
raises the further important question. It is assumed that the semantic properties for every sentence
in a language are given by the model.
includes a truth predicate, so it follows that every
sentence which includes a truth predicate must be interpretable within the model as the model
must assign it a truth value.
condition: If
MT |= T rpP q.
each model
To put it dierently, since every valid model
MT |= P ;
then it must also satisfy the condition: If
must satisfy the
However, this condition is only coherent if there is an interpretation of
for otherwise
MT |= T rpP q
literally does not make sense.
Without being given any information about
interpretation of
in any model is for any
it is not possible to determine what the
The interpretation depends clearly on the
relevant language and model. However, we can identify a natural condition it should satisfy. Quite
simply, whenever
is true in
Interpretation Principle:
M, T rpP qshould
If this principle is not satised, then the
of truth in the model.
also be true in
That is:
predicate cannot be said to represent the concept
If it was not true then there would be some sentence that is true in a
model, but the assertion that it is true is not true in that model. Or alternatively, there would
be a sentence that is not true in the model, but the assertion that it is true is true in the model.
Both of these alternatives would imply that the truth predicate is not a genuine truth predicate.
Thus the interpretation of any formal truth predicate within the relevant model must satisfy this
principle. We would normally expect that this implies the interpretation of
way depend on the interpretation of
2 We
will in some
but this is not a necessary assumption.
are not assuming anything about the denition of negation, except that it is not paraconsistent.
We have therefore identied three principles which must hold within a Model Theoretic framework, if a formal truth predicate is to represent the concept of truth within model theory in any
meaningful sense. These Principles are not independent, however, as Satisfactory Truth Principle 1
can be derived from the
Interpretation Principle, and the denition of a model. Thus although
they can be independently motivated, they follow from a basic restriction on the properties a truth
predicate must hold in a model.
However, these Principles are not sucient for a discussion of
the Liar Paradox, since we do not yet have any rules governing the behaviour of negation within a
model theoretic context. Nevertheless, they allow for a discussion of the Truth Teller within Model
Theory, which is an important related phenomenon.
3.2.3 The Truth Teller
A Truth Teller Sentence is one which asserts of itself that it is true. In order to investigate the
properties of a Truth Teller sentence within the model theoretic framework, we have assumed
that our language
includes the sentence
T r(a),
which has the name
Principle, which is meant to place a necessary condition on the interpretation of
Int. for
MT |= T r(a)
a, is the following:
This is trivial but vacuous as it does not tell us anything. The two Satisfactory Truth Principles
are likewise vacuous:
Satisfactory Truth Principle 1:
T ` T r(a)
T ` ¬T r(a)
MT 2 T r(a)
They do not tell us anything more than the fact that
Obviously if we try
to use any of these principles in reasoning about the Truth Teller, we simply get back to what we
already know about the Truth Teller. It is true if and only if it is true, and it is not true if and
only if it is not true. However, it adds a new dimension to our understanding, when we consider
the question within the model theoretic context.
With model theoretic semantics, the sentence
the model assigns a truth status to
is true (or has any other truth value) only if
Moreover, for the majority of sentences of the form
T rpP q,
Interpretation Principle is itself sucient to establish the truth value that the sentence
should have.
is only a valid sentence if
is a sentence in
then it must have some truth status with respect to
will follow from this. The problem in the case of
interpretation or truth value in
determine a truth status for
its truth value in the model.
The status of
is that its denition (as
with respect to
T r(a))
does not x an
The denition in itself is not enough to allow the model to
or in other words the information contained in
underdetermines
This is not a problem if we are considering a xed model
MT , since this model will presumably
contain many sentences without a determinate truth value, and the model assigns truth values to all
such sentences. However, when we consider truth in all models, a diculty arises. Sentences which
get an arbitrary truth value assigned will have one truth value in some models and a dierent truth
value in other models. This is to be expected when the sentences are contingent, and therefore
have a truth value that depends on the relevant facts.
on the other hand does
not appear contingent, and it seems weird to suppose that this sentence could have dierent truth
values in dierent contexts. The only ways to x the truth value of
are an intended model, or to
add some extra principle(s) to the language and/or model which assign truth values to sentences
Either approach requires justication, and requires appealing to information beyond that
contained in
to determine a truth value.
This problem is surmountable but requires justication as to the correct extra principles or the
correct intended model.
In one sense, this simply conrms the observation above that the Liar
Paradox itself does not dictate a solution. We must assess any solutions against criteria external
to the Liar Paradox. However, this analysis reveals something of the structure of the problem, and
identies an area where more serious problems arise when consider Liar Sentences. However, in
order to that we must understand the basic properties of negation within Model Theory.
Negation, and how to understand it is a very large topic, particularly in the context of discussions of
the Liar Paradox. Many dierent solutions to the Liar Paradox require dierent assumptions about
the correct properties of negation. However, as pointed our in Chapter 1, the key to solving the
Liar Paradox cannot be in our understanding of negation as there are languages with no negation
that can be aected by the paradox. For the sake of this analysis, it is necessary not to prejudge
these discussions as much as possible. However, analysing the situation requires something and
we will make the assumption that we are dealing with consistent models (at least with respect to
the relevant negation). This is a non-trivial assumption in the context of discussions of the Liar
Paradox, yet is widely accepted and justiable. To be more precise, we will assume that:
Consistency of Models: For any sentence
in a language
with a model
at most one of
This assumption has signicant consequences, particularly via a couple of obvious corollaries.
Firstly, it follows from the Consistency of Models Principle that if
the case that
MT |= P .
That is, if
MT |= ¬P ,
MT 2 P .
then it cannot be
Secondly, if we assume that a
model assigns a truth value to every sentence, in other words that the model is complete, it follows
that if
MT 2 P ,
MT |= ¬P .
This principle and its corollaries, combined with the assumptions on a satisfactory denition
of the truth predicate in a model theoretic framework, give us enough to begin an investigation of
the Liar Paradox. This will be split into several parts.
3.2.5 The Liar Sentence itself
The rst focus of investigation will naturally be the archetypal Liar Sentence
As stated before,
¬T r(b).
is the sentence
b in our language LT .
This sentence is the most simple form of liar sentence,
and we wish to evaluate this sentence with respect to its semantic properties in the model,
As a valid sentence within the language
Interpretation Principle for
should have a truth value in
bring out the reasons that
MT |= ¬T r(b).
(which is
¬T r(b))
However, the problem is more serious than this. Firstly, in order to
does not hold in
we assume that
does hold in
i.e. that
We get the following valid argument on the principles developed here:
MT |= ¬T r(b)
(Defn of
MT 2 T r(b)
(Corollary to Consistency of Models)
MT 2 ¬T r(b)
However, the
This is obviously contradictory, and obviously means that neither
can hold consistently in
MT |= T r(b)
LT , b
(Contraposed
Interpretation Principle i.e. if
does not hold, then
cannot hold)
That is, from the principles we have established, if we assume that
holds in the model, then it
follows that it does not hold. Given that it does not make sense that a sentence both holds and
does not hold in the model, our assumption must be incorrect, and so it should follow that
not hold in
MT 2 ¬T r(b),
However, if we assume that
that is we assume that
then the following argument holds:
Assuming that
Interpretation Principle,)
for any given model,
leads to the conclusion that it must hold. This means that,
can hold in the model if, and only if, it does not hold in the model.
other words, the basic conditions imposed on the denition of a formal truth predicate above lead
to a completely contradictory characterisation of the semantic properties of the liar sentence
we assume that these conditions are valid, which is dicult to argue against without undermining
the idea of a model theoretic semantics, the sentence
with respect to the model
cannot consistently have any properties
That means that it cannot be consistently semantically interpreted
within the model theoretic framework.
Furthermore, even if we ignore the contradictory nature of the denition, we notice that none
of these lines provide any more information about the semantic properties of
than the original
assumption. As in the case of the Truth Teller, the Principles in question do not allow the derivation
of any useful information about the semantic status of the sentence in question. Thus
provide sucient information to allow a semantic evaluation with respect to a model.
Given some innocuous assumptions about the nature of truth and negation within a model, it
is not possible for the model to coherently assign a consistent semantic status or evaluation to the
Liar Sentence
This is partly because it is not possible for the model
to be interpreted within
to provide any relevant
However since
represent a language with the basic semantic framework set up by a standard model theoretic
denition, the structure of model theory itself impedes a consistent semantically closed solution to
3.2.6 Sentences about a Liar Sentence
Although it is clear from the consideration of liar sentences that there are limitations inherent
within the structure of model theory, the problems that commonly arise mean that it is important
to investigate another type of problematic sentence within the same context, that is sentences
about a liar sentence. To do this, we will adopt the same framework as the previous sections with
¬T r(b)
with the same governing principles. We will also consider the
a sentence
T r(b).
and the sentence
¬T r(b) ∧
within this context are identical to those of
Interpretation Principle, it follows
b does not have a consistent semantic
This means that, since
cannot have a consistent semantic evaluation within this structure, and that neither
nor its negation (however that is interpreted) can be true. Similarly, the semantics properties
assuming the obvious principle that
time an application of the
By two applications of the
MT |= d i MT |= T r(b) i MT |= b.
evaluation,
d;
Unsurprisingly, the semantic properties of
whose name is
So neither
M |= A ∧ A
M |= A,
are identical to that of
Interpretation Principle with the denition of
MT |= b.
holds exactly when
does, but
leads to:
MT |= c
cannot consistently hold.
can have a consistent semantic evaluation.
These two sentences, however, represent the two key options for reporting the truth value of the
b within a language.
b is either true or not true, then either d or c should be true.
the structure of model theory, since all the semantic properties are determined with respect to the
model, cannot dierentiate between liar sentences and any other sentences about a liar sentence
since none of them can have a semantic status with respect to the model. This causes problems
for the reporting of truth values within the language and hence for semantic closure.
3.2.7 The Liar Sentence and Semantic Closure
A model theoretic approach to semantics cannot consistently assign a Liar Sentence any semantic
properties within the model. However, this does not prevent semantic closure since this fact may
be sucient, for example, to justify that
From this it would follow that it is possible for
the language to be semantically closed, for the crucial test which we are applying is whether one of
T r(b), ¬T r(b)
T r(b) ∧ ¬T r(b)is
justied as true within the language. However, the conditions
that were justied on a Satisfactory Denition of truth within a model theoretic context prevent
this from occurring.
To see this, we will rst repeat the relevant Principles for reference:
` T rpP q
` ¬T rpP q
M |= T rpP q
M2P
On this basis we can prove the following theorem:
Theorem.
If a language
has no complete semantic model.
Proof. Firstly, we assume that
is semantically closed, and it includes the Liar Sentence
` T r(b)
M |= b,
` ¬T r(b)
(for some negation in
has a semantic model, then for all provable sentences within
is interpretable and holds in
is semantically closed, then
M 2 b.
has a model
M |= b.
` ¬T r(b) and that LT
If follows then that
Hence if
LT ).
We note
M |= P .
M |= T r(b).
It follows that
From this it follows, as shown previously, that
contradiction, it is not possible that both
However we have shown above that if
the conditions on negation in a model theoretic context. Then, by the
Thus, it is not possible that both
Similarly, we assume that
Interpretation Principle, it follows that
dened previously. It follows that either
has a model.
M 2 T r(b), by
Interpretation Principle,
That is, on pain of
is semantically closed, then it can have no complete model.
is here being used in the sense discussed above: a complete model is one in which every sentence in
the language has a truth value.
one assumes the necessity of the model theoretic approach to formal semantics, then it provides
a clear proof of Tarski's conclusion that a semantically closed language is impossible.
other hand, if we believe that there are alternative approaches to dening the semantics of formal
languages, then this conclusion shows that we need change the way that formal semantics are
dened, if we wish to achieve semantic closure. In order to work out whether this is possible, it is
necessary to consider conceptually where the problems arise in Model Theory, which is the focus
of the next section.
3.3 The Conceptual Structure of Model Theory
The result just proven demonstrates that adopting a Model Theoretic Semantics necessarily places
limitations on what can be achieved by a formal truth denition.
If we accept the necessity of
Model Theoretic Semantics, then semantic closure is not possible. Any formal truth denition that
aims to be semantically closed must therefore, at least, fundamentally modify some aspects of the
Model Theoretic approach. Nevertheless, the success and enormous utility of Model Theory as an
account of formal semantics demonstrates that, even if there are problems with it in the particular
context of formal truth denitions, any attempt to resolve these problems should attempt make
as few changes to the standard structure as are necessary. That is, we will attempt to adhere to a
principle of minimal mutilation, changes will only be made where necessary and the theory should
revert back as much as possible to the standard theory when we take out considerations about
formal truth denitions.
In order to achieve this, it is necessary to have a clearer understanding of the reasons why there
are problems with the orthodox semantic approach. For this purpose, it is vital to examine the
conceptual structure which supports the model theoretic approach in order to identify the causes
of the identied problems.
When one considers the conceptual structure of Model Theory, the
problems arise very intuitively, and in turn they suggest a strategy by which we might resolve
them. This can be made particularly clear if we consider the relationship between formal language
(or syntax) and model as analogous to the relationship between (natural) language and the world.
As has been made clear, within the Model Theoretic semantic approach, the idea is that the
semantic information (at a minimum: truth) for sentences in a language is only determined by
the Model(s), which is (are) separate from the syntax of the language. If we translate this into
the analogy between language and world, it means that all of the semantic information, and in
particular truth values, about sentences within a language are purely determined by what is the
case in the world; and that the language and the world are strictly separate.
Furthermore, the
semantic properties of sentences within the language are xed by how they are interpreted with
respect to the world.
If we then ask the question, for example, whether a Truth Teller Sentence would be true or not
on this conceptual picture, the formal problem identied above immediately arises. If the Truth
Teller has any semantic properties, it must be interpreted within the world.
However, a Truth
Teller Sentence says that it itself is true, which is a claim about the semantic properties of an
element in the language. That is, it does not say anything that can be directly interpreted within
the world, since the language and world are strictly separate on the model theoretic conceptual
structure. For ordinary truth ascriptions, say of the form `D is true', this problem is resolved since
the sentences make indirect claims about the way the world is via the sentence D. In other words,
they are grounded. A Truth Teller, however, only makes a claim about the semantic properties
of a sentence which makes the same claim about the semantic properties of the same sentence.
That is, it cannot, in principle, be interpreted as about the world and since world and language
are separate, it is not possible to evaluate the truth value of a Truth Teller sentence with respect
to the world.
This problem obviously aects Liar and other similar sentences equally, and the reasoning
involved exactly mirrors the argument given above.
Furthermore, it follows that any sentences
about a Liar Sentence also cannot be interpreted as about the world, and hence they have the same
semantic status with respect to the world/model.
Thus the model theoretic semantic structure
cannot dierentiate between paradoxical sentences and sentences about paradoxical sentences.
This demonstrates that the conceptual structure of model theory, and in particular the way that
semantic information is determined for all sentences with respect to the model, is incapable of
dealing with the Liar Paradox in an intuitively satisfactory way.
The problems with the conceptual structure, in turn, illustrate why semantic closure is not
possible within a model theoretic framework.
Semantic closure requires that one of
is provable within the language. However, within a model theoretic framework, if we can
assert either
T r(b) or ¬T r(b), then the truth value of these sentences must be denable with respect
to a model. It follows that if one of
evaluate the sentence
is provable, we must be able to semantically
with respect to the model; otherwise there is a truth that is expressible
within the language but not in the model, which contradicts the assumptions of model theory.
However, if
is a liar sentence, this is not possible, and hence semantic closure is not possible
within a model theoretic framework.
This analysis of the problems with the conceptual structure of model theory also identies
why there are problems with Liar (and related) Sentences, but not with other self referential
sentences. If we take, for example, the sentence This sentence has thirty one letters, the semantic
interpretation of this sentence does not depend on the semantic evaluation of the sentence itself.
It is determined by other properties of the sentence which can be identied independently of the
semantic properties of the sentence - in this case by counting the letters.
In the case of a Liar
Sentence, the semantic interpretation of the sentence depends on being able to independently
identify the semantic interpretation of the same sentence, which is impossible. Thus self-reference
itself is not a problem, only the type of semantic self-reference that is exhibited in the Liar Paradox.
This fact that that self-reference is only a problem in the context of semantic predicates such as
truth, indicates that, if we are to adhere to a principle of minimal mutilation, it is only necessary
to change the way that our semantic account of formal languages deals with semantic predicates.
How this can be achieved will be left to the next chapter, as there is the other assumption inherent
in modern formal semantics which needs to be examined.
3.4 Recursivity
At the beginning of this chapter, it was argued that there are two key assumptions present in the
orthodox approach to the formal denition of semantics: the concept of a semantic model and
the assumption that semantic values are dened recursively. These assumptions were deliberately
separated as each assumption has dierent consequences when we want to dene a formal truth
predicate. The previous sections have shown that assuming that all semantic properties are dened
with respect to a model is problematic when we are trying to dene a formal truth predicate. Importantly, this was shown without making any assumptions as to whether the semantic denitions
are recursive. This section will look at the assumption of recursivity.
Assuming that the denition of the semantic properties of sentences is recursive is one of the
most productive and important assumptions with the orthodox approach to formal semantics.
That is, the assumption that the semantic properties of the atomic sentences, or (depending on
the type of language) the most basic sentence components, are initially dened and the semantic
properties of complex sentences are iteratively dened from the semantic properties of the most
basic sentence components by dening the way that the rules of sentence construction transmit
semantic values. Thus, to take a very simple example, if
is true exactly when
is a sentential logic, and
are atomic sentences, then a recursive denition works as follows:
1. Assign truth values to
- say T, T and F respectively..
2. Dene the semantic properties of connectives. For example,
A∧B
are each true.
3. Iteratively assign truth values to complex sentences (such as
(P ∧ Q) ∧ (R ∧ Q))
by means
of the truth values dened in Step 1, and the properties of the connectives dened in step
2. Thus, in our example, we rst determine that
From this it turns out by a further iteration that
P ∧Q
has value T and
(P ∧ Q) ∧ (R ∧ Q)
R∧Q
has value F.
When this process is repeated indenitely, it will normally assign truth values to every possible
sentence within the language.
One of the most attractive features of a recursive denition is that in order to determine the
semantic properties of any particular sentence, it is only necessary to decompose the sentence into
its most basic components and then use to recursive denition to calculate the semantic properties
of the relevant sentence. Thus, in our simple example, if one wishes to work out the truth value of
(P ∧ Q) ∧ (R ∧ Q),
one simply decomposes it and notices that it contains the atomic sentences
From the basic denition we can determine the truth value of these sentences, and then
use the denition of
to calculate the truth value of
(P ∧ Q) ∧ (R ∧ Q).
It has already been shown that the very concept of a model against which all the semantic
properties of sentences in a language are dened prevents a completely semantically closed formal
truth denition. We will, for the purposes of examining recursivity, ignore this conclusion and look
purely at the consequences of assuming a recursive semantic denition. The conclusions reached
here will apply beyond model theoretic semantics to any recursive semantics.
that we have the language
¬T r(b) ∧ ¬T r(b)
and the sentences
a, b,
We will assume
as dened above (T r(a),
respectively).
This process of decomposition that can normally be used in a recursive denition, however, does
not work when we examine sentences such as
The semantic properties of the sentence
on a recursive denition, depend on the semantic denition of the
properties of
predicate and the semantic
This sets up an immediate circularity, which means that the sentence
decomposed into atomic sentences, or any sort of basic sentence components which have dened
properties. The Liar Sentence
suers from exactly the same problem - its semantic properties
depend on its own semantic properties within a recursive denition. Therefore an ordinary recursive
denition cannot assign any semantic properties to Sentences like
This observation has been often made before, and is the basis of the Kripkean approach to
formal truth denitions. Kripke (and Herzberger before him) labeled those sentences which can
be decomposed into basic sentence components with dened semantic properties Grounded sen-
4 His method of denition was, on this basis, to allow grounded sentences to be assigned
tences.
semantic properties according to an ordinary recursive semantics. For ungrounded sentences he
dened a third truth value for ungroundedness, and allowed the recursive denition to continue
transnitely, in order to have a complete denition of it.
However, the application of Kripke's
denition was limited by what he referred to as the ghost of the Tarskian hierarchy.
The essential problem that Kripke identied with his denition is that sentences about a Liar
Sentence are also ungrounded and therefore on his denition have the same semantic properties
Hans G. Herzberger. Paradoxes of Grounding in Semantics. In: The Journal of Philosophy 67.6 (1970),
pp. 145167; Saul Kripke. Outline of a Theory of Truth. In: Journal of Philosophy 72.6 (1975), pp. 690716
5 Kripke,
as the Liar Sentence.
This problem is one that necessarily follows a recursive approach to the
denition of semantics. On this type of denition, the semantic properties of a sentence, such as
which refers to another sentence will necessarily depend on the semantic properties of the sentence
it refers to, in this case
In fact, the semantic properties of
purely determined by the properties of
assign any semantic properties to
¬, T r, ∧
¬T r(b) ∧ ¬T r(b),
have to be
However, if a recursive denition cannot
then it equally cannot assign any semantic properties to
This means that a recursive denition semantics cannot semantically dierentiate between a liar
sentence and any sentence about a liar sentence, such as
This is a serious problem since, according to an intuitive examination of a sentence such as
its semantic properties cannot be identical to those of
the example above, that is,
on a recursive denition,
have assumed that
is true, if and only if,
is consistent,
We will assume that
is dened as in
are each true. This means that,
is true, i.e. when
is not true. As we
c cannot have the same semantic properties as b.
stated, a recursive semantic denition cannot semantically dierentiate between
However, as
b and c so
it must
assign them the same semantic properties and therefore truth value. Thus a recursive semantic
denition cannot capture the logical relationship between
and therefore cannot allow a
semantically closed truth denition.
It is important to note neither the sentence
nor the connective
are unique in this re-
spect. There is a long list of sentences which cause the same problem. Some examples include:
¬T r(b) ∨ ¬T r(b); (P ⊃ P ) ⊃ ¬T r(b);
- with a name other than
(if that is
possible). Thus, if we assume that we have a consistent language with a recursive semantic denition, then that language cannot contain a semantically closed truth denition. The assumption of
Recursivity, independently of any other assumption about the nature of formal semantics, prevents
a semantically closed truth denition.
3.5 Sentences as Truth Bearers
While we have shown that a recursive denition of semantics cannot properly capture the logical
relationship between various sentences, there is a weaker assumption that also has similar problems.
One of the consequences of assuming a recursive semantics is that every identical string of symbols
within the same language necessarily has the same semantic properties and hence the same semantic
value. This is because two identical strings of symbols have the same syntactic structure and the
same atomic sentence components, and therefore under a recursive semantics must have the same
semantic properties. This means, to put it in the language of philosophy of language, that sentences
(or sentence types) are the truth bearers if we assume a recursive semantics. This assumption within
formal languages is reected, for example, in the assumption that every sentence has a unique, or
canonical, name, and we can use that name exclusively to refer to the sentence. Assuming that
sentences bear truth values is a weaker assumption that recursivity, since it is possible to have a
non-recursive semantics where sentences bear truth values.
Assuming that sentences are truth bearers, and that every sentence has a singular canonical
name, has similar consequences to assuming recursivity for the denition of a truth predicate. In
order to demonstrate the consequences of this assumption, we will assume that it is false and show
that assuming that sentences are the truth bearers is a non-trivial assumption that has problematic
consequences. We will therefore consider the situation where there are two instances of the sentence
in the language
which we will identify by dierent names,
test whether they potentially have dierent properties.
and in order to
If the assumption that sentences bear
truth values does not have any consequences for formal truth denitions or is trivial, then we can
unproblematically assume that
b = g.
However, when we consider the denitions of
we can assume
b = g
of the sentence
then it should not be expected that
without suering any adverse consequences.
we consider the sentence
If we consider the instance
then it is obviously a classic liar sentence. However, if
in isolation from the denition of
a perfectly unproblematic sentence whose truth value depends on the truth value of
then it is
(whatever
that is). Thus the reference structure of these two sentences is dierent, since one sentence is selfreferential and the other is not, even though they are dierent instances (or tokens) of the same
sentence. This dierence leads a signicant problem, which demonstrates that a formal language
for which sentences are truth bearers cannot be semantically closed.
We will rst consider the sentence
if, and only if, the sentence
By denition,
should be true
(whatever it is at this point) is not true. However, if one assumes
sentences are the truth bearers, then
must should have identical semantic
properties. Given that in a consistent semantically closed formal language, exactly one of
¬T r(b) is true, it follows, as above, that a language in which sentences bear truth values cannot
consistently include a truth predicate and be semantically closed. This is the identical problem to
the situation for recursivity.
This fact, that any formal language which assumes that its semantics is recursive or that assumes
that sentences bear truth values cannot be semantically closed, partly explains why semantic
closure has not been achieved in any formal denition that currently exists.
For every existing
truth denition assumes at least one of these principles, with the majority of denitions assuming
recursivity and hence both. The question that this raises is what a non-recursive semantic denition
for a formal language in which sentences do not bear truth values could look like.
The existence of systematic limitations in modern formal truth denitions as shown in Chapter 2
has turned out to be unsurprising, given that all of these denitions assume a standard recursive,
model theoretic semantic denition. Both the assumption of recursivity, and the assumption that
truth is to be dened with respect to a model inherently limit what can be achieved with a formal
truth denition. Interestingly, the reasons that each of these assumptions cause the limitations is
independent, so we have two separate problems that need to be addressed.
For this reason, consideration of the method that can resolve these problems will be split into
two parts. The rst part, addressing the problems within Model Theory, will cover chapters 4 and
5. Chapter 4 will look at how to address the problem and chapter 5 will provide a formal denition
to show how it can be done. The consideration of recursivity and the assumption that sentences
are truth bearers will be left to chapter 6. It is a less complicated, and more controversial change,
and builds on the work in chapters 4 and 5. Given the two problems are separate, the solutions
are also separate and each can be implemented without the other.
Outline of a New Approach
As noted before, a philosophically satisfactory formal solution to the Liar Paradox requires the
denition of a logical language which can satisfy the grammatical conditions of natural languages.
That is, the logical language must include its own truth predicate, and there are no restrictions on
applying this predicate to the sentences in the language. Moreover, a philosophically satisfactory
formal solution should satisfy the condition of Semantic Closure, that is, for any sentence in the
language, it is possible to construct sentences which assert that the relevant semantic properties
(or at least truth values) for the original sentence hold and that these sentences in turn have the
correct truth values. In the previous chapter, it was demonstrated that adopting a standard Model
Theoretic semantics for a formal language is inconsistent with semantic closure. The structure of
standard Model Theory prevents the satisfactory evaluation of the truth value of certain sentences
within the relevant language. A philosophically satisfactory formal solution to the Liar Paradox is
therefore not possible if we accept a standard Model Theoretic semantics.
Usefully, two key assumptions were identied in the previous chapter that individually prevent
a formal language with a standard Model Theoretic semantics being semantically closed. The rst
assumption was that the semantic properties of all the sentences within a language are dened
directly against the semantic model.
The second assumption was that sentence types are truth
bearers, or that identical syntactic entities have identical semantic properties, which is necessary
for the standard recursive denition of semantic properties. Given that each of these assumptions
prevents semantic closure by themselves when we are dealing with the Liar Paradox, a satisfactory
solution to the Liar Paradox is only possible if our understanding of the semantic structure of
formal languages can be changed.
The two assumptions that need to be questioned are independent of each other, and therefore
need to be investigated separately. The second assumption, that sentence types are truth bearers,
has already been challenged in the literature on the Liar Paradox,
1 with varying degrees of success.
example, see Brian Skyrms. Denitions of Semantical Reference and Self-Reference. In: Notre Dame
Journal of Formal Logic XVII.1 (1976), pp. 147148; Laurence Goldstein. 'This Statement is Not True' is Not
Moreover, as argued in the Introduction, changing the account of the correct truth bearer does
not address the problems in dierentiating between the paradoxical and non-paradoxical, which is
crucial to solving the Liar Paradox. For these reasons, discussions of this assumption will be left
to Chapter 6.
The focus of this chapter is on the structural assumption within Model Theory that the semantic
properties of all sentences are dened solely with respect to the relevant Model. Given that this
assumption has been shown to be incompatible with a philosophically satisfactory solution to
the Liar Paradox, the aim of this chapter will be to develop an alternative understanding of the
semantics of formal languages that avoids this assumption.
Once this is achieved, the formal
implementation of this alternative understanding will be explored. It should be noted that just as
the relevant assumption is independent of any particular avour of modern logic, the alternative
approach developed will also be independent.
This means that, if successful, the new approach
and the resulting truth denition will be applicable to any formal system of logic and any formal
The development of the approach will also follow an important principle, namely that since
the Liar Paradox does not directly aect the vast majority of sentences within a language, any
solution should directly aect as few sentences as possible. This means that the approach adopted
will be to rene the standard Model Theoretic approach to semantics, rather than to entirely
replace it. This is not meant to represent any deep philosophical commitment to the correctness
of Model Theoretic Semantics as the correct semantics for formal or any languages. Instead it is
more conceptually illuminating, and easier formally, to see where the standard approach needs to
be changed, rather than attempting an entirely new approach.
The strategy for developing an alternative approach, and an alternative truth denition, is to
begin by considering what the concept of Truth should actually represent in a formal language.
From this we can consider the nature of the truth predicate and therefore how to understand the
structure of truth and the denition of the truth predicate.
4.1 Truth in Formal Languages
The concept of Truth is one of the more controversial concepts in the history of philosophy, and it
is beyond the scope of this project to resolve the debates about the nature of Truth or the correct
theory of Truth. Our focus is more narrow: the concept of Truth in formal languages, and how
we are to understand and work with this concept. This narrowing of focus reduces the scope of
the problem, since what we are interested in this context is what it is for a sentence to be true.
While in one sense this problem cannot be answered without an answer to the broader questions
True. In: Analysis 52.1 (1992), pp. 15; Alan Weir. Token Relativism and the Liar. In: Analysis 60.2 (2000),
about what truth is, there is a formal or structural sense in which this problem can be addressed
independently of any Theory of Truth. That is, we can develop a clear sense of certain structural
facts about truth that are neutral with respect to the large philosophical debates about truth.
4.1.1 What does it mean for a sentence to be true?
A sentence, read purely as string of symbols, does not in itself possess any truth value. It is only
when the string of symbols has, or is assigned, a meaning that it makes sense for a sentence to
have a truth value. The meaning of a sentence is a major determining factor in the truth value
of a sentence, as it provides truth conditions for the sentence. That is, the meaning of a sentence
determines conditions under which the sentence will be true.
A sentence is therefore true whenever its truth conditions (as determined by the meaning) hold,
or are the case. What it exactly means for truth conditions to hold relates directly to the broader
questions of Truth which are beyond this work. However, the structure outlined here requires that
there is something against which the truth conditions of a sentence can be tested to determine
whether they hold or not. In order that the concept of truth be meaningful and not circular, this
something must be independent of the sentence in question. There must be something outside the
sentence (and perhaps the language) against which the truth of the sentence is determined.
Thus, for example, the truth of It is raining outside is tested against the current external
weather conditions; the truth of Bachelors are married men is tested against the system of
meanings of English words; and the truth of 2+2=4, depending on one's account of mathematical
truth, is tested again an external mathematical reality, a socially constructed denition, or its
derivability from a set of self-evident axioms.
The truth of a sentence is always tested against
something external to the sentence itself, and this thing which determines truth values is usually
external to the language the sentence is expressed in.
Model Theoretic Semantics oers a formalisation of this principle, with the simplifying assumption that what the truth of sentences is tested against is completely external to the language. In
this case, if we have a formal language
and a sentence
is true exactly
is true exactly when its meaning
(interpretation) generates truth conditions that are decidable in
M and moreover they hold in M.
when it is the case that
M |= p.
That is, a sentence
is by denition external to the language
which illustrates the way that Model
Theoretic Semantic is a particular formalisation of this fundamental principle that the truth of
sentences is determined (via the sentence's meaning) by something external to the sentence.
4.1.2 The Truth Predicate
This fact that the truth of a sentence is decided by whether the truth conditions of that sentence,
as determined by the meaning, hold or not against something external to the sentence allows us to
explore the necessarily properties of any satisfactory truth predicate. A satisfactory truth predicate
must reect this fact in the way it is dened, otherwise it cannot correctly be a truth predicate.
Now, say we have a sentence
the meaning (or interpretation) of
some language, if
a must hold.
a is
true then the truth conditions given by
Thus, asserting that a is true asserts a particular
type of relationship between the sentence and something external to that sentence. Exactly how
we are to understand this relationship of the truth conditions of a sentence `holding' is not a matter
that will be investigated here. Dierent theories of truth will give dierent answers to this.
However, this gives a clue to the problems faced by attempts to dene a truth predicate in formal
language. The sentence
is a syntactic entity, and on the normal approach to the semantics of
formal languages, the external thing which determines truth values is the Model. The assertion
that a is true is therefore the assertion of a particular type of relationship between an element of
the language and the Model. To be more precise, the assertion that a is true would normally be
understood as the assertion that the interpretation of the sentence
in the Model holds. From a
structural point of view, one would therefore think that the assertion that a is true could therefore
only be made in a language which includes both the model and the original language. The attempt
to dene the truth predicate in the original language, as formal truth denitions typically do, seems
therefore to attempt to assert the relationship between two things, one of which (the Model) is not
expressible within the language.
This problem is not fatal to the project at hand. Indeed, if it were fatal then the use of the
truth predicate in natural languages would also be doomed, as we normally measure the truth of
sentences against things that are not directly representable in language. However, it shows that
we must understand the truth of sentences which include a truth predicate dierently to normal
sentences. We can see this in natural languages, where we approach assertions such as a is true
and their truth value dierently to other sentences. For example, let
is hot. In order to evaluate the truth value of
be the sentence The kettle
we identify the conditions under which
be true, namely the relevant kettle is hot, as evidenced by a number of possibilities such as the
steam rising out of it, or that it feels hot if we put our hand on or near it. We can then test to
see whether these states of aairs exist which enables us to judge whether the truth conditions of
hold, and therefore to evaluate the truth value of
In evaluating the truth value of the sentence a is true, however, we cannot directly test its
truth conditions against a relevant external thing. For the truth conditions of a is true are that
the truth conditions of
hold. We must therefore rst evaluate the truth value of
and then use
this to evaluate the truth value of a is true. The evaluation of any sentence of the form a is
true is therefore a two step process, we must rst evaluate the truth value of the sentence
then we can evaluate the truth value of a is true.
The existence of this two step process reveals a problem with standard Model Theoretic Se-
mantics. Denitions of Model Theoretic Semantics normally assume that every sentence has an
interpretation in the Model, and therefore that the evaluation of the truth value of every sentence
occurs directly in one step against the Model. The evaluation of sentences such as a is true does
not function in this way, as there is the two step process, which is not possible under the normal
assumptions of Model Theory.
4.1.3 Understanding the Liar Paradox
This two step process is crucial for understanding the Liar Paradox, for the existence of this extra
step creates the `space' in which the Paradox works. In essence the Liar Paradox occurs because
there are certain sentences for which this two step process can never complete.
A requirement on the denition of a truth predicate is that there is some means of referring to
sentences within the scope of the truth predicate. Typically in a formal setting names of sentences
are used.
As soon as there is this dierence between a name (or other means of referring) and
the sentence it identies, the combinatorics of the situation mean that it is possible that there are
sentences which include their own name, or at least sentences which refer (by means of names) to
another sentence which includes the name of the original sentence. This possibility of circularity
is equally possible if we use descriptions or some other means of referring to sentences.
This circularity is, however, fatal when it occurs in the scope of the truth predicate. Suppose
includes b is true as a
part. According to the two step process outlined above, the truth value of
b depends on evaluating
we have two sentences,
the truth value of
includes c is true as a part and
However, the truth value of
depends in turn on evaluating the truth value
Evaluating each sentence depends on being able to move to the second step of the process for
the other sentence and therefore evaluating the other sentence. However, because there is this two
step process for the truth predicate, and because of circularity of reference, this is an example of
a situation where the evaluation never reaches the second step in the process. No sentence whose
truth value is evaluable can be found, and the two step process fails.
The truth value of these
sentences cannot be evaluated.
This breakdown of the two step evaluation process occurs in the case of the Liar Paradox,
and explains the way that any consideration of the Liar Paradox starts heading round in circles.
The situation in the case of the Liar Paradox is more pathological, since not only does this two
step process never complete, but the truth conditions generated at dierent steps in the process
contradict each other.
Both of these factors demonstrate that there is no way of denitively
evaluating the truth value of a liar sentence through the normal two step process for sentences
which include a truth predicate.
4.1.4 What does it mean for a sentence to be not true?
Although the standard process for evaluating the truth value of sentences which include a truth
predicate breaks down in certain cases, it does not mean that we cannot say anything about the
truth value of such sentences. For the breakdown of this process demonstrates denitively that
these sentences cannot be true. There is nothing external to the sentence which veries the truth
conditions of the sentence.
Instead the truth conditions of the sentence lead us into an innite
loop which can never be escaped. Thus although sentences such as
are not evaluable on
the standard two step process for sentences which include the truth predicate, this fact that they
are not evaluable in this sense means that we can conclude that
are not true.
This means that there are two dierent ways that we would judge a sentence to be not true. A
sentence is obviously not true if its truth conditions do not hold. Thus for example
if the kettle is not hot and which could be veried by me touching the relevant kettle in my bare
hands and my hands not feeling hot or getting burnt. However, the case above involving the truth
predicate shows that there are cases where the truth conditions of a sentence do not allow us to
test the truth of the sentence, as nothing testable is ever identied.
This conclusion is of fundamental importance in tackling the Liar Paradox.
Since there are
multiple ways in which a sentence can fail to be true, any formal truth denition needs to represent
these multiple ways. The standard understanding of truth, the T-Schema, only allows for sentences
to be not true if what they say is not the case. That is, the T-Schema only allows for the rst
way of sentences not being true identied above. It does not allow for the failure of the standard
evaluation process for sentences which include a truth predicate. As the Liar Paradox turns exactly
on this type of situation, the T-Schema needs to be revised if a satisfactory formal truth denition
is to be achieved.
4.1.5 The Strengthened Truth Schema
It has just been shown that there are at least two ways in which a sentence can fail to be true
in a formal language which includes a truth predicate. A correct formal denition of truth must
correctly reect these dierent ways that it is possible for a sentence not to be true, however, the
T-Schema cannot. The T-Schema, that `p' is true i
provides a single condition on the truth
of a sentence, and therefore a single condition on a sentence being not true.
The Liar Paradox
demonstrates clearly that, in at least special limit cases, there are at least two ways in which a
sentence can fail to be true. The second condition is that the sentence fails the standard truth
value evaluation procedure. For the sake of identifying this easily, we will say that such sentences
are not evaluable.
While we have sketched an intuitive account of what Evaluability represents, namely that the
normal truth value evaluation procedures, we have not given any precise denition. This will be
investigated in a later section, as here we are only interested in exploring the use of Evaluability as
a second condition on the truth of a sentence. As argued above, this is necessary to deal with the
Liar Paradox, and the T-Schema needs to be strengthened to include this extra condition. Adding
this condition in gives the following Strengthened version of the T-Schema:
`P ' is true i (P and `P ' is evaluable)
Strengthened T-Schema:
is true in exactly the cases when
Or to translate it into a formal language
(what
says holds) and
is evaluable.
we assume that there is a new predicate
EvalpP q
which identies the evaluable sentences and get the following:
T rpP q ≡ (P ∧ EvalpP q)
Str. T-Schema:
This Strengthened T-Schema captures very neatly the idea that there are two ways that a sentence
can fail to be true, and therefore that there are two conditions which need to be satised for a
sentence to be true. If we treat this as a denition, then a sentence
is evaluable. If either of these fail, then it is not true. As a note on the notation adopted here,
we have used capital letters such as
names. The
to represent sentences and lower case letters will represent
notation here is, as is standard, a name-forming device.
While this Strengthened T-Schema has been developed in response to a conceptual need, it has
some very elegant formal properties. Firstly, it reduces to the normal T-Schema in the case when
the sentence is evaluable. If it is the case that P is evaluable, then the Strengthened T-Schema
is equivalent to `P ' is true i
Given that ordinary sentences are assumed to be evaluable,
the Strengthened T-Schema fullls the criteria of minimal mutilation, since it reduces back to the
normal T-Schema in all unproblematic cases.
Secondly, if evaluability is appropriately dened, it provides a very neat method of formally
resolving the Paradox.
We will consider an archetypal formal liar sentence, say the sentence
with the syntactic name
¬Eval(b)
is a theorem for
as an illustration. We will assume that
Str. T-Schema for
b: T r(b) ≡ (¬T r(b) ∧ Eval(b))
is therefore:
However, since we have assumed that
¬Eval(b),
the following derivation holds:
T r(b) ≡ (¬T r(b) ∧ Eval(b))
Str T-Schema
¬(¬T r(b) ∧ Eval(b))
Classical Derivation
Modus Tollens lines 1,3
is dened so
and that these sentences are dened in a Classical Logic. The
Eval(p)
is dened appropriately, it follows from the Strengthened T-Schema that
However, this does not license the conclusion that
T r(b),
which occurs if we assume
the standard T-Schema. On the standard T-Schema,
¬T r(b),
it follows immediately that
T r(b) ≡ ¬T r(b)
once we have proven that
However, on the Strengthened T-Schema, we can only
T r(b) if both ¬T r(b) and Eval(b) hold.
Only one of these hold, and therefore the standard
argument to the contradictory conclusion is blocked.
To put it dierently, a sentence of the form
logic if both
(¬T r(b) ∧ Eval(b))
are false.
A ≡ (¬A ∧ B)
can be consistently true in classical
That means that the Strengthened T-Schema for
] can be consistently true in classical logic if both
T r(b) ≡
Eval(b)
false, which is what follows from our denition. While this does not prove that a formal system
which includes the Strengthened T-Schema as a truth denition is consistent, it shows that it is
classically consistent with an archetypal liar sentence. This is a remarkable result, that a simple
truth denition such as the Strengthened T-Schema allows a classically consistent solution to at
least archetypal liar sentences.
The exibility of the Strengthened T-Schema can be further demonstrated by the following
derivation. Suppose we have a formal language which has a Truth predicate and an Evaluability
predicate, but the
predicate has not (yet) been dened.
sentence whether it is Evaluable or not.
a truth denition for the sentence
That is, we cannot say of any
Despite this, if we use the Strengthened T-Schema as
as dened above, the following holds in a standard natural
deduction system for classical logic:
Str. T-Schema
¬T r(b) ∧ Eval(b)
⊃E
∧Elim
T r(b) ∧ ¬T r(b)
∧Intro
Modus Tollens 1,2
¬¬T r(b) ∨ ¬Eval(b)
DeM 3
T r(b) ∨ ¬Eval(b)
Double Negation 4
Disj. Syll 2,5
That is, even if
¬T rpbq
¬Evalpbq.
Intro 2-5
is not dened, it follows from the Strengthened T-Schema that both
The denition of
over, so long as the denition of
is not crucial for deriving this conclusion. More-
is such that it agrees with the Strengthened T-Schema on
which sentences are not Evaluable, we can expect that the resulting system would be consistent.
This means that within a particular language, if every sentence which is paradoxical upon the introduction of the T-Schema is not evaluable, then the introduction of the Strengthened T-Schema
will ensure that the system remains consistent. This in turn provides a very simple criterion on the
denition of a satisfactory evaluability predicate, namely that every normally paradoxical sentence
is not evaluable.
That is, if we dene truth using the Strengthened T-Schema, and all of the
`normally paradoxical' sentences are not evaluable, the denition will be consistent.
These results demonstrate that the conceptual conclusion that there needs to be two conditions
on the truth of a sentence is very promising from a formal sense, and promises the possibility of a
much improved formal truth denition. In particular, the Strengthened T-Schema can potentially
provide a classical, consistent denition of a truth predicate within its own language. The eectiveness of such a denition depends, however, on a satisfactory denition and explanation of the
concept of Evaluability that is being applied.
4.2 Evaluability
The previous section has shown that introducing the concept of Evaluability as a second condition
on the Truth of sentences is a promising formal approach to the Liar Paradox.
Moreover this
concept was introduced from a conceptual analysis of the concept of Truth and Truth Predicates,
not merely as a formal device.
This concept could therefore play the key role in a satisfactory
formal truth denition. Before this is attempted, however, the exact concept needs to be claried,
particularly as the concept of a sentence not being evaluable has been introduced more clearly
than the concept of Evaluability.
The basic reason for introducing this concept is clear. It was observed that the process for evaluating the truth value of sentences which contain a truth predicate is dierent to other declarative
sentences. The process of evaluating the truth value of a sentence which contains a truth predicate
requires us to rst evaluate the truth value of the sentence which is being referred to in the scope
of the truth predicate, and then evaluate the original sentence. As pointed out before, there are
sentences for which this process fails to complete. Given that the standard process for evaluating
the truth value of such sentences fails, it follows, in some sense at least, that these sentences are
not evaluable.
However it is not clear how to precisely understand what it means for a sentence to not be
evaluable. For, as argued above, we can say something denite about the truth value of sentences
which are not evaluable, they are denitely not true.
But presumably that would make them
evaluable because we can now evaluate their truth value.
There are two dierent concepts of evaluability at play here, and they need to be carefully
dierentiated. In the rst case, we were discussing evaluability in the sense of whether the standard
process for evaluating the truth value of the sentence succeeds. By standard process, we mean the
process for evaluating the truth value of a sentence as determined by its truth conditions.
the case of sentences which contain a truth predicate, their truth conditions depend on the truth
value of another sentence or other sentences. The standard process for evaluating these sentences
therefore involves rst determining the truth value of the other sentence(s), which is not possible
in certain cases. These sentences are not evaluable in the sense that their truth conditions are not
evaluable. Thus the rst sense of evaluability is evaluability of a sentence's truth conditions.
In the second case, the concept of evaluability at play is the question whether it is possible to
work out the truth value of a sentence at all, whether or not the truth conditions for that sentence
are evaluable. Thus, for sentences such as Liar Sentences, their truth conditions are not evaluable,
yet this means that they must be not true, and so they are evaluable in this second, more general
sense.
Every meaningful sentence is evaluable in this second sense. For if a sentence is meaningful,
then it has determinate truth conditions. If it has determinate truth conditions, then it must be
the case that either the truth conditions can be evaluated or they can not. If they can be evaluated,
then the sentence has a truth value, and the sentence is evaluable in the second sense. If the truth
conditions cannot be evaluated, then the sentence is not true, and the sentence is again evaluable
in the second sense.
As far as the Strengthened T-Schema, and any formal denition of truth, is concerned, it is
the rst sense of evaluability that is relevant. So to be more precise, the Strengthened T-Schema
should read: `p' is true i (p and the truth conditions of `p' are evaluable).
Since the ordinary
declarative sentences that we are mostly concerned about are assertions about facts, it is not
possible that the truth conditions of these sentences can be not (at least in principle) evaluable.
The only sort of sentences we have found which may have truth conditions that are not evaluable
are sentences which contain a truth predicate.
While it may be possible in other contexts that
there are other types of sentences whose truth conditions are not evaluable, the focus of this project
is on the Liar Paradox and we will therefore only consider its application to the truth predicate
and ordinary declarative sentences. We therefore need to identify the conditions under which the
truth conditions of a sentence of these types are or are not evaluable. Since this is the only concept
of evaluability that is relevant for the following discussion, unless it is made clear otherwise, the
concept of evaluability being applied below is equivalent to the truth conditions being evaluable.
4.2.1 Groundedness
As discussed so far, the concept of Evaluability is very similar to the concept of Groundedness, as
identied by Herzberger
3 and Kripke4 . This is the concept that sentences which contain a truth
predicate are grounded if they eventually refer only to sentences which do not include a truth
predicate, that is to sentences expressing facts. The motivation behind this is that it is the facts
that determine the truth values of sentences, and therefore sentences which refer (eventually) to
a matter of fact, it may not be possible for humans to evaluate them.
evaluable.
Herzberger, Paradoxes of Grounding in Semantics, see n. 14
Kripke, see n. 9
However, they are in principle
facts must have a truth value. That is, to put it in terms of Evaluability, the truth conditions for
sentences which refer (eventually) to facts are evaluable, and therefore these sentences must have
a truth value. Grounded sentences are therefore evaluable sentences.
Herzberger and Kripke, however, go further and claim that grounded sentences are the only
sentences that can have a truth value. This means, in terms of evaluability, that sentences which
refer (eventually) to facts are the only ones whose truth conditions are evaluable. This is a significant claim that does not follow from the fact that grounded sentences are evaluable, and is not
justiable.
For a sentence a is true, its truth conditions require us to rst determine the truth value of
in order to determine the truth value of a is true. If the only way that
can have a truth value
is that it eventually refers only to facts, then the Herzberger/Kripke position on Groundedness
is justied. However, we identied a sentence above which does not refer to any facts but which
was identied as not true - the Liar Sentence. Given that the Liar Sentence is not true, the truth
conditions of the sentence The Liar Sentence is not true are evaluable. The Liar Sentence has a
truth value and therefore we can work out the truth value of The Liar Sentence is not true. It is
in fact true. This means that there are evaluable sentences which are plausibly not grounded.
This occurs since, although the Liar Sentence's truth conditions are not evaluable, the sentence
itself is evaluable in the broader sense of evaluability outlined above, and therefore has a truth
value. This means in turn that the truth conditions of the sentence about the Liar Sentence are
evaluable, and therefore this sentence about the Liar Sentence is evaluable even though it is not
grounded. Grounded sentences are therefore a subset of Evaluable sentences.
4.2.2 When is a sentence not evaluable?
Given the role of the Evaluability predicate in the Strengthened T-Schema, the most crucial part
of dening the Evaluability predicate is ensuring that it identies the correct sentences as not
evaluable. This means that we need to be able to identify more precisely the sentences whose truth
conditions are not evaluable.
As noted above, for this project we are assuming that sentences
which contain a truth predicate are the only sentences which can be not evaluable in this sense.
The core feature of the truth conditions of sentences which contain a truth predicate is that
they generate a two step process. For a sentence of the form a is true, we must rst evaluate the
truth value of
and then we can evaluate the truth value of a is true. This means that we must
run a truth value evaluation process for
Running the process for
are evaluable and
rst, and then separately run the process for a is true.
gives us three possible types of conclusions: i) the truth conditions of
has truth value
Z;
ii) the truth conditions of
is not true; or iii) cannot tell if the truth conditions of
are not evaluable and therefore
are evaluable and have to run the
process for some other sentence(s). In the rst two cases, we have some determinate information
about the truth of
and therefore the process for a is true will succeed. The truth conditions of
in these two cases are therefore evaluable.
In the third case, upon repeating the process possibly multiple time we will normally eventually
only reach sentences that fall into either i) or ii), and therefore this information will transmit back
up the line and a is true will be evaluable. However, it is possible that it will turn out that a is
true is not evaluable. We will look at the situations in which a sentence will turn out to be not
evaluable next.
A sentence is not evaluable if its truth value evaluation process cannot successfully complete.
Given that among the sentences we are considering, this can only occur for sentences which include
the truth predicate, we can focus on these sentences. The evaluation process for these sentences
involves progressing to the sentence(s) referred to and evaluating the truth value of these sentences.
As argued above, when we move to these other sentences, we will either get a denite truth value of
some sort, or we will be required to move to further sentences. This pattern of a sentence referring
to other sentence(s) which refers to further sentence(s) sets up what we will refer to as a chain of
A chain of reference will halt if a sentence in the chain does not refer to any further sentences,
that is if it is a sentence which does not include a truth predicate. This means that the chain of
reference for a particular sentence will be nite if it eventually refers to only sentences which do
not include truth predicates, that is to sentences about facts. That is, if the chain of reference for
a particular sentence is nite, then that sentence is grounded.
Now if the chain of reference for a sentence is nite, the evaluation process will succeed immediately for those sentences on the end of the chain, and will therefore all of the sentences in
the chain will be evaluable. Sentences that are not evaluable must therefore have, at a minimum,
innite chains of reference.
In an innite chain of reference, there must be either a nite number of sentences innitely
repeated, or an innite number of sentences.
If there is an innite number of sentences, the
situation must obviously include something like the following:
In this case, none of the sentences can be evaluable, since the evaluation process can obviously
never halt. There is no way to determine a truth value for these sentences by following the chain
of reference, as the truth conditions on each sentence require us to do.
If there are a nite number of sentences innitely repeated, then there must be a loop in the
chain and the chain continually repeats each sentence in the loop. The reason for this is that for
any repeated sentence, that sentence must refer to the same sentence(s) every time it appears in
the chain. Therefore the section of chain after a repeated sentence must be identical after every
time it repeats. It follows that there must be xed loops in the chain that keep reoccurring. The
following is an example:
is not true.
The chain of reference in this case will follow the pattern:
T1 , T2 , T3 , T1 , T2 , T3 , ...
indenitely.
Obviously in this case, none of these sentences can be evaluable as there is no way of following
the chain of reference and getting outside of the loop. Thus sentences which belong to a loop of
reference like this are also not evaluable. Liar Sentences belong to this category.
There is, however, another possibility (aside from combinations of these two) which will generate
an innite chain of reference. For example, we can take the previous example, and add a sentence
which is not part of the loop, but refers to one of the sentences in the loop:
The chain of reference which starts at
will be innite, but in this case, it does not mean that
is not evaluable. If we take the evaluation process for
T2 .
When we run the evaluation process for
of reference, and therefore
evaluation process for
we must determine the truth value
we can see that
belongs to a circular loop
is not evaluable and hence not true. This however means that the
can succeed, and therefore that
is evaluable and in fact true.
We have therefore identied two situations in which a sentence is not evaluable, and each of
these depend purely on the chain of reference of the sentence in question.
The rst is that the
chain of reference generates a loop which the original sentence is a part of, and the second is
that the chain of reference is an innite chain which has innitely many unique sentences in it.
There is a major computational dierence between these two situations which is relevant to what
follows. In the rst case, one needs to only go through a nite number of steps until one reaches the
original sentence in the chain. In the second case, there is an innite chain and the type of nite
computation available for the rst step is not possible. One needs to be able to reason through
quantication and/or induction to be able to conclude that this case holds. These dierence aect
how evaluability can be dened in dierent formal languages.
4.3 Application to Formal Languages
We have developed a conceptually justiable approach to the denition of Truth in the Strengthened T-Schema which depends on the concept of Evaluability which has been claried in the
previous section. The challenge now is to apply these to our understanding of the semantics of
formal languages, and to dene them formally in a way that captures the concepts as set out above.
Fortunately there are no major obstacles to achieving this.
The key observation in achieving this is that the distinction between evaluable and not evaluable
sentences depends, for the types of sentences we are interested in, purely on the chains of reference
of sentences. That is, if we know what sentences refer to which other sentences, it will be possible to
compute Evaluability. This information about reference is, at least in formal languages, normally
dened as part of the language syntax, and there are therefore no conceptual barriers to representing
this information in the relevant syntactic language.
The Strengthened T-Schema, moreover, is
easily denable as an axiom in a formal language, if Evaluability is denable in the language. So
there are no large technical barriers to achieving this type of denition, and more details will be
discussed in the next section.
There is, however, the remaining conceptual problem of how to understand the semantics of
formal languages which include a truth predicate.
The rst point is to note that, as argued
above, the truth conditions of sentences which include a truth predicate are dierent to ordinary
declarative sentences.
The truth conditions of sentences which include a truth predicate relate
to whether the truth conditions of another sentence are satised, not to whether some state of
aairs holds. It is therefore reasonable to expect that the formal semantics for these sentences are
dierent to ordinary sentences.
Moreover, for all sentences which do not include a truth predicate, we have not identied any
good reasons for abandoning the standard model theoretic semantics, so it should be kept for these
sentences. For sentences which include a truth value, on the other hand, we have shown that their
semantic value depends on both the semantic value of the sentence(s) referred to and whether or
not the sentence is evaluable. Since evaluability is a property that depends on reference which is
dened within the language in formal languages, the truth value of the sentences which include a
truth predicate cannot be determined by a model in the standard sense. Normally, models do no
contain any information about what sentences in the language refer to other sentences
This leaves two options, the model must be extended to include information about sentences in
the language and what sentences refer to what other sentences, or sentences which include a truth
predicate must remain uninterpreted and therefore are not measurable against the model. Thus,
on the rst option, facts about what sentences refer to what other sentences must be included in
the model, and therefore the evaluability of sentences is testable against the model. On the second
option, the facts about what sentences to refer to other sentences is purely encoded in the syntax
of the language and therefore evaluability and truth (as dened by the Strengthened T-Schema)
are properties that are not interpretable in the model as the model does not contain the relevant
Both approaches are justiable, and the choice does not impact the denition of the syntax.
Moreover, on both options the truth and evaluability predicates are not interpretable in the standard model for a language. On the rst option, these predicates are interpretable within a model
which comprises the standard model plus extra information about the reference structure of the
On the second option, these predicates are simply not interpretable.
As noted, the
choice between these does not impact on the denition within the language, or the properties of
the denition within the language. Therefore no denitive choice will be presented here. The key
point, on both options, is that the truth and evaluability predicates are not interpretable on the
standard models for formal languages.
4.3.1 Dening an Evaluability Predicate
The analysis of Evaluability above identied two conditions under which a sentence is not evaluable,
either the sentence is part of a circular reference loop, or part of an innite reference chain that
never loops.
While both of these conceptually lead to a sentence not being evaluable, only one
of these is involved in the Liar Paradox - the circular reference loop.
So long as we do not
have quantication over innite sets of sentences, as in Yablo's Paradox, the case of an innite
reference chain is not paradoxical. We will therefore focus on dening an Evaluability Predicate
that deals with circular reference loops, and will leave the innite case to one side.
the major technical advantage of allowing a truth denition that does not require quantication,
which broadens the applicability of the denition. The innite case can obviously be included in
languages which include quantication and its basic denition will be discussed here.
The primary focus of the denition of Evaluability is whether a sentence is part of a circular
reference loop. In order to use this concept for a formal truth denition, we need to be able to
test whether this is the case within the relevant system of reasoning. In existing formal logics, it is
not possible to test this, since their denition does not allow both the necessary information to be
expressed and reasoning about this information to occur within the language. As a historical note,
this fact that existing formal languages are not capable of dealing with this type of reasoning is a
little curious. It is widely acknowledged that circular (or self ) reference is one of the causes of the
Liar Paradox, yet there has been little eort put into being able to represent this information as
part of a formal denition of truth. Nevertheless, the necessary concept of reference is intuitively
simple and it is not dicult to provide a precise formal denition.
4.3.1.1 Direct Reference
To illustrate this, we will use an arbitrary language
which includes sentences
a standard notation to identify names of those sentences within
pq.
A, B, C....,
These brackets can be
read either as a name-forming device, in the way that quotation marks are used in English, or as
a meta-theoretic notation which stands for whatever the actual name of the relevant sentence in
the object language is. In order to develop a denition of reference, we suppose that the language
includes a number of predicates which take the names of sentences as their arguments, say
P pq, Qpq, Rpq....
exactly when
We can then say that a sentence,
directly refers to another sentence,
A includes at least one of the predicates P pq, Qpq, Rpq... as a part and an argument
of that predicate is the name
at least one of
P pBq, QpBq
pBq.
RpBqetc
directly refers to
as a part of the sentence
exactly when one of
A includes
To give a short example to illustrate this denition: Say we interpret
Rpq
as meaning that is an atomic sentence, then the sentence B is not an atomic sentence and it is snowing directly
while the sentence It is snowing and the grass is frozen does not. This matches what
we mean by reference, and importantly for formal denitions, can be determined from the syntax
A number of useful facts follow from this denition. Firstly, there will be many sentences that
do not directly refer to any sentence, at a minimum all of the sentences which do not include one
of the relevant predicates.
As in our example, It is snowing and the grass is frozen does not
directly refer to any sentence, as no sentence is mentioned within the sentence. Secondly, even if
the sentence includes a relevant predicate, direct reference is not generally reexive or transitive.
So for example, if
an atomic sentence,
is the sentence B is not an atomic sentence and
is the sentence C is
does not directly
Thirdly, a sentence may directly refer to many other sentences, and if the language
allows predication over names, then it may directly refer to an innite number of other sentences.
Fourthly, a standard Liar Sentence such as
can be seen since
T rpq
¬T rpBq with the name B
directly refers to itself. This
is a predicate which takes names of sentences as arguments, and
T rpBq
The concept of direct reference picks out all of the other sentences that a given sentence mentions
directly in its syntax. We need however a broader denition of Reference to be able to deal with the
Liar Paradox in general, since it often arises by means of a sentence referring to another sentence
which may (directly) refer back to the rst sentence.
4.3.1.2 N-Reference
One can use the denition of direct reference to inductively dene an indexed notion of reference
to identify the cases where a sentence refers to a sentence which itself refers to another sentence.
We will say that a sentence,
Further, a sentence
1-refers to another sentence,
n-refers to a sentence
1-refers to
if there is a sentence
Thus, in the example above,
(n-1)-refers to
That this denition corresponds to our intuitive concept of Reference can be seen in the fact
that it provides a neat denition of Groundedness. A sentence
does not n-refer to any sentences.
is grounded, if there is some n
That is, whatever sentences
in turn directly refer to other sentences which eventually refer to sentences which do not refer to
any other sentences. In other words, there is some m such that the sentence
m-refers only to
sentences which do not refer to other sentences, i.e. statements of direct fact. This corresponds to
the denition of Groundedness.
For the purposes of this project, this concept of Reference allows a very natural denition of a
circular reference loop, namely: A sentence
n such that
n-refers to
belongs to a circular reference loop i there is some
This denition is very natural, and allows us to prove a number of
useful results.
From these denitions, we can show a couple of easy but important lemmas:
Lemma 1.
m-refers to
b n-refers to c,
Proof. If
then there is some sentence
d1 ,
each sentence
(m+1)-refers to
d1 .
(m+n)-refers to
b (n-1)-refers to dn−1
dn−1
d1 , d2 ,
.....,
di+1 ,
it follows by the denition of n-reference that
By iteratively repeating this process, it is necessarily the case that
and therefore that
(m+n-1)-refers to
a belongs to a circular reference loop,
dn−1 .
However, since
(m+i)-
there are innitely many
n such that a n-refers
belongs to a circular reference loop, then by denition
However, that means that
dn-1
follows that
This process can be iteratively repeated to nd a series of sentence
Now, since
(n+n)-refers to
2n-refers to
since by 1, if
By mathematical induction it follows that
for some n.
(kn)-refers to
for all natural
It is not possible that a sentence is grounded, and that it belongs to a circular reference
loop.
Proof. If as sentence
Thus for any
belongs to a circular reference loop, there is some m such that
follows by Lemma1 that
natural number j,
k-refers to, for k<m, since
(m+k)-refers to
(jm+k)-refers to
m-refers
also k-refers to
By repeating this process it follows that for every
This means, in particular, that there can be no number i,
does not i-refer to any sentence.
If, on the other hand, a sentence
is grounded, there is some n such that
any sentences. This obviously contradicts the consequences of
loop. Hence if
belongs to a circular reference loop,
does not n-refer to
belonging to a circular reference
cannot be grounded, and vice versa.
However, it is possible to have ungrounded sentences which do not belong to a circular reference
loop. A good example is the following two sentences:
d,
n-refers to itself for all n. Therefore
refers to itself, and hence
¬T rpdq
T rpdq. c
for all n. It, however, never
does not belongs to a circular reference loop. This fact is important,
since it allows us to distinguish potentially paradoxical sentences, such as the Truth Teller sentence
from sentences about them, and hence give them dierent semantic properties.
4.3.1.3 Reference Sets
While the denition of a circular reference loop given above allows us to easily prove things metatheoretically, it has a particular formal limitation: it is an existence statement that can be technically
hard to falsify. In order to prove that a sentence is not part of a circular reference loop, we have to
show that there is no n such that
which may require checking an innite number
For this reason, we will introduce 2 further denitions, the n-reference set and the complete
reference set. The n-reference set of a sentence
is the set of all sentences that
all m less than or equal to n. Thus, the 2-reference set of
m-refers to for
a is the set of all sentences that a 1-refers
to and 2-refers to. This set will be empty in many cases. It also has the property that in most
cases it stabilises. That is, after some n, all of the n-reference sets for larger n are identical. When
this occurs, we will label this stabilised set the complete reference set.
Using these concepts, we can oer an equivalent denition of a circular reference loop, namely
that a sentence
n-reference set of
belongs to a circular reference loop i there is some n such that
This means, moreover, that if
belongs to the
has a complete reference set, then
belongs
to that set. However, these concepts also allow us to more easily compute when a sentence does
not belong to a circular reference loop. If there is a complete reference set for a sentence
5 If
we allow innite sets, this will always exist.
does not belong to the completed reference set of
for any n. Therefore,
cannot belong to the n-reference set of
does not belong to a circular reference loop.
The concept of a Complete Reference Set also enables us to provide a precise denition of the
other case considered above, where there is an innite non-repeating reference chain. If there is an
innite reference chain that does not loop, then there can be no nite n such that the n-reference
sets stabilise. Each stage in the innite reference chain will introduce a new sentence to the nreference set. Moreover, if there is no innite reference chain, then there must be a nite n, such
that no new sentence is referred to, and therefore the n-reference sets stabilise. Thus a sentence
is part of an innite non-repeating reference chain, if, and only if, there is no nite Complete
Reference Set for that sentence.
4.3.1.4 Evaluability
Assuming that the vocabulary of a formal language is increased somewhat, there is no information
within the denition of a sentence being part of a circular reference loop or part of an innite
reference chain that is not representable within a language. The most radical change required is
that some basic set notation and axioms are required, however, we only need some basic axioms.
Assuming we have a language in which we can represent the required concepts, it is now possible
to give a simple denition of Evaluability that covers both possibilities dened above:
is evaluable i
has a nite Complete Reference Set and
is not a
member of its Complete Reference Set.
Thus we have a denition of evaluability that depends purely on features that are denable
within the syntax, and which is possible to compute in a language which contains the relevant
concepts. However, testing for whether a sentence has a nite Complete Reference Set involves a
number of concepts that cannot be dened in the more simple systems of logic.
We will therefore introduce a more restricted notion of nite evaluability. The idea of this concept is that a sentence is nitely evaluable if we can determine that it is evaluable in nite number
of computational steps and without quantication or induction. Similarly, a sentence is nitely not
evaluable, if we can determine that it is not evaluable in a nite number of computational steps
and without quantication or induction.
The concept of nite evaluability is co-extensive with the concept of evaluability.
Proof. Assume a sentence
is evaluable if, and only if, it is nitely evaluable.
a is evaluable.
Therefore, by denition, it has a nite Complete Reference
Set. As its Complete Reference Set is nite, it is possible to calculate this set in a nite number
of steps.
We need only calculate the n-reference set for the n where the reference sets cease to
grow. Once we have calculate the Complete Reference Set, we need only one more step of checking
is a member of the Complete Reference Set, which by denition, it cannot be. We can
therefore compute that
Similarly if
is nitely evaluable.
is nitely evaluable, then we must be able to compute that
is not a member of
its Complete Reference Set in a nite number of steps. This implies that the Complete Reference
Set is nite. Thus
The sentences which are nitely not evaluable are, however, not co-extensive with the not
evaluable sentences. Obviously, there will be sentences which do not have nite Complete Reference
Sets, and these cannot be computed nitely. However, it is possible that a sentence does not have
a nite Complete Reference Set, yet is nitely not evaluable. The reason follows from the following
Lemma:
If a sentence
Proof. Assume that
is a member of one of its n-Reference Sets, then
is a member of its k-Reference Set, for some xed k.
By the denition
of n-reference set, the k-reference set is a subset of every (k+i)-reference set, for any i>0. This
means, in particular, that if
has a Complete Reference Set,
Reference Set and therefore
must be a member of its Complete
does not have a Complete Reference Set, it cannot have a nite Complete Reference Set,
and therefore again it is not evaluable.
However, it is always possible to compute nitely whether a sentence is a member of a nreference set. So if a sentence is a member of an n-reference set, then it is nitely not evaluable.
On the other hand, if a sentence if nitely not evaluable, then it must have a nite Complete
Reference set, and be a member of that Complete Reference set. That, however, means that there
is some j, such that the sentence is a member of its j-reference set.
We can therefore oer the following denitions:
is nitely evaluable i
not a member of its Complete Reference Set.
If we are in a language which only allows nite sets, we alter the denition to remove the rst
clause on the right hand side. This however, removes the equivalence:
is nitely evaluable, if
is not a member of its Complete Reference Set.
We can, moreover, oer a denition of nitely not evaluable:
a is nitely
not evaluable, if
a is a member of its k-Reference Set, for some
nite k.
Both of these denitions are formally denable in a language with nite sets but without
any quantication or induction.
Thus the concepts of evaluability and nite evaluability are in
principle denable in a very wide range of logics. This suggests that, if the Strengthened T-Schema
in combination with these denitions functions as outlined above, these will provide a formal truth
denition for the same wide range of logics.
4.3.2 Are these denitions adequate?
There are a number of dierent criteria against we should evaluate the potential adequacy of these
denitions. Firstly, and most importantly for the project, is whether they are sucient to block
Secondly, we need to check that other sentences, outside of the paradoxical
sentences are also given the correct truth value. Given that we are only oering a template for a
new formal truth denition, and not an actual formal denition in this chapter, these discussions
can only be indicative.
These denitions of evaluability and nite evaluability can only block the Liar Paradox, in
combination with the Strengthened T-Schema, if all possible paradoxical sentences are come out
as not evaluable. A core feature of the Strengthened T-Schema is that the standard T-Schema holds
for evaluable sentences. If any paradoxical sentences are evaluable, then the paradox will arise in
the normal way. It was, however, argued in the rst chapter that one of the dening features of the
Liar Paradox is that it can only be generated when the paradoxical sentences belong to a circular
reference loop. The denitions of Evaluability and Finite Evaluability are dened, however, so that
every sentence which belongs to a circular reference loop is not evaluable.
Hence, the standard
mechanism by which the Paradox bites is prevented on these denitions.
This does not prove
consistency, that cannot be done without a more explicit denition, but it demonstrates that the
standard paradoxical argument form will not hold if we adopt these denitions.
Every evaluable sentence will, on these denitions, get the same truth value as it would under
a dierent truth denition, since the normal T-Schema holds for all of these sentences. However,
there are some esoteric sentences which are identied as not evaluable on these denitions although
intuitively they seem true. One example is:
(a ⊃ a) ∨ ¬T rpkq
This sentence would normally be true as one disjunct is a tautology. However, on this denition it
is not evaluable as it is part of a circular reference loop. It will therefore come out as not true on
the Strengthened T-Schema. The denition of evaluability outlined above is not sensitive to the
internal syntax of sentences, whereas our intuitions here with regards to truth are that the truth
of one disjunct overrides the non-evaluability of the other disjunct.
This problem could certainly be resolved by a more ne-grained denition of evaluability. This
will not be undertaken here however for a couple of reasons.
Firstly, it is not an easy project
and a satisfactory denition of evaluability of this form will require more sophisticated concepts
that we want to allow for our approach.
the logic involved.
The exact denition will also be highly dependent on
In classical logic, for example, the interdenability of the connectives needs
to be adequately dealt with but other logics do not have this.
Secondly, it makes consistency
much harder to justify and prove, as one needs to be very careful that no paradoxical sentences
with a non-standard sentence structure are counted as evaluable. Thirdly, the ultimate aim of this
denition is to show that the Strengthened T-Schema and a plausible concept of evaluability will
provide a consistent truth denition. There is undoubtedly room for a more ne grained denition
of evaluability, if it can be found.
In any case, we will see in future chapters that the situation for sentences as these is not as
bad as it appears here. The results of Chapter 6, in fact, will suggest an alternative way to deal
with these sentences which unies their treatment with paradoxical sentences. We will come back
to sentences such as these in future chapters.
On the basis of the two simple observations, a new approach to the formal denition of truth
has been developed in this chapter. This denition of truth, if implementable, potentially oers a
classically consistent solution to the Liar Paradox without any restrictions on the sentences which
can be assigned truth values.
The two observations were that i) the truth of a sentence is determined against something
external to that sentence, and ii) the truth of sentences which contain a truth predicate are ordinarily assigned a truth value through a two step process. Following the second observation through
means that there must be sentences which cannot be evaluated using this ordinary process since
the process can never complete. Liar Sentences are one type of example of these sentences. This,
however, imposes a second condition on a sentence being true to the ordinary one expressed in the
standard T-Schema, namely the question of whether it is evaluable through the ordinary means.
If it is not evaluable through these means, then it cannot be true.
This in turn gives rise to a new principle governing the denition of a truth predicate, the
This denition makes use of an Evaluability predicate, and a couple of closely related ways to
dene this predicate were motivated. The dierent denitions are appropriate to systems of logic
with dierent internal powers of expressibility. The exact denition to be adopted will naturally depend on the language involved, and as it involves explicit reasoning about the patterns of reference
dened in the language, this will normally required extra vocabulary in the formal language. Examples will be given in the next two chapters of how this can be achieved for a Classical Sentential
Logic.
Most usefully, the analysis in this chapter also provides a test by which we can judge whether
a denition of Evaluability will satisfactorily deal with the Liar Paradox. The key feature of the
Strengthened T-Schema is that if a sentence is not evaluable, then a Paradox cannot be derived
in any of the normal ways as the Standard T-Schema does not apply.
This will be used in the
following chapters as the basis of a consistency proof, so it is useful to state this more precisely:
An evaluability predicate
any sentence
a such that T rpaq ≡ a ` ⊥ (i.e.
` ¬Evalpaq
(i.e.
in some language
is satisfactory if, and only if, for
a contradiction follows from the ordinary T-Schema),
is not evaluable).
While this will ultimately guarantee that the denition is consistent, the key motivation in
this project is to achieve as great a level of Semantic Closure as possible. The Strengthened TSchema promises to provide a very high level of semantic closure as there are no restrictions on
the sentences it applies to. However, the key test for semantic closure is whether the truth value
of every sentence within the language is truthfully reportable within the language. That is, for
every sentence, if we can metatheoretically identify its truth value from the truth denition in the
language then the statement that asserts that that sentence has the relevant truth value is true in
That is, a language
from the denition, then the sentence
(or
) is provable within
of this investigation, the most important test is whether for a Liar Sentence
the correct one of
T rplq
¬T rplq
should be provable in
is true (or not)
In the context
¬T rplq (with name l),
The Strengthened T-Schema does
not guarantee that this level of semantic closure will occur, as will be shown in the next chapter.
The method of dening truth motivated in this chapter will be used to provide formal truth
denitions for Sentential Classical Logic in the next two chapters.
First Formal Denition
In the previous chapter a new approach to the formal denition of a truth predicate was developed
on the basis of the observation that the truth of a sentence depends on something other than that
sentence, and a careful analysis of the properties of a truth predicate. This new approach holds
signicant formal promise as it was shown that a classically consistent denition of truth in its own
language, which does not restrict the application of the truth predicate to any sentences, ought
to be possible. The aim of this chapter is to prove that such a denition is in fact possible, by
providing a consistent formal truth denition for Classical Sentential Logic. Naturally, if such a
denition is possible for Classical Sentential Logic, it is possible for any classical logic and arguably
any logic whatsoever, since no features of the denition depend crucially on Classical Logic.
The approach outlined in the previous chapter depends on dening truth with the Strengthened
T-Schema, which requires the denition of an Evaluability predicate. Evaluability, in turn, depends
on the pattern of reference set up by the naming conventions, or more generally, what refers to
what within the scope of relevant predicates.
These concepts of evaluability and reference are
not denable within ordinary Classical Sentential Logic, so it will be necessary to dene a new
language which adds vocabulary and denitions to deal with these concepts. Given the role that
the concept of reference plays in the Liar Paradox, it is highly plausible that a good denition of
truth must deal with this concept in its denition. Thus there is nothing counter-intuitive about
adding extra vocabulary to deal with the extra concepts required.
5.1.1 Conditions on an ideal Truth Denition
Importantly, if this extended language is to provide a denition of truth for the original language,
it must not change the character of the original language. So, for example, none of the denitions
which govern the logical connectives can be changed or added to; and no sentences should have
FIRST FORMAL DEFINITION
a truth value in the extended language when they have a dierent truth value in the original
language. We will use the term of an adequate denition to identify truth denitions in extended
languages which satisfy these properties, and we can dene this concept more precisely. Suppose we
have our original formal language
and its formal truth denition is dened within the extended
Then we have the following denition:
A language
which extends a language
if the following three conditions hold for any sentence
L ` P,
LT ` P .
LT ` T rpP q.
is as consistent as
(where
provides an adequate truth denition for
expressible in
is the truth predicate)
L.1
That is, the denition is adequate, if every theorem in the original language is also a theorem
in the extended language, every theorem in the original language is true in the new language,
and the extended language does not introduce any inconsistencies that were not in the original
language. If these conditions hold, then we are justied in saying that the predicate
an adequate truth denition for
T rpq provides
Adequacy, as dened here, is a minimal, very weak set of
criteria and only demonstrates that a proposed denition can reasonably be a truth denition.
It does not guarantee that a truth denition is good or philosophically satisfying. Many existing
truth-denitions satisfy these criteria. For example, if we interpret a Tarskian meta-language as
an extension of the original language, then the Tarskian Meta-language will satisfy the denition
of an adequate truth denition.
However, one ultimate philosophical aim in this project is not to construct truth denitions
for one language within another, but to construct a truth denition which is adequate for its own
language. We can almost use the same denition, and replace
L with LT
(or vice versa). However,
doing that does not quite work, since both the rst and third conditions become tautological.
While the rst condition is not necessary, we need to alter the third condition slightly. The point
of this condition is that the truth denition does not introduce a contradiction into the language.
This means that we need to compare the consistency of the whole language with the consistency
of the T-free part. That is, all of the language that does not include the truth predicate. In that
case we get the following:
means that if
of inconsistencies as
2 Or
is too. But if
is paraconsistent, then
only contains the same type
at least, in the case of certain paraconsistent languages, a contradiction that was not already there.
provides an adequate truth denition for itself if the following two
conditions hold for any sentence
LT ` P ,
LT :
is as consistent as the T-free part of
The other large philosophical motivation for this project is the idea of semantic closure. That is in
the context of truth denitions, it is possible to assert the correct truth status of every sentence in
the language itself. Put more precisely, for a sentence
truth status for
(e.g.
T r(p))
p, another sentence which asserts the correct
is provable in the language itself. The ultimate philosophical aim
is to develop a formal language which is semantically closed in this sense and which includes an
adequate truth denition for itself.
While the previous chapter provided the template for a formal truth denition which has the
potential to both adequate and semantically closed, we cannot we cannot give a denition for
in general, either of an adequate extended language
language itself.
or of a denition in the
A truth denition based on the template from the previous chapter depends
necessarily of the properties of
In this chapter, however, we will only be supplying a truth
denition using Classical Sentential Logic as a base logic, which we will label as
SL. SL
contain sucient vocabulary to express the required concepts outlined in the previous chapter, so
we have to dene the truth denition in a extended language, which we will label
language will be dened to oer a truth denition for both
and itself.
SLT 1 .
That is, the truth
predicate will apply to sentences in its own language, and if adequate, the truth denition will be
classically consistent.
5.1.2 Evaluability in SLT 1
In terms of dening
SLT 1 ,
there is one main question that we need to answer with reference
to the blueprint oered in the previous chapter.
Two dierent formal concepts of evaluability
were oered, evaluability and nite evaluability.
The denition of the more general concept of
evaluability contains an implicit quantication which is not removable, and the denition
does not allow quantication, so we will use the concept of nite evaluability to dene a formal
Evaluability predicate in
That is, we will use the following denition:
evaluable, i
a is not a member of its Complete Reference Set.
It must be noted that this denition only works if our language is restricted so that it only
includes nite sets of names, which is reasonable for
SLT 1
since it does not include any quanti-
cation. We must, therefore, include only nite sets (of names) in our denition of
means that only nite Complete Reference Sets are possible in
Moreover, since we will be
using nitely evaluable for the denition of the formal Evaluability predicate in
we must
use the concept of nitely not evaluable for the negation of the formal concept. That is:
is nitely not evaluable, i
is a member of its k-Reference Set, for
some nite k.
The reason for this, as discussed previously, is that there are cases where we can decide in a
nite number of steps that a sentence is not evaluable because it belongs to a circular reference
chain, but that sentence does not have a nite Complete Reference Set.
A disadvantage of using the concepts of nitely evaluable and nitely not evaluable is that
these concepts do not exhaust the possibilities. There are possible sentences which do not have a
nite Complete Reference Set, so the denition of nitely evaluable cannot apply, and they are not
part of a circular reference loop. However, it is not possible to deal with this sentences without
either quantication or a concept of innity within the language, and these cannot be introduced
without fundamentally changing the logic. None of the sentences are paradoxical in the
cases that we are considering in this thesis.
With these concepts in place, we can provide a formal denition of a truth predicate for Classical
Sentential Logic.
To make the denition easier to follow, the denition and proofs will be only
be provided for the Logic without any substantive theories. The denitions and proofs work in
the same way if we introduce a consistent Theory into Classical Sentential Logic. For the sake of
clarity of presentation, however, we will only consider a truth denition for the bare logic.
5.2 Classical Sentential Logic with Truth
5.2.1 Basic Denitions
be a Classical Sentential Logic.
{¬, ∧, ∨, ⊃, ≡}
We let
P, Q, R, ....
stand for atomic propositions and
be the set of connectives dened in the normal way.
metatheoretic propositional variables.
natural deduction system.
We will use
is dened as a standard Fitch style
⊃I
P ∨Q
3 Choosing
¬P
3 It contains the following introduction rules:
¬I
A, B, C, ...
P ⊃Q
a Natural Deduction system to work in admittedly restricts the logical machinery available. However,
it has two signicant advantages here: the resulting system can be more easily used to understand natural languages;
and the aim is to show how a formal truth denition is possible in a very unlikely logical system.
And the following Elimination Rules:
¬E
/ MP
We will extend
SL to a language SLT 1
which includes a truth denition. This requires adding
certain new vocabulary, in particular some predicates (Eval(),
T r(), Refn ()
a system of reference that allows one to refer to sentences within the language
REF ())
For this
language we will dene a calculus of names to handle reasoning about the system of reference. This
will include a new class of individual constants (p, q, r, ...) which are names; nite sets of names,
{};
and rules governing these.
5.2.2 Grammatical Sentences
Firstly, we need to dene what a grammatical sentence in
the same atomic propositions in
SL,
is. We let
and the set of (propositional) connectives are
naturally dened by the same derivation rules. We will use the letters
as metatheoretic
variables for names.
• P
A basic proposition in
an atomic proposition;
• Eval(a)
for some name
• T r(a)
• Refi (a) = {b, c, d, ..., f }
(or replacing
• Refi (a) = Ref1 (b)
REF )
is any string which takes one of the following forms:
for a name
a nite number of names
b, c, d, ..., f
for some names
and a number
We can use basic propositions to dene the grammatical sentences of
will let
A, B, C, ....
function as metatheoretic sentential variables within
A grammatical sentence in
recursively. We
is either
a basic proposition;
• ¬A
for some grammatical sentence
• A ∨ B; A ∧ B; A ⊃ B;
A≡B
for some grammatical sentences
Now we assume that the atomic names
p, q, r, .... are assigned to sentences within SLT 1
in some
way. The method of assignment is not here important, only that it assigns at least every relevant
sentence a unique name. The condition of the uniqueness of the name is a stronger condition than
normal, but is necessary in this context. As argued in Chapter 3, the name of a sentence can have
an eect on its truth value and potentially have paradoxical consequences. This possibility will be
discussed in the next chapter. Hence in order to ensure that no problems arise in this chapter, we
will only allow sentences to have one name. As a matter of convention we will let the metatheoretic
name variable
be the name of the sentence
A; b
Furthermore, for notational convenience, especially in the denition of axiom schema, we will
adopt a further metatheoretic convention.
brackets are used as a notational variant for
names, where the name of the relevant sentence has not been specied. Every sentence in
has a dened name within the class
p, q, r, ...,
however we have not specied which name which
sentence has. Where we want to refer, metatheoretically to the name of, say
the notation
pP ∧ ¬Qq
P ∧ ¬Q,
as it refers to whichever member of the class of names belongs to that
sentence. Similarly, in the Axiom Schema, something like
of the relevant sentence of the form
¬A.
p¬Aq
In this context, the
will be used to identify the name
pq need not be read as a name-forming
device, but rather simply a a metatheoretic convention to pick out a name that is xed by the
denition of the language but that has not been identied meta-theoretically.
5.2.3 Name Calculus
The Name Calculus is a set of rules and axioms that allow us to reason about the concept of
Reference introduced in the previous chapter, and therefore to be able to dene Evaluability. The
basic elements of the Name Calculus are the class of names,
including the empty set
before,
a, b, c,...
a series of predicates
Refi (),
nite sets of names
and a further predicate
REF .
{a, b}
As noted
will be used as metatheoretic sentential variables. In order to reason about sets
of names, our system includes set union
∪,
inclusion in a set
∈,
and equality between sets
are dened in the standard way, and equal sets are always substitutable.
The series of predicates
is not a single predicate but an innite series of predicates, which
primarily take names as an argument and dene sets. We will however extend its denition to take
sets of names in an obvious way. The
predicates are dened recursively over the construction
of sentences as follows:
Ref1 (a) = {}
Ref1 (pT r(b)q) = {b}
Ref1 (p¬Aq) = Ref1 (pAq)
Ref1 (pA ∨ Bq) = Ref1 (pAq) ∪ Ref1 (pBq)
Ref1 (pA ∧ Bq) = Ref1 (pAq) ∪ Ref1 (pBq)
Ref1 (pA ⊃ Bq) = Ref1 (pAq) ∪ Ref1 (pBq)
Ref1 ({a, b, ..., c, d}) = Ref1 (a) ∪ Ref1 (b) ∪ ... ∪ Ref1 (c) ∪ Ref1 (d)
Refi+1 (a) = Refi (a) ∪ Ref1 (Refi (a))
predicates dene the concept of
Importantly, the
after some
if the sentence denoted by
set includes the
is a basic proposition but not of the form
n-Reference
Sets discussed in the previous chapter.
set, which means that in many cases the set will stabilise
This allows us to dene a Complete Reference Set (REF ()), where it is nite:
[Refi (a) = Refi+1 (a)] ⊃ [REF (a) = Refi (a)]
That is, once the set of sentences in an
set stabilise, no more new sentences can be
added. Therefore we have the complete set of sentences that the original sentence refers to. The
predicates can be used to dene introduction rules for
a ∈ Refi (a) ` ¬Eval(a)
¬(a ∈ REF (a)) ` Eval(a)
for some
predicate:
Thus, as per the denitions of nitely evaluable above, a sentence is not evaluable if it belongs
to its own
set for any
and is evaluable if it does not belong to it own complete
reference set. It should be remembered that
only allows nite sets, so there will be some
sentences which are not classied as either evaluable or not evaluable.
Since the
predicate identies the sentences that we want it to as evaluable and the others
as not evaluable, we can use it to dene the
equivalently
T r(a) ≡ A ∧ Eval(a)4
T rpAq ≡ A ∧ EvalpAq
predicate by the axiom schema:
Thus we have a denition which corresponds closely to the blueprint in Chapter 4. This should
mean that if sentences are evaluable, then the Tarskian T-Schema, that is
sentences are not evaluable, then it follows that
¬T r(a).
T r(a) ≡ A,
holds and if
This in turn, if the blueprint is correct,
will guarantee consistency. Naturally, the proofs are below.
An immediate criticism of this denition is that it appears circular.
terms of
Eval; Eval in terms of Refn ; but Refn depends on T r.
the denition. The denition of
says that the truth value of
denition of
in terms of
This does not, however, undermine
is a denition of the truth value of
depends on the truth values of
Refn
We have dened
provides the truth values for
Eval.
T r(a).
Eval(a).
Similarly, the
Refn (a)
not have a truth value, but is only a notational variant for a nite set of names. Furthermore, the
T r.
in terms of instances of the
predicate does not in any way depend on the
It only depends on the name/sentence which falls within the scope of the
predicate. From a formal point of view, this is a purely syntactic property. Thus the denition
of the truth value of
depends on which names fall within the scope of that predicate in the
relevant sentences, but that is what we should expect.
5.2.4 Basic Results
Before we move to an analysis of any paradoxes and the consistency of
some basic properties of the system.
rules of
Every theorem of
is a theorem of
we will rst note
These will be explored in the form of a series of Lemmas
The rst establishes one of the properties that need to be satised if
are valid in
is to be
SL.
is also a theorem of
then there is a proof of
Therefore there will be a proof of
However, all the derivation
The next Lemma shows that all sentences which do not include Truth Predicate are evaluable.
This shows that the denition follows from the intuitions established in the previous chapter. It
is only sentences with a Truth Predicate which have a dierent truth value evaluation procedure
which leads some sentences into paradox.
It is therefore only these sentences which should be
aected by the formal concept of evaluability.
Proof. Let
Every T-free sentence is evaluable.
be a T-free sentence. That is,
does not contain any instances of the
From the denition of grammatical sentences, this means that either
which is not of the form
Basic Proposition
Ref1 (pAq) = {}.
is a Basic Proposition
or it is composed of Basic Propositions of this form.
A, Ref1 (pP q) = {}.
predicate.
It follows from the recursive denition of
For any
From this fact, we can prove that
is evaluable:
Ref1 (pAq) = {}
Ref2 (pAq) = [Ref1 (pAq) ∪ Ref1 (Ref1 (pAq))]
Defn of
Ref2 (pAq) = [{} ∪ Ref1 ({})]
Substitute 1 into 2
Ref2 (pAq) = [{} ∪ {}]
Den of Ref()
Set Union
Ref2 (pAq) = Ref1 (pAq)
REF (pAq) = Ref1 (pAq)
REF (pAq) = {}
Substitution from 1 in 7
¬(pAq ∈ REF (pAq))
Eval(pAq)
Eval 2
Thus every T-free sentence
Refi+1
Axiom and MP on 6
The next Lemma simply shows that if a sentence is evaluable, then the ordinary T-Schema
holds, and all normal intuitions about truth are correct.
Eval(a) ` T r(a) ≡ A.
Proof.
Eval(a)
T r(a) ≡ (A ∧ Eval(a))
Strengthened T-Schema
A ∧ Eval(a)
⊃Elim
T r(a) ⊃ A
⊃Intro
A ⊃ T r(a)
T r(a) ≡ A
Intro 7,11
The next Lemma illustrates the power of the Strengthened T-Schema, for it establishes formally
the conclusion in the previous chapter that if a sentence is not evaluable, then it is not true.
¬Eval(a) ` ¬T r(a)
¬Eval(a)
Eval(a) ∧ ¬Eval(a)
¬(A ∧ Eval(a))
¬Intro
¬T r(a)
The following Theorem establishes that
every sentence that is a theorem in
Theorem 10.
For any sentence
Therefore, by Lemma 7,
indeed provides a truth denition for
is T-free, since there is no Truth Predicate within
is evaluable, i.e.
is true. Since
is a reasonable extension of
is a theorem, this means that
is true. Therefore, by the Strengthened T-Schema, so is
and the truth denition satises the rst two
conditions on it being an adequate truth denition for
The third condition of consistency
must wait until after the treatment of the relevant paradoxes. Since Classical Logic is consistent,
we must show that
is also consistent.
5.3 Paradoxes
Given the limited expressive resources of
and in particular the existence of only four predi-
cates, we have a limited range of Liar Sentences to consider. In particular, we do not need to worry
about Quinean Liar Sentences, since the concepts involved are not expressible in
will focus on the archetypal, self-referential Liar Sentence in
is a sentence,
Thus we
¬T r(l),
has name l.
diction in
¬T r(l)
(which has name l) does not immediately produce a contra-
Proof. The properties of the sentence
are determined by the Strengthened T-Schema,
since no other Axiom Schema or derivation rules are dened for the Truth Predicate.
Therefore the question of whether
whether the sentence:
is consistent in
T r(l) ≡ ¬T r(l) ∧ Eval(l)
is equivalent to the question of
Firstly, we can note that a sentence of the form
A ≡ ¬A ∧ B
is consistent in Sentential Logic.
If we consider it in terms of truth tables, if the truth values of
is true. Therefore if
fact that we can prove both
¬Eval(l)
T r(l)
Eval(l)
are both false, the
are both false, the sentence
is supported by the
are presumably both false.
¬T r(l):
T r(l) ≡ (¬T r(l) ∧ Eval(l))
¬T r(l) ∧ Eval(l)
T r(l) ∧ ¬T r(l)
We can also prove
from the Strengthened T-Schema using only the
This means that
Firstly, we prove that
is consistent with Sentential Logic.
The consistency of the sentence
¬Eval(l):5
See Previous Proof
¬(¬T r(l) ∧ Eval(l))
¬¬T r(l) ∨ ¬Eval(l)
T r(l) ∨ ¬Eval(l)
These proofs are simple applications of the rules of Sentential logic.
If Sentential Logic is
consistent, the model theoretic argument above means that no contradiction can be derived purely
from the Strengthened T-Schema (for the Liar Sentence in question) using the rules of
Furthermore, there are no other rules or axioms which govern the behaviour of the Truth
Predicate, and therefore the conclusion that
cannot be directly contradicted in
However, there are other rules governing the behaviour of the
If these rules
contradict the conclusion, then the Liar Sentence will be inconsistent, otherwise it should be
consistent. That means if
is a theorem, it is inconsistent, but if
is consistent.
We therefore compute the value of
Eval(l):
can be proved independently of the previous proof, but the proof is more involved.
is a theorem it
Ref1 (l) = Ref1 (pT r(l)q)
Defn of Ref (n.3)
Ref1 (pT r(l)q) = {l}
Ref1 (l) = {l}
Substitutivity 1,2
l ∈ Ref1 (l)
Therefore the
rules are consistent with the conclusions drawn from the Strengthened T-
Schema and the rules of
be derived in
from the sentence
are consistent, then no contradiction can
with the name l.
As can be seen from the proof, a contradiction is not derivable because of the
If we just have the normal T-Schema, once we can prove that
T r(l).
it immediately follows that
However, with the Strengthened T-Schema, it only follows that either
Given that the point of
¬Eval(l).
is to identify which sentences are not evaluable, and hence when the
normal T-Schema cannot hold, this is reasonable.
Formally, the
predicate prevents the derivation of the contradiction directly from
T r(l),
without having to compute whether the sentence is evaluable or not. This has a couple of important
Firstly, it means that the burden of preventing a contradiction moves from the
to the denition of
is dened so that it is false for all the sentences
which produce a contradiction when we assume the normal T-schema, then no contradiction can
Secondly, this shows that the basic approach to resolving the Liar Paradox
adopted here is sound.
sentence for which
If the system turns out to be inconsistent, then there must be some
is true, but from which a contradiction can be derived from the normal
T-Schema. This means that we have not dened
that we did not notice. Changing the denition of
correctly, as there is some paradoxical case
does not require a great change in our
conceptual apparatus, and given the complex nature of the ways large sets of sentences can refer
to each it would not be a serious problem if our initial denition failed to take into account all the
possibilities.
However, given the central role that
has in preventing the Liar Paradox, it is possible
that the Revenge Problem will bite here in terms of the
through which it could bite are the sentences
¬Eval(g)
Fact.
by Lemma 8,
¬Eval(f )
predicate. The most likely sentences
¬T r(g) ∧
However, neither of them produce a paradox.
Proof. We begin with
p¬Eval(f )q)
f. f
is T-free, so by Lemma 7,
` T r(f ) ≡ ¬Eval(f ).
is not true. Given that
intuitively correct solution.
p¬T r(g) ∧ ¬Eval(g)q
is evaluable (i.e.
` Eval(f )
produce a paradox.
` Eval(f )).
, it follows that
¬T r(f )
is provable,
says of itself that it is not evaluable, and it is evaluable this is the
We now turn to
follows therefore that
From the denition of
Ref ,
is not evaluable. Thus, by Lemma 9,
Refi (g) = {g}
¬T r(g)
i ≥ 1.
is provable. Although what
says is correct, it does not produce a paradox from here.
The Strengthened T-Schema is:
T r(g) ≡ (¬T r(g) ∧ ¬Eval(g)) ∧ Eval(g)).
¬Eval(g) is provable, Eval(g) is not, so we are in the identical situation to l.
Thus if
l is consistent
The proof that multiple sentence versions of the Liar Paradox are not evaluable and hence not
true is essentially the same as the proof for l. These cases will be included within the proof below
is consistent. However, there another type of Paradox to deal with rst, namely Curry's
Paradox. Thus we consider the sentence
T r(k) ⊃ Q
whose canonical name is
k,
and where
arbitrary.
Lemma 12.
is not paradoxical.
Proof. We begin by observing the following derivation:
T r(k) ≡ ((T r(k) ⊃ Q) ∧ Eval(k))
T r(k)
(T r(k) ⊃ Q) ∧ Eval(k)
MP ln.1,2
Thus the sentence
because from a proof of
Elim ln.3
Eval(k)
we can derive
ln.2-5
is provable within
and therefore
Normally, the paradox arises
However, in
is derivable. Therefore we will compute the value of
Ref1 (k) = Ref1 (T r(k)) ∪ Ref1 (Q)
Ref Rule 7
Ref1 (T r(k)) = {k}
Ref1 (k) = {k} ∪ Ref1 (Q)
Substitutivity ln 1,2
k ∈ Ref1 (k)
Defn
¬Eval(k)
be proven within
SLT 1 we
can only
Eval(k):
is derivable and hence by Lemma 9,
¬T r(k)
is derivable. Therefore,
Thus the denitions within
Paradox. We have not shown that
deal equally well with the Liar Paradox and with Curry's
is consistent, just that the ordinary methods by which
an inconsistency arises have been blocked.
inconsistency, then
Unless there is some other means for deriving an
will be consistent (on the assumption that
showing this requires a far more detailed proof.
is consistent). However,
5.4 Consistency
The standard method in formal logic for demonstrating the consistency of a formal system is to
prove the system is sound with respect to a (consistent) model. That is, one proves that there is a
model of the relevant formal system, and since the model is consistent and the provable sentences
are a subset of the true sentences in the model, the formal system must be consistent.
motivation for the development of
however was the negative result in Chapter 3, namely
that a semantically closed formal language cannot be sound with respect to a model. If this is the
case, and
is semantically closed in something close to the relevant sense, then it would follow
cannot have a model, and therefore we cannot prove consistency using the standard
The argument in Chapter 3, however, relied on a couple of assumptions which do not, or
need not, hold for
M |= p)
Those assumptions are the
Interpretation Principle (M
of traditional T-Schema, and the T-Schema does not hold in
Interpretation Principle does not hold. In fact,
Interpretation Principle as a type
it is not surprising that the
cannot have a consistent model if the
Interpretation Principle holds, since on that principle it must be true, for
M |= T r(b)
and replace it with a `Strengthened'
as dened above, that
M |= ¬T r(b).
The obvious move in this context is to drop the
and the assumption that a model must be complete (i.e. every sentence in the language
has a truth value in a model). Given that one can read the
|= T r(p)
is evaluable).
Interpretation Principle from Chapter 3,
Interpretation Principle, namely:
M |= T r(p)
i (
To do this, however, is to undermine the philosophical basis of a semantic
model. The idea is meant to be that the semantic model denes what is means for sentences to
be true or not true. If we accept the introduction of the extra clause around evaluability, which
depends on syntactic information about names, then we seem to be admitting that the model for
cannot completely dene what is and is not true.
We can however treat a model purely as a mathematical device for proving consistency via
soundness, and this is all that we need here to prove consistency.
If we do this, however, we
can consider dening the concept of evaluability within the model, so that sentences of the forms
T r(p)
have a truth value within the model. Evaluability, however, is a property that
depends purely on which sentences have which names. In formal languages, the denition of what
sentences have what names normally belongs to the syntactic denition of the language. It is not
part of the normal denition of a model.
have a model in which the `Strengthened'
If we are to have a model of
which means we
Interpretation Principle holds, then the model must
include information about what sentences are in
and what names those sentences have.
This is not dicult to do, and a model of this sort will be presented below. Nevertheless it comes
at the cost of blurring the neat philosophical distinction between the syntactic language and the
model that has existed. On this approach, syntactic entities are necessary for semantic properties
and therefore must be included within the model.
The second relevant assumption in the argument in Chapter 3 is the assumption that every
sentence must have a truth value in the model. While the role of the model is to x the semantic
values of the sentences within the language, it is possible that there may be sentences whose value
cannot be xed directly against the model but require some (non-recursive) process in the language
to x their semantic value. Given that the problems identied in Chapter 3 are with the semantic
interpretation of the
predicate, a natural candidate for a set of sentences who do not have a
truth value in the model are sentences (or at least some sentences) which include the
The idea here would be that since for the T-free part of the language provability and truth must
coincide, then one can use the provable sentences to ensure that the correct sentences which include
predicate are provable.
If we take this approach, that the T-free part of
has a model, but all of
does not,
we do not have to include information about sentences within the semantic model. However, we
cannot use soundness as a consistency proof.
Nevertheless, a consistency proof is still possible.
This proof is however more intricate and less philosophically illuminating so it is presented in the
It is also possible to take the approach that the T-free part of
sentences which include the
predicate has a model, while the whole of
plus a dened set of
does not. The
most obvious way to make this idea work is to allow use the type of model Kripke dened to let
all grounded sentences in
have a truth value in the model, and then use the approach in the
Appendix to prove the whole language is consistent.
For ease of exposition, and for the philosophical illumination it sheds on the structure of
we will focus on building a model for all of
It should be remembered that this relies on a
broader concept of semantic model than is normally adopted, since information about sentences
and their names must be a part of the model.
5.4.1 Building a Model for SLT 1
We will construct the model for
The rst sub-model of
are theorems of
out of two smaller models for fragments of the language.
will obviously be the standard model for
SL, since all of the theorems
The second sub-model will be a model that deals with the denition
of Evaluability within the model. To do this, we will construct a model of the Name Calculus part
This sub-model will be entirely separable from the model of
since the only logical
connective that gets used within reasoning within the Name Calculus is
⊃,
and that is to allow a
conditional denition. The model of the Name Calculus can include this without needing the full
denition of sentential logic. Once we have a model for the Name Calculus, the model for
can be built from the union of this model with the model for
The key challenge to that is
integrating the two in a way that is consistent with the Strengthened T-Schema.
5.4.1.1 A Model for the Name Calculus
A model for the Name Calculus is not dicult to construct. An illuminating way to think about
it is as a collection of points with names and arrows which connect points. The points are names
of sentences, and an arrow from one point to another means that the sentenced named by the
rst point refers to the sentence named by the second point. In the context of
that the name at the second point occurs within the scope of the
this means
predicate within the sentence
named by the rst point.
Points may have have none, one or nitely many arrows leading away from them, and every
point will have many arrows pointing towards it. For any
T r(p), ¬T r(p),
T r(p) ∧ T r(q),
there are many sentences, such as
that refer to it. As every name in
(and vice versa) these arrows are well dened in the model. The set
which one can get to by following up to
arrows from
of points that one can get to by following arrows from
set. That is, there are nitely many points accessible from
p and end up back at p via other arrows.
Refi (p)
REF (p)
is the set of points
is the complete set
If we follow the denitions through, a
sentence/point in this model will be evaluable if it has a nite
identies one sentence
set, and
itself is not in the
p and one cannot follow an arrow
We will refer to a series of arrows which follow
from each other at points as a path.
has a nite
(i.e. sentences) from
p is nite.
set, this means that one can only reach a nite number of other points
This can occur under two dierent situations. The rst is when every path
This means that if we follow an arbitrary sequence of arrows from
p, we will always
reach a point which does not refer to any other sentences. These sentences which do not refer to
other sentences are sentences which do not have a
predicate in them. In this rst case,
be grounded. The second possibility is that there is a path from
which continues indenitely,
but which eventually only repeats points which have already occurred on the path. In this case,
there will be a circular loop in the arrows. Naturally there will be sentences that have nite
sets, and that have some paths which terminate and others which lead to circular loops.
On the rst possibility, if every path from
is nite, then
itself cannot be in the
will be evaluable, by the denition. This part of the denition of evaluability corresponds
to the denition of groundedness. On the second possibility, if there is a path from
a circular loop,
can only be evaluable if
is not in this loop. Otherwise,
set. This case, which is also evaluable on the denition, where
which has
will be in the
refers to sentences in a circular
reference loop but is not itself part of the denition, goes beyond the concept of groundedness. It
is this case which plays a crucial role in resolving the Paradox in a philosophically satisfying way.
This denition of Evaluability corresponds to the more general denition of Evaluability dened
in Chapter 4, not to the concept of Finitely Evaluable dened in
We are not restricted to
nite reasoning within the Model, and it is easy to check that the denitions of Evaluable coincide.
Using this denition ensures that everything is either evaluable or not evaluable within the model,
which means that every sentence will have a truth value within the model. It is easy to see that
this model is consistent. There cannot be a situation where a sentence/point is both evaluable and
not, since one can either follow arrows away from a point and back to it, or one cannot. Therefore
the model of the name calculus is consistent.
5.4.1.2 A Model for SLT 1
from the two sub-models by using the two sub-models to
dene a base and then use a recursive denition on top of the base.
This procedure is similar
to the recursive denition introduced by Kripke, but also draws heavily on approaches such as
Herzberger's where he extended Kripke's denition.
6 Whereas Kripke used the facts in the base,
which eectively corresponds here to the model of
we will use a more complicated base which
also includes separate sets of true and false sentences, as in Herzberger's approach. We will generate
the model for
by the following steps:
1. Include all of the true sentences from the models of
and the Name Calculus in the set
in the base; and include all of the false sentences from the models of
Calculus in the set
in the base.
2. For any sentence for which
3. Close both
(a) If
is in
and the Name
¬Eval(p)
is true in the base (i.e. in
T0 ),
F0 .
by using the following recursive denitions for the connectives in
F0 ).
(b) For all other connectives use the standard recursive denitions.
The Recursive Denition
1. We begin with
Fn ,
and add elements to form
Tn+1
Fn+1
according to the
following.
2. We add
(i.e. unless
Fn+1 )
that is in
Fn ),
is not evaluable
is true in the base).
Hans G. Herzberger. Notes on Naive Semantics. In: Journal of Philosophical Logic 11 (1982), pp. 61102
F1 , Tn−1
Fn−1
are not dened and so are taken to be empty sets.
are closed by using the same recursive denitions of the connectives as above.
If we take the union of all levels on this denition, we will get a model for
will have a truth value. For every sentence in
SLT 1 and every sentence
is either evaluable or it is not. If it is
not evaluable, it will be false according to the model. If it is evaluable, there are two possibilities,
it is grounded or it is not. If it is grounded then it will appear in the hierarchy in exactly the same
way as all grounded sentences appear in Kripke's hierarchy. If it is not grounded, but is evaluable,
then it must refer to a sentence which is not evaluable as it belongs to a nite reference loop.
However, that not evaluable sentence is false and included in the Base, and therefore the evaluable
sentence will have a derived value in the hierarchy.
We have, therefore, a model in which every sentence has a truth value. If this model is consistent, and it is genuinely a model, i.e. everything that is provable in
is true in this model,
5.4.2 Is the model consistent?
We will follow the two stage construction of the model and use mathematical induction in order
to show that the model is consistent.
showing that the sets
Ti .
The key thing in showing that the model is consistent is
are consistent, that is there is no sentence such that both
are consistent then the
will also be consistent, as the negation rules guarantee
that any inconsistency in one is transferred to the other. To do this, we will show that the base
(T0 ) is consistent, and then show that if at any level
n, Tn
is consistent, then at level
n + 1, Tn+1
The two sub-models are obviously consistent, so we must rst check that adding
¬T r(p)
for every sentence which is not evaluable preserves consistency. All of the sentences in
Name Calculus are T-free, hence they are evaluable. This is because a T-free sentence does not
refer to another sentence and hence is grounded. This means the set of sentences that are true in
the sub-models and the set of not evaluable sentences are mutually exclusive sets. Therefore, there
will be no sentence
that is both true in one of the two sub-models, and the sentence
by Step 2 of the denition. This means that
¬T r(q)
is consistent before the closure over
the connectives.
The only connectives that could cause trouble is negation, since the recursive rules for the
other connectives cannot introduce a contradiction.
The key point about the negation rule, is
that it ensures that the negation of every false sentence is true and that the negation of every
true sentence is in
The issue therefore is whether there is some sentence which is in
whose negation is inconsistent in
T0 ;
or some sentence in
T0 ,
F0 ,
We know that the submodels are consistent and do not share vocabulary, therefore the only
possibility must involve the sentences of the form
which were introduced into
are not evaluable. However, as argued, sentences that are not evaluable and sentences from the submodels are mutually exclusive. Therefore the negation rules cannot introduce any inconsistency.
We will now assume that
introduces sentences of the form
is consistent for some
T r(p) into Tn+1 .
The second step of the recursive denition
are consistent, the introduced sentences will
be consistent, which means that the introduced sentences will also be consistent when closed under
negation. Closing under the other connectives in classical logic cannot introduce an inconsistency
if the starting set of sentences is consistent.
This means that if
can only be inconsistent if one of the introduced
sentences contradicts a sentence that already has a truth value in level
Fn ).
(i.e. is in either
We will go through the dierent cases identied with regards to Evaluability to check that
none of these can introduce an inconsistency.
The dierent cases all depend on the a key insight. In the model for the name calculus, the
reference structure for each sentence is represented by arrows leading from that sentence, which
sets up a complicated network when all sentences are considered.
The important aspect to the
network when considering evaluability is the tree leading from any particular sentence. If the tree
and all paths in the tree are nite, the sentence is grounded.
If the tree includes nitely many
sentences, but has innite paths (i.e. loops), it is evaluable if the original sentence is not in any
loop and not evaluable if the original sentence is included in a loop. If the tree includes innitely
many sentences, it is not evaluable.
Not evaluable sentences are the easiest case to deal with.
that is not evaluable is
not introduced at any level apart from the base, due to the condition on the second step. Hence
these sentences cannot introduce an inconsistency in the recursive denition.
We will consider the situation with grounded sentences next.
For every grounded sentence,
there will be a longest path in the tree leading from it. For a T-free sentences, that path length
will be 0, and every sentence whose longest path length is 0 will be T-free, for any sentence that
includes the truth predicate will refer to another sentence and so will have a non-zero maximum
path length. This means that any sentence with maximum path length 0 will be in the Base.
Any sentence with maximum path length 1 will have at least one part of the form
is T-free, and no parts of the form
T r(q)
is not T-free. This means that every sentence
with maximum path length 1 will be introduced into the model by the recursive denition in level
1, i.e.
and not in any earlier level. One can easily see that this pattern will continue,
and that all sentence with maximum path length
will be introduced at level
and not at any
earlier level.
Now since
Tn−1 ⊂ Tn ,
the introduction rule (T r(p) for every
will be `reintroduced' into
Tn+1 .
Tn ),
every sentence of the
Since these are sets this does not aect any
properties of the sets.
The key thing to focus on are the sentences that are introduced by the
recursive denition at level
which are not introduced at any previous level.
every sentence with maximum path length
n in the name calculus model are introduced at level n.
However, we also know that every sentence with maximum path length less than
at a smaller level and those with maximum path length greater than
This means that the `new' introductions at any level
n (for grounded sentences).
We know that
are introduced
are introduced at a higher
are exactly those with maximum
This means that no grounded sentence introduced at level
can be in contradiction with a sentence introduced at a lower level, since the sentences with
smaller maximum path length are a distinct set of sentences. This demonstrates that grounded
sentences cannot introduce an inconsistency.
The nal category to consider are sentences that are evaluable, but are not grounded. Similar
considerations apply to grounded sentences, except that we measure the maximum length to a
not evaluable sentence in which none of the sentences in the path are themselves not evaluable.
In this case, the Base will contain all sentences with maximum path length
introduce all sentences with maximum path length
and level
However, as in the case with
grounded sentences, the set of sentences introduced at each level is distinct, and so no sentence
can be introduced at a level which contradicts a sentence at a lower level. Therefore no sentence
that is evaluable but not grounded can introduce an inconsistency in the recursive hierarchy.
This exhausts the cases, and hence it follows that
is consistent, if
is consistent. By
induction, every level of the hierarchy is consistent and the model is therefore consistent.
5.4.3 Is every provable sentence true in the model?
We must again follow the two stage construction of the model to show that every provable sentence
is truth in the model.
The rst step is to note that as the models for
base, all provable sentences in these two parts of
only part of
and the Name Calculus are part of the
are automatically in the model.
that is not in one of these is the Strengthened T-Schema.
that the Strengthened T-Schema holds for every sentence in
If we can show
the fact that we close the
model under the recursive denition for the connectives at each level means that all of the classical
consequences from the Strengthened T-Schema will also hold in the model. This will mean that
everything provable in
Every sentence in
is true in the model.
is either evaluable or it is not evaluable.
are true in the base, so
For every non-evaluable
T r(b) ≡ (b ∧ Eval(b))
is true by the
recursive denition of the connectives since both sides are false. This means that the Strengthened
T-Schema is true in the base for every sentence that is not evaluable.
is evaluable,
Eval(c)
is true in the base and the standard T-Schema will hold as
Also it means that either
is grounded, in which case every path from it ends with a
T-free sentence, or that at least one path ends in a circular loop which does not contain
is grounded, then every path ends with a T-free sentence, but every T-free sentence in
member either of
or the Name Calculus. All the T-free sentences will have a truth value xed
in the base, and the recursive process will only transmit this up to
denition will mean that either
T r(c).
c and T r(c) are both true or are both not true.
The recursive
c is not grounded,
and has a path that ends in a circular reference loop, all of the sentences in that circular loop will
be not evaluable and hence not true in the base. The relevant values will transmit up the levels and
recursive denition will mean that either
T r(c)
both cases the Strengthened T-Schema will hold as
are both true or are both not true. Again in
This means that the Strengthened T-Schema is true in the model for every sentence in
and hence every provable sentence in
5.5 Final Results
We have shown that
includes a consistent truth denition within a classical sentential logic
that does not place any restrictions on which sentences the truth predicate can apply to. That
is, any sentence can be within the scope of the truth predicate, and the resulting sentence will
have a truth value.
This makes this a very promising formal truth denition, particularly for
philosophical applications, but we need to test it against the criteria on a good denition that were
dened above.
Fact 13.
in the sense dened above.
Proof. The denition of an adequate truth denition included three conditions. The rst was: If
consistent as
This rst condition is proven by Lemma 6. The second condition was that:
This is proven by Theorem 10. The third condition, that
Proof. While
does not provide an adequate truth denition for itself.
LT ` T rpP q
satises the consistency condition, the rst condition, namely If
does not hold.
In particular, we know that
sentence dened above. However it does not follow that
identical to
is as
follows immediately from the consistency proof.
However, the same does not apply for
and it is not the case that
SLT 1 ` T r(b).
SLT 1 ` ¬T r(b),
SLT 1 ` T rp¬T r(b)q
the liar
p¬T r(b)q
This could only be the case if
inconsistent.
Thus while
SL, and allows every sentence which
includes a truth predicate to have a truth value, it does not provide an adequate truth denition
for itself. In other words,
is not semantically closed in the desired sense. The form of this
problem suggests that Kripke's problem has simply been pushed up a level. Although we can now
assert that the Liar Sentence is not true; we cannot assert that it is true that the Liar Sentence is
not true. To this problem, we could take Kripke's solution and suggest that at this point we still
need a ghost of the Tarskian hierarchy, and have to resort to a metalanguage. However, another
solution to this will be explored in the next chapter and involves changing the way we understand
dene names within a formal logic. For the moment though, it is enough to note that although
T rp¬T r(L)q
is not true within the system,
T rp¬T r(l) ∧ ¬T r(l)q
is a provable theorem.
In this chapter we have provided a consistent and adequate truth denition for Classical Sentential
Logic. Most remarkably, this was achieved without changing any features of the underlying logic
and simply by adding extra machinery to deal with the reference relations between sentences and
the truth predicate. The success of this extra machinery means that it is clear that an equivalent
denition can be added to any more sophisticated logic.
The key to this denition is the approach to the semantics of the language, which includes the
denition of truth. Truth was taken to be about the relationship between syntactic entities and
semantic facts, which means that it depends on more than one condition. One of these conditions
is traditionally semantic and the other traditionally syntactic, however both are required for a
successful denition of a truth predicate. This overlap of the traditional separation meant that,
in the approach adopted here, information that was traditionally syntactic is required within
the semantic model of the language. While this approach undercuts traditional formal practise,
it accords with natural language practise. If one wants to work out the truth value of The rst
sentence in this chapter is true, the information about what sentence is being referred to is relevant
semantic information and necessary to working out the truth value of the sentence. Including the
information about what sentences refer to what in the model is simply including this type of
The truth denition presented in
problem generated by the Liar Paradox.
is a viable candidate for solving the philosophical
It is consistent, and the language satises the four
conditions on a Grammar-Only language being aected by the Liar Paradox, as natural languages
do. The truth denition is not tied to a particular logic, which is also a positive. However, it fails
to be fully semantically closed, as there are sentences which intuitively ought to be true but which
cannot be true in
This problem will be dealt with in the next chapter.
Second Formal Denition
The formal truth denition oered in Chapter 5 is remarkable in that it allows a consistent and
plausible denition of truth within a classical logic. Moreover, it does not require any restrictions
on grammatically acceptable sentences, and allows all grammatically acceptable sentences to have
a determinate truth value. This means it is a language whose Grammar-Only part satises the
conditions on a Grammar Only language being aected by the Liar Paradox.
Moreover, any
sentence can be referred to within the scope of the truth predicate without contradiction. Thus
Chapter 5 oers a Logical Language whose Grammar Only part fullls the philosophically desirable
conditions and the Logical Language is not aected in the stronger sense.
is nothing in extension of
Importantly, there
that depends on working with a classical logic, so the
denition can be adapted to any formal system of logic. We have a formal truth denition which
consistently satises the linguistic conditions on a solution to the Liar Paradox, which means that
the assumptions about truth embodied in this truth denition are plausibly the correct ones for
our understanding of natural languages. However, as noted in the previous chapter, this denition
does not achieve everything that an ideal formal denition should achieve.
A key aim for a formal truth denition is that it achieves semantic closure. That is, the correct
semantic properties of every sentence in the language can be stated (by true sentences) within the
language. Unfortunately, this does not hold in
as there are sentences which have certain
semantic properties, but it is not possible to assert those properties within the language.
example, given the sentence
can prove that
it follows in
is not true, i.e. we
However, it does not follow that an assertion that `b is not true' is true is
true. That is, it is not possible to assert the truth of this conclusion within
that the sentence which expresses `b is not true' is true in
The problem is
which is false because
is provable. While we can assert that `b is not true', we cannot assert the semantic fact
that it is true that b is not true within the language. This problem is obviously a limitation in
the application of this approach to natural languages. If it is the case that
is not true, we would
SECOND FORMAL DEFINITION
expect the sentence `b is not true' is true to be true. Otherwise we would not have correctly
captured the concept of truth. This means that there is still a gap between the formal concept of
truth denition in
and our understanding of the concept of truth.
Fortunately as a point of analysis, in terms of overcoming this limitation, the denitions in
Chapter 5 are based on only half of the analysis in Chapter 3. In Chapter 3, two main types of
problems were identied with the standard model theoretic approach to semantics in the context
of the Liar Paradox. The rst was the denition of truth purely with respect to a model, which
was addressed in Chapters 4 and 5 by including certain facts about the reference structure of the
language in the denition of truth through the concept of evaluability. However, the second type
of problem was the inability of a language with a recursive semantics, or in fact a language that
assumes that sentence types are truth bearers, to be semantically closed.
The truth denition in the previous chapter is, at least in a strict sense, a non-recursive definition. The truth value of a sentence does not purely depend on the semantic properties of the
components of the sentence, but also on the name of the sentence and the interaction between
the name and the general reference structures in the language. The model dened as part of the
consistency proof, however, shows that the semantic properties of sentences can be dened recursively, so long as facts about evaluability are taken to be primitive. In any case,
was dened
so that each sentence (i.e. grammatical string of symbols) had only one name, and therefore it
follows that all sentences of the same type in
of Chapter 3 predicts that this means that
must have the same truth value. The analysis
cannot be semantically closed, which has been
conrmed above. Therefore, the analysis in Chapter 3 shows that if we want to achieve semantic
closure, then we need to modify or replace
and do away with the assumption that sentence
types are the truth bearers.
6.1 Is this necessary?
Before setting out to nd a way of modifying
so that sentence types are no longer the truth
bearers, it is important to consider whether this is a plausible account of similar situations in
In terms of the philosophical motivation of this project, these is little point
pursuing a truth denition which assumes that sentence types are not truth bearers if natural
languages demonstrably make this assumption.
The debate about the correct truth bearers in
natural languages is a vigorous and detailed debate that cannot be covered here, beyond saying
that there is no universal consensus on the matter.
The fact that there are live options in the
debate which argue that sentence types are not truth bearers is perhaps sucient justication for
pursuing this part of the project. It could therefore be a reasonable position to take, and if it can
be shown that a formal language which takes something else as truth bearers can be semantically
closed, then that would be evidence that sentence types are not the primary truth bearers.
We can, however, oer more direct evidence that the type of change being considered to the
formal languages is a reasonable one in the context of considering natural languages. One way of
seeing this is to consider the following situation:
The country of Xodarap has in recent history had a change of government. The previous President was a highly mendacious and rarely, if ever, told the truth. The current
President, in contrast, is transparently honest, and made this honesty the cornerstone
of his campaign. His rallying cry during the campaign was the assertion that What
the current President of Xodarap is saying is not true. Given the legacy of his predecessor, the current President legislated that if the President of Xodarap ever uttered a
falsehood, he must be removed from oce.
One day, the President fell ill and was gripped by a terrible fever. After lying in bed
almost all day, unable to speak and barely able to move.
Suddenly, and with great
energy, he sat up straight and deantly repeated his campaign rallying cry: What the
current President of Xodarap is saying is not true. The illness then regained control
of his body, he sunk to his bed and could not speak for the rest of the day.
The situation is further complicated by another comment that was made at the same
time on the same day. The President's doctor, after assessing the President's peculiar
fever, pronounced that, in his medical opinion the nature of the fever will mean that
What the current President of Xodarap is saying is not true.
The most important task, for our purposes here, is to consider the truth status of the doctor's
assertion.
Given that both assertions were made at the same time, then the doctor's assertion
will be true if the President's assertion is not true and vice versa. However, the doctor made an
assertion of exactly the same sentence type as the President, which appears to show that these two
assertions of the same sentence type cannot have the same truth value. It is commonly accepted
that sentences of the same type which contain indexicals will dier in truth value, since the meaning
of the sentence will depend on the meaning of the indexical in the context. In this case, however,
while there is an indexical in the sentence, `What the current President of Xodarap is saying', the
indexical refers to exactly the same thing in both assertions, and hence there is no dierence in
meaning due to indexicality. Thus this natural language example provides a situation where the
same sentence type does not always have the same truth value which is not captured by indexicality.
It is reasonable to pursue a formal solution which allows sentences of the same type to dier in
meaning, as has been proposed.
It might be objected that this analysis has only used a very simplistic conception of sentence
types, and say that accounts of sentence types which take into account some role for context would
not have the same problem.
Firstly, the point of this example is only to show that questions
about sentence types arise in the context of the Liar Paradox and that it is plausible to look at
alternatives to sentence types as truth bearers in this context. Secondly, and more importantly,
standard formal languages implicitly adopt this simple conception of sentence types that identical
strings of symbols have identical truth values, and moreover there is normally no dened concept of
context in formal languages that can play a role. If a more sophisticated account of sentence types
is necessary to deal with this type of situation in natural languages, then an alternative approach
must also be adopted in a formal language, if we are to provide an account of truth and reasoning
in natural languages within a formal language.
The fact that situations such as that with Xodarap provide situations where it is plausible
to move away from sentences types as truth values means that it is reasonable to look for an
alternative approach to formal languages so that sentence types are not (always) truth bearers, at
least when considering the Liar Paradox.
6.2 Truth-bearers
The idea being advanced is to work with a formal denition in which sentence types can have
dierent truth values, at least in certain circumstances. Given the success of
the Liar Paradox, and the fact that the formal denition in
identied, we propose to modify
in addressing
addresses one of the key issues
so that it is not assumed that identical sentence types have
identical truth values. The immediate question is then what should play the role of truth bearer.
While the normal alternatives to sentence types as truth values are propositions or sentence tokens,
as argued in the Introduction each of these options is problematic in this context, at least on a
standard account of either.
On the one hand, standard understandings of propositions are not ne-grained enough to
distinguish between the statements made by the President and the Doctor in the example above.
Each person has uttered the same sentence in virtually the same context and would normally
be taken to have expressed the same proposition. On the other hand, any account which takes
sentence tokens as truth bearers has to account for the following problem, particularly in a formal
setting where context does not play much of role. Argument and derivations typically assume that
if we assert a new sentence token of a type already proven, then that new token is still proven or
true. If this is not always the case since dierent tokens can have dierent truth values, then it
is necessary for an account that uses tokens to be able to account for when such a move is valid
and when it is not. To put it dierently, tokens of the same type normally have the same truth
value, how can we know when they do and when they do not? While this problem may not be
insurmountable, it requires a rethinking of the whole approach to proof in a formal context.
The story of the President, however, oers a hint of a dierent approach to understanding
truth values.
The sentence What the current President of Xodarap is saying is not true as
the President's assertion seems to have one truth value, while the same sentence as the Doctor's
assertion seems to have a dierent truth value. The person who uttered the sentence, at least in far
as this aected the place of the sentence within the reference structure, aected the truth value.
The fact that one sentence is identied as the President's assertion means in the context that that
sentence is self-referential. The fact that the other sentence is the Doctor's assertion takes it out
of the circular reference loop, which means that it can potentially have a dierent truth value.
If we take the concept of evaluability as dened in previous chapters, the distinction between
the President's assertion and the Doctor's assertion is clear.
The sentence, as the President's
assertion, is not evaluable as it belongs to a circular reference loop: the sentence refers to itself as
that is the only relevant sentence. However, the sentence, as the Doctor's assertion, is evaluable
as it refers to the President's assertion which refers to itself. The Doctor's assertion is not in a
circular reference loop, and so is evaluable.
While in this particular example the key fact is who asserted the relevant sentence, other
examples show that the important information is the method we use to refer to or identify a
particular sentence. For example, if we have a piece of paper with a single sentence written on
it, the following are only a few examples of the ways we can identify the same sentence: The
rst sentence on this page; The only sentence on this page; What is written on this page.
If, moreover, that sentence reads The rst sentence on this page is not true, then the denite
description used to identify the sentence appears to aect the truth value of the sentence. While
the sentence itself, i.e.
The rst sentence on this page is not true, is paradoxical; there does
not seem to be any reason to suppose that The only sentence on this page is not true should be
problematic. This denite description identies a clear sentence and no circular reference arises.
This is the same sort of situation as in the Xodarap example. In both cases, the way of identifying
sentences aected the possible truth values that particular sentences could hold. It follows therefore
that we need to look at the combination of sentence, with the means of identifying the sentence,
as a truth bearer.
Within formal denitions, the standard means by which sentences are identied and referred
to is through names.
Sentences are assigned names of some sort, and those names are used to
refer to the sentence. Analogously to the Doctor and the President and the denite descriptions,
the name of a sentence will determine whether it belongs to a circular reference loop or not. The
obvious step for a formal language is therefore to treat the combination, or pair, of a sentence and
a name as the truth bearer. That is, it is sentences as identied by names that truth is predicated
of. In
where every sentence has one unique name, this distinction is meaningless as there
is no real distinction between a sentence and its name.
As pointed out above, in natural languages there are many cases where we can refer to the same
sentence, and even the same sentence token, in dierent ways. To take the example of the piece of
paper with a single sentence written on it, the following are only a few examples of the ways we
can identify the same sentence token: The rst sentence on this page; The only sentence on this
page; What is written on this page. These need not be strictly names in the way we understand
them in a formal language, but they play the same role as names in the formal language. Moreover,
as in the example given above, if that sentence reads The rst sentence on this page is not true,
we can see the same type of behaviour as in the Xodarap example. The sentence under one method
of reference is problematic as it sets up a circular reference. To use the terminology of the last
couple of chapters it is not evaluable.
However, the same sentence token considered under the
other methods of reference are not problematic and are evaluable. This will be taken to mean that
there is a dierence in truth value here.
The dierence in truth value depends on the method of referring to a sentence, which in a
formal language is its name.
Truth values therefore depend both on the sentence and, at least
in a formal context, the name being used to refer to the sentence. In order to implement this in
a formal language, we will therefore take sentence/name pairs as the basic truth bearers in the
following formal denition.
6.3 Preliminaries for a Formal Denition
Since we are adopting the convention that the primary truth bearers are combinations of a name
and a sentence, it necessary to allow single sentence types (or grammatically correct strings of
symbols) to have multiple names.
Moreover, it is possible that dierent names for the same
sentence type will dier in whether they are evaluable and therefore have dierent truth values.
Importantly, this possibility provides a way around the semantic closure problem identied above
Suppose now that we have two sentence-name pairs, one being the sentence
and another being the same sentence
of evaluability,
If we look at the denition
is evaluable, as it does not exist in a circular reference loop. This fact is crucial,
as it means that the standard T-Schema holds for
then we can prove that
c, which means that if we can prove that ¬T r(b)
In other words, it will be possible to prove the assertion that It
is true that b is not true within the formal language. It is this assertion that was not provable
since in
consistently proven.
this assertion could only be represented by
which cannot be
Thus, taking truth bearers as name/sentence pairs, which allows the same
sentence to have dierent names with dierent truth values, allows us to address the semantic
closure problems with
Moreover, this solution mirrors the intuitive solution to the Xodarap example above. In that
case, it is very plausible to say that there is something dierent between the Doctor's assertion
and the President's assertion, and therefore the two dierent assertions can have dierent truth
values. That would allow that dierent assertions of the same sentence type in the same context
can have dierent truth values, in this case because of dierent speakers.
Allowing the same sentence type to have dierent truth values when identied by dierent
names has therefore the potential to achieve semantic closure in the sense that, for any sentence
in the relevant language, for some name
will be provable. This
is possible because, at least for the cases relevant to the Liar Paradox, for any name/sentence pair
that is not evaluable, it is always possible that there is another name for the same sentence type
that is evaluable.
In Chapter 4, two types of situations were identied which meant that a name/sentence pair
could be not evaluable. The rst case, which is the case relevant to the Liar Paradox, is when the
name/sentence pair is within a circular reference loop. Two tokens of the same type must refer
to the same sentences and therefore have identical reference sets. Thus, if we have name/sentence
pair that is not evaluable, so long as there is another name for the same sentence which is not
within the reference loop, then this second name/sentence pair will be evaluable.
Moreover, whenever there is a name/sentence pair that is not evaluable for this reason, it will
be possible to choose another name for the same sentence that is evaluable provided the language is
suciently liberal with naming. Suppose a name/sentence pair is not evaluable because it belongs
to at least one circular reference loop. Since we are not considering cases where quantication over
sentences is allowed, the name/sentence pair must belong to nitely many circular reference loops.
Moreover, a circular reference loop can only include nitely many name/sentence pairs within the
loop. Therefore there can be only nitely many name/sentence pairs within the circular reference
loops that the relevant sentence belongs to. It will therefore be possible to identify the original
sentence by means of a name that is outside the nite number of names within the circular reference
loops, and the original sentence paired with this name will be by denition evaluable. It does not
belong to any of the relevant circular reference loops.
This means that whenever there is name/sentence pair that is paradoxical, it will be possible
to assert that the sentence type involved is true or not true using an alternative name for the same
sentence type. If this works, it will be possible to non-paradoxically assert the truth status of every
sentence in the language.
The second situation where a name/sentence pair is not evaluable is of less interest as it does
not lead to paradox. However, it gives rise to dierent problems with semantic closure, which are
not resolved in the same way by the change to name/sentence pairs. Suppose we have an innite
series of sentences such as the following:
All of these sentences are not evaluable as the semantic process that governs the truth predicate
does not ever identify a sentence with a denite truth value. It follows, if we adopt the Strengthened
T-Schema, that each of these sentence is not true. However, say we have a further sentence
says that S1 is not true.
combined with the series
also makes an innite series which will
face exactly the same problem as the innite series that only included the
Sn .
Thus although we
would conclude that
is not true, we cannot conclude within the language that
though that is what
asserts.
sentence which refers to any
is true, even
Moreover, there is nothing special about the sentence
Sn will
A which
have exactly the same problem. Thus the approach outlined
here that works for the paradoxical cases with circular reference cannot provide semantic closure
in these cases.
This will be simply accepted here as a limitation of this method, which has no real consequences
when the Liar Paradox is considered. These types of cases involving innite series are only paradoxical if we allow quantication over sentences, which has not been considered. So consistency
is not threatened if this problem is left out.
these types of sentences were not given
any truth value, since the computation of their evaluability requires quantication or innite sets,
neither of which were available in the sentential logic. This limitation in achieving semantic closure
will therefore not aect an extension of
which does not change the underlying logic.
The important point is that semantic closure is possible for all sentences that relate to the Liar
Paradox if we shift to taking name / sentence pairs as truth bearers.
Semantic closure in this
context would have to mean that any semantic (i.e. to do with the truth denition) fact that is
true for the language can be asserted as true within the language using an evaluable name/sentence
pair which expresses this fact. Thus every semantic fact is assertible within the language, just not
every instance of a sentence type which expresses that fact is true.
This limitation is rstly fairly minor, and secondly reects what seems to actually occur in
natural languages. For example, consider the following liar sentence:
Sentence 2) is not true.
Now suppose that a detailed discussion and argument follows this sentence, and that this argument
leads to a denite conclusion about the truth status of this sentence. It is quite possible that the
conclusion of the argument could be expressed in the following sentence: Sentence 2) is not true.
In terms of ordinary use of language, there is nothing problematic about this form of asserting
the conclusion. If, however, we accept that the conclusion can be stated in this form, then there
is nothing strange about making distinctions between the truth values of dierent instances of
a sentence type.
For this conclusion is stated using the same sentence type as the paradoxical
sentence.
to give a new formal language
that allows sentence types to be associated with more than one name. The predicates in
The approach to be adopted is therefore to alter
which take names as arguments will therefore refer to name/sentence pairs, rather than sentence
types as in the case with
in which sentences had canonical names. This means that whenever
a name appears within a predicate in
it should be read as referring to a name/sentence
pair, rather than simply a sentence. This will allow
to be semantically closed in the way
described above.
Before this system is presented, however, it is necessary to consider the problem posed previously with respect to adopting sentence tokens as truth bearers in formal systems.
derivations normally work on the basis that a sentence type is proven, and it follows that any
repeated assertion of the same sentence type will also be proven. The problem was that if sentence
tokens are the truth bearers there is no guarantee that, when one token of a sentence type is proven
and therefore true, that another token of the same type will be true. This same point also applies
to name/sentence pairs, as two dierent pairs with the same sentence may have dierent truth
values. This could mean that a radical change in argument structure is required.
However, there is a natural way of structuring the formal system so that this is not required,
and this can be philosophically and intuitively justied. Firstly, we should note that we are moving
to name/sentence pairs as truth bearers because of particular problems that arise when predicating
over sentences. This means that the possibility of having two name/sentence pairs with the same
sentence with diering properties is only relevant in cases where the sentence includes a predicate
that predicates over sentences. This means that the majority of sentences are not aected.
However, due to the nature of the truth denition being implemented, a careful treatment of the
remaining sentences is not required. The Strengthened T-Schema, for a name
name/sentence pair
a/A;
when fully spelt out would read a/A is true i
This means to prove that a/A is true, we need to prove that
and that a/A is evaluable. The
rst of these works purely on sentence types, as it always has in formal logic, and at least in
the second has a series of rules which govern its derivation. Only Truth and Evaluability need to
take into account name/sentence pairs, and these are catered for already in the denitions. There
does not need to be any change to the proof structure as the relevant distinctions are already
tracked within the language.
While it is completely coherent to treat provability as a property of sentence types and truth
and evaluability as properties of name/sentence pairs, this may seem like a sleight of hand or
conceptually unappealing. If this is the case, the practice to be adopted below of proving sentence
types but predicating over name/sentence pairs can be seen as a notational convenience. As argued
above, every name/sentence pair with the same sentence type that is evaluable will have the same
truth status. Moreover, for every sentence type, it is possible to ensure that there is a name for that
type which is evaluable. It follows, therefore, that proofs of sentence types can be considered to be
proofs of any name/sentence pairs including the same sentence type where the name/sentence pair
is evaluable. This fact could be included notationally, but it would be necessary to identify a name
for every line of proof and then invoke rules about when one can use the same sentence with a
dierent name. This would add notation and a vast amount of work without actually changing any
of the proofs or the conclusions. Given that the equivalent proofs hold and that it is far simpler,
the system will be dened so that proofs work on sentence types.
6.3.1 Adequate denition of truth
will be judged against the same criteria as
provides adequate truth denitions for
namely as to whether it
SL and itself, and the extent to which it oers a semantically
closed solution. The following repeats what was dened at the start of Chapter 5.
It should be noted that we need to be careful as to how we understand the second condition
in this context, where name/sentence pairs are acting as truth bearers. In this context we need to
consider the situation where one sentence can have dierent names with dierent truth values and
properties. Given this, we can either allow that the second condition holds for all name/sentence
pairs which include the relevant sentence, or that it holds for at least one name/sentence pair
which includes the relevant sentence. If the second condition held for all name/sentence pairs in a
particular language, then for any provable sentence in
all assertions of its truth with all names
would also be provable. However, this would not allow for sentences to have dierent truth values
under dierent names, which is a key to the approach being adopted here. Admittedly, if
not include a truth predicate, then the distinction will be irrelevant, but the aim of this denition
is to be as general as possible. We will therefore consider the second condition to be satised, if
there is some name/sentence pair that satises it. The denition will therefore be the following:
then for some name
p, p
is a valid name of
LT ` T r(p).
The same considerations hold for, and are in general more relevant to, the denition of an
adequate truth denition for itself:
In the previous chapter it was shown that
failed to provide an adequate truth denition
for itself, and for that reason it was not semantically closed.
The problem was that there was
a particular sentence that was provably true, but the assertion that it was true was not true in
The aim in the construction of
is to avoid this problem, so a dening test of
will be whether it succeeds in this.
6.4 Classical Sentential Logic with Truth (2)
6.4.1 Denition
As in the previous chapter, we let
propositions;
be a Classical Sentential Logic, with
the set of connectives dened in the normal way; and
metatheoretic propositional variables. Furthermore, we assume that
is dened as a standard
Fitch style natural deduction system. It contains the following introduction rules:
as atomic
We extend
to a language
of individual constants
p, q, r, ...
which includes a truth denition.
We rst add a class
(and corresponding metatheoretic variables
which will function as names for sentences within
a, b, c, ...)
We will assume no particular method
of attaching names to sentences. Furthermore, in order to be able to identify within the system
which sentences have which name, we will introduce a two place predicate,
Names as its rst argument and Sentences as the second argument.
as a is a name of the sentence:
One can also understand
N (a, A),
N (a, A)
which takes
will simply be read
as meaning that
valid name/sentence pair.
(Eval(),
also requires adding certain new vocabulary, in particular some predicates
T r(), Refn (), REF ()
) and a calculus of names that allows reason about the refer-
ence relationships between sentences (or to be more precise, between name/sentence pairs) within
This will include a new class of individual constants (p, q, r, ...) which are names; nite sets
of names,
could be left to the metatheory, or rules of application of the system as in the previous chapter, but it is
more informative to be able to express this within the language,
and some grammatical sentence
an atomic proposition
• N (a, A)
It should be noted that the names within the scope of the various predicates denote name/sentence
pairs, rather than only sentences. We can use basic propositions to dene the grammatical sentences of
function as metatheoretic sentential variables
recursively. We will let
We will assume that the naming conventions of
the form
N (a, A).
are xed by an explicit list of axioms of
This list of axioms is subject to the unambiguity condition that no name can
be attached to two sentences. As previously stated, we will generally assume that there is at least
one axiom of the form
for every sentence
This list of naming axioms will be taken
as primitive in the system, however, it could be generated, for example, by a Gödel Numbering
system, as long as these conditions are satised.
Reference introduced in Chapter 4, and therefore to be able to dene Evaluability.
elements of the Name Calculus are the class of names,
the empty set
p, q, r, ..., nite sets of names {a, b} including
In order to reason about sets of names, our system includes set union
and set inclusion
∈.
The basic
∪, equality between sets =
Both are dened in the standard way, and equal sets are always substitutable.
predicates are dene recursively over the construction
N (a, T r(b)) ⊃ Ref1 (a) = {b}
(N (a, ¬B) ∧ N (b, B)) ⊃ Ref1 (a) = Ref1 (b)
(N (a, B ∨ C) ∧ (N (b, B) ∧ N (c, C))) ⊃ (Ref1 (a) = (Ref1 (b) ∪ Ref1 (c))
(N (a, B ∧ C) ∧ (N (b, B) ∧ N (c, C))) ⊃ (Ref1 (a) = Ref1 (b) ∪ Ref1 (c))
(N (a, B ⊃ C) ∧ (N (b, B) ∧ N (c, C))) ⊃ (Ref1 (a) = Ref1 (b) ∪ Ref1 (c))
is a basic proposition not of the form
predicates again dene the concept of
includes the
Sets. Importantly, the
set, which means that in many cases the set will stabilise after some
This allows
us to dene a Complete Reference Set, where it is nite:
predicates can be used to dene introduction rules for the
is in any
set then
is not evaluable. If
and is not a member of its own reference set, then
has a complete reference set
is evaluable. In this way, the
identies the correct sentences as evaluable and not evaluable. We therefore use it to dene the
N - Strengthened T-Schema:
N (a, A) ⊃ [T r(a) ≡ A ∧ Eval(a)]
The key dierence between the Strengthened T-Schema in
is that this version explicitly
checks in the language that the name is a name of the correct sentence.
This is necessary as
sentences can have more than one name in
It should also be remembered that
holds, is to be read as saying that the name/sentence pair
a formal language
is true. We have dened
which includes a truth denition and allows there to be multiple names
for the same sentence. This was introduced in order to allow a semantically closed formal truth
denition. Obviously we now need to check whether that is achievable.
The basic properties of the system are identical to those of
and the basic proofs are almost
identical. We will therefore in general only give detailed proofs for results where the proof diers
signicantly because of the dierent naming convention.
From this it quickly follows that
A is evaluable, as its reference sets are all empty.
The full proof is identical to that of the equivalent proof in
It follows that every T-free
The next proof makes explicit use of the
Predicate, and therefore will be given, although
it is essentially identical to the equivalent proof in the previous chapter. This Lemma shows that
whenever a name/sentence pair is evaluable, the standard T-Schema holds.
N (a, A), Eval(a) ` T r(a) ≡ A.
Proof. The proof is the following formal derivation:
N (a, A) ⊃ [T r(a) ≡ (A ∧ Eval(a))]
The next Lemma shows that if a name/sentence pair is not evaluable, then it is necessarily not
true. Again the proof makes explicit use of the
N (a, A), ¬Eval(a) ` ¬T r(a)
The following Theorem demonstrates one of the key results to show that
adequate truth denition of
provides an
is a theorem
Therefore, by Lemma 16,
by Lemma 15,
is T-free, since there are is no Truth Predicate within
Furthermore, since we are also given that
T-Schema that
Thus both
are theorems.
is a theorem, then it follows from the N-Str
is a theorem.
thus far satises all of the properties that were satised by
We now need
to check how it works when faced by the Liar Paradox.
6.4.5 Paradoxes
the archetypal, self-referential Liar Sentence is dened by the existence of a naming
axiom of the form:
N (l, ¬T r(l)).
We will therefore show that adding this axiom to
have any undesirable consequences.
N (l, ¬T r(l))
as a theorem/axiom in
does not immediately produce
Proof. We rst note that the N-Str T-Schema, in the relevant context, has the following form:
N (l, ¬T r(l)) ⊃ [T r(l) ≡ ¬T r(l) ∧ Eval(l)].
it follows by Modus Ponens that
Thus the question of whether
of whether the sentence:
the proof in the case of
Given the assumption that
is equivalent to the question
are both false, the truth value
is true, and hence this sentence is consistent with Sentential Logic.
we prove both
from the Strengthened T-Schema.
N (l, ¬T r(l)) ⊃ [T r(l) ≡ ¬T r(l) ∧ Eval(l)]
MP 1,2
The proof is identical to
If we consider it model theoretically, if the truth values of
` T r(l) ≡ ¬T r(l) ∧ Eval(l).
¬Eval(l):
These proofs are simple applications of the rules of Sentential logic, and hence if Sentential
Logic is consistent, then no contradiction can be derived purely from the Strengthened T-Schema
(for the Liar Sentence in question).
contradict the conclusion, then the Liar Sentence will be inconsistent, otherwise it will be consistent.
That means if
Therefore no contradiction can be derived from the sentence
This shows that the system
is a theorem it is consistent.
shares the most of same properties as
in terms of how
it deals with paradoxes. However, crucially for the success of this project, it avoids the problem
of semantic closure that was identied for
in the way it was designed to. The idea behind
moving to name/sentence pairs as truth bearers is that it is possible for the same sentence to have
dierent truth values when it is paired with dierent names. With regards to the Liar Paradox,
the idea was that while the pair
the case that any other pair
expressed within
l/¬T r(l)
p/¬T r(l)
would not be true as it is not evaluable, it ought to be
would be true, and therefore the relevant truth could be
The following proves that this is in fact the case:
N (p, ¬T r(l))
are theorems, then
Proof. The rst stage is to compute whether
name for the sentence
and we choose
` T r(p)
` ¬T r(l)
is evaluable. To do this it is necessary to assume a
N (q, T r(l))
(N (p, ¬T r(l)) ∧ N (q, T r(l))) ⊃ Ref1 (p) = Ref1 (q)
Ref Axiom 3
N (q, T r(l)) ⊃ Ref1 (q) = {l}
N (p, ¬T r(l)) ∧ N (q, T r(l))
Ref1 (p) = Ref1 (q)
Ref1 (q) = {l}
Ref1 (p) = {l}
Substitution, 9,10
Ref2 (p) = Ref1 (p) ∪ Ref1 (Ref1 (p))
Ref2 (p) = {l} ∪ Ref1 (l)
Substitution 11,12
Ref2 (p) = {l} ∪ {l}
Ref2 (p) = Ref1 (p)
Subst. 11,16
(Ref2 (p) = Ref1 (p)) ⊃ (REF (p) = Ref1 (p))
REF (p) = Ref1 (p)
MP 17,18
REF (p) = {l}
¬(p ∈ REF (p))
Then we use the Strengthened T-Schema and the fact that
in the proof for Theorem 20 to prove that
Intro ln.1,5
N (l, ¬T r(l)) ` ¬T r(l)
T r(p):
Previous Proof
N (p, ¬T r(l)) ⊃ (T r(p) ≡ (¬T r(l) ∧ Eval(p)))
N-Str T-Schema
T r(p) ≡ (¬T r(l) ∧ Eval(p))
MP ln 1,3
¬T r(l) ∧ Eval(p)
MP 2, 22
Thus, we can prove that
T rp¬T r(l)q,
was proved
so long as
p¬T r(l)q
is not l. That is, we can prove that
l is not true, so long as the name of l is not true is not l. This fact will allow the system to be
semantically closed in the way required. All of the other results proven in
hold for
such as the fact that Curry's Paradox also does not bite. However, the key question is whether
is still consistent.
In Chapter 5, the consistency of
was proven by constructing a model of
This model
was more complicated than is normally the case as it included information about what sentences
refer to what. To be more precise, the model contained information about what names referred to
what names, but as each sentence has a single name in
this is equivalent. As
SLT 1 , it is unsurprising that we will adopt the same approach to proving consistency.
a model for
SLT 1 and SLT 2 is that the truth bearers in SLT 2 are name/sentence
pairs, rather than sentences.
however involves including even more information in the model.
The key dierence between
bearers in
It is worth pointing out though that we could interpret the truth
as being name/sentence pairs. As
has the restriction that every sentence
has one unique name, the name/sentence pairs uniquely correlate with the sentences, and so there
is no real formal dierence between treating sentences and name/sentence pairs as truth bearers
however there is a signicant dierence as we allow there to be non-identical
name/sentence pairs with the same sentence. That is, one sentence can have multiple names, and
the dierence names have consequences for the semantic properties of the name/sentence pairs.
This means that in the model of
it is necessary to assign truth values to name/sentence
pairs rather than sentences. This is a signicant conceptual change, and requires some care in the
denition of the model, but the basic structure of the denition is identical to the denition of the
model for
As in the case of the model for
models for fragments of the language.
standard model for
standard model of
since all of the theorems of
out of two smaller
would obviously be the
except that the
SL has sentences as truth bearers rather than name/sentence pairs.
sub-model will be a model that deals with the denition of Evaluability within the model. To do
this, we will construct a model of the Name Calculus part of
Name Calculus, the model for
same way as the model for
The key fact about
Once we have a model for the
can be built from the union of these two sub-models, in the
is built.
is that it is name/sentence pairs, rather than sentences, which have
truth values. However, in the denition of
this distinction is only relevant in the scope of
the relevant predicates. We explicitly prove sentences, rather than name/sentence pairs in
The role of a semantic model is to demonstrate the consistency of the system of proof in the
language, which means that we only need to be concerned with true sentences in the model, rather
than name/sentence pairs. This means that we can use the standard model for
as a sub-model
and that we only need to be concerned about name/sentence pairs in the model of the
Name Calculus.
Nevertheless, a model for the Name Calculus is not any more dicult to construct than the case
We will think about it is as a collection of points and arrows which connect points. The
points are names/sentence pairs, and an arrow from one point to another means that the sentence
at the rst point refers to the sentence at the second point by means of the name at the second
point. In the context of
scope of the
this means that the name at the second point occurs within the
predicate within the sentence at the rst point.
point will have many arrows pointing towards it.
¬T r(p);
T r(p) ∧ T r(q)
arrows are well dened in the model. The set
following up to
to by following arrows from
there are many sentences
T r(p);
identies a sentence these
is the set of points which one can get to by
is the complete set of points that one can get
If we follow the denitions through, a point (i.e. name/sentence
pair) in this model will be evaluable if it has a nite
set. That is, one cannot follow an arrow away from
and end up back at
via other arrows. We
will refer to a series of arrows which follow from each other at points as a path. This denition is
almost identical to the denition for
and so has the same properties. In particular, every
name/sentence pair is either evaluable or not evaluable; and the evaluable name/sentence pairs are
split between those that are grounded and those which are not grounded.
In order to make the dierences between this denition and the one for
clear, it is
useful to consider the situation where there are two name/sentence pairs which include the same
sentence. The rst point to note is that the arrows from this point depend purely on the syntax of
the sentence. Therefore the two points which represent these two name/sentence pairs will have
arrows to exactly the same points. This means that the reference structure of these two points is
identical, after the rst arrow. This means in particular that if one of these points is grounded,
then the other must also be grounded.
However, it does not mean that if one of these two points is evaluable, then both of them are
evaluable. We will take the sentence
works. In this case, both of the points (<
goes to the point
< a, T r(a) >.
{< a, T r(a) >},
in its
and pair it with the names
a, T r(a) >
This means that the
which will equal the
< c, T r(a) >)
to see how this
have a single arrow that
sets of both of the points are identical
set for each of the points. However,
set, and so is not evaluable. On the other hand,
and it is therefore evaluable.
< c, T r(a) >
< a, T r(a) >
is not in its
Despite this change, it is still easy to see that this model is consistent.
There cannot be a
situation where a point is both evaluable and not, since one can either follow arrows away from a
point and back to it, or one cannot. Therefore the model of the name calculus is consistent.
in the same way as for
that is by using the two
sub-models to dene a base and then use a recursive denition on top of the base. To repeat, we
generate the model for
is true (i.e. in
T0 )
in the base, add
will have a truth value.
The T-free sentences in
will all have a truth value in
the base, so they are not a problem. The truth value of any sentence which contains the truth
predicate will depend on whether the name/sentence pair within the scope of the truth predicate
is evaluable or not. However, every name/sentence pair is either evaluable or not in the model,
and so every sentence containing a truth predicate will have a truth value in the model of
If this model is consistent, and it is genuinely a model, i.e. everything that is provable in
true in this model, then
The argument that the model is consistent is the same as the argument for
however it will
be repeated here for completeness. We will follow the two stage construction of the model and use
mathematical induction in order to show that the model is consistent. The key thing in showing
that the model is consistent is showing that the sets
such that both
are in
are consistent, that is there is no sentence
will also be consistent, as
the negation rules guarantee that any inconsistency in one is transferred to the other. To do this,
we will that the base (T0 ) is consistent, and then show that if at any level
at level
is consistent, then
are automatically in the model. The only
that is not in one of these is the Strengthened T-Schema. If we can show that the
Strengthened T-Schema holds for every sentence in
the fact that we close the model under
the standard recursive denition for the connectives at each level means that all of the classical
Every name/sentence pair in
evaluable
is either evaluable or it is not evaluable. For every non-
b, both ¬T r(b) and ¬Eval(b) are true in the base, so T r(b) ≡ (b ∧ Eval(b)) is true by the
is true in the base. Also it means that either
is grounded, in which
case every path from it ends with a T-free sentence, or that at least one path ends in a circular
loop which does not contain
every T-free sentence in
is grounded, then every path ends with a T-free sentence, but
is a member either of
or the Name Calculus. All the T-free
sentences will have a truth value xed in the base, and the recursive process will only transmit
this up to
The recursive denition will mean that either
or are both not true. In both cases the Strengthened T-Schema will hold as
are both true
is true. If
is not grounded, and has a path that ends in a circular reference loop, all of the name/sentence
pairs in that circular loop will be not evaluable and hence dened as not true in the base. The
relevant values will transmit up the levels and recursive denition will mean that either
c and T r(c)
are both true or are both not true. Again in both cases the Strengthened T-Schema will hold as
The formal truth denition in this chapter is an improvement on the denition in the previous
Chapter, since we have provided a consistent truth denition for Classical Sentential Logic that can
be plausibly semantically closed. To conrm this, we need to rstly check that the truth denition
is adequate in the senses dened above.
Proof. The rst condition is satised by Lemma 15. The second condition is satised by Theorem
19 and the third condition is satised by the consistency proof.
Thus the truth denition is
for itself, assuming that
is adequate for
SLT , and it can also be shown to be adequate
SLT 2 is suciently liberal with naming.
As argued above, for any sentence,
it is always in principle possible to nd a name/sentence pair for that sentence which is evaluable.
The reason is that a name/sentence pair is not evaluable if it is in at least one circular reference
loop. Given that a name/sentence pair can only exist within nitely many circular reference loops
and each loop can have at most nitely many names, then assuming there are
innitely many names, there will always be some name outside the reference loops. This means
that, for any sentence, it is always possible to nd a name which would be evaluable in a pair with
that sentence. We will assume in what follows that the denition of the naming axioms in
ensures that for every sentence there is a name/sentence pair which is evaluable.
provides an adequate truth denition for itself in the sense dened above.
Proof. The second condition, that
the consistency proof, since both the T-free part and
The rst condition is that if
SLT 2 ` T r(p).
SLT 2 ` P ,
We know that it is possible to nd some name
q/P ,
SLT 2 ` N (q, P ),
the standard T-Schema holds. This means
SLT 2 ` T r(q),
then it follows that
is satised by
are consistent.
is evaluable. For this name/sentence pair,
as required. Hence the rst condition is
satised.
The major motivation for developing
was that
was not semantically closed, since
there was a sentence which was intuitively true and provable, but which could not be asserted as
true within
The fact that
provides an adequate truth-denition for itself means that
this precise problem cannot occur. For every provable sentence in
could alternatively amend the denition of
it is possible to assert
to allow the introduction of Naming Axioms within proofs
in such a way that the name/sentence pair created is evaluable.
that that sentence is true. However, this does not guarantee semantic closure. We need to show
that it is possible to assert that untrue sentences are not true in
To see this, we will prove
the following Lemma:
SLT 2 ` ¬P
SLT 2 ` N (p, P )
SLT 2 `
¬T r(p).
Proof. For any
we know that there is some
evaluable. It follows, by Lemma 8, that
SLT 2 ` T r(p) ≡ P .
p which is a name of P ;
which is a name of
and that if
is provable, it
is provable then
is also
¬T r(p) is also provable,
is semantically closed.
satises the full list of desiderata for a formal truth denition.
formal truth denition for
is semantically closed for the truth predicate.
provable, for some
SLT 2 ` ¬T r(p).
Proof. From the previous two proofs, we have shown both that if
It both oers a
and itself, it is consistent and is semantically closed for the truth
Something obviously had to be given up to reach this.
What has been given up in
comparison to other approaches to the formal denition of truth are the T-Schema (for particular
sentences), and the idea that sentence types are truth bearers, at least for the same particular
sentences. These two changes have nevertheless been well-motivated, t with our intuitions and
only apply to certain sentences.
Importantly, the machinery that allows this to occur does not depend on classical logic, but
can be dened in any suciently expressive logic. This has a number of consequences, with the
most signicant being that this approach to the denition of truth should apply equally well to
natural languages. As noted, the techniques adopted are naturally expressed in natural languages,
and the idea of taking the name/sentence pair as a truth bearer (at least in certain circumstances)
is a natural response to certain natural language situations.
This also means that while
plausibly embodies the correct formal approach to the def-
inition of truth in natural languages, it does not follow that
itself is the correct logical
analysis of truth in natural languages. The application of this denition to natural languages will
be discussed further in the next chapter. What is most signicant about this Chapter is that the
formal goal articulated in the opening chapters has been achieved, we have a formal truth denition
which satises all of conditions identied previously that hold for a natural language, and which
is consistent and semantically closed.
Application to Natural Languages
The truth denitions in the previous two chapters have provided consistent formal truth denitions
within classical logic including one that, in the case of
By doing
this, the approach to the denition of truth that has been adopted appears to meet all of the
requirements to be a philosophically satisfactory truth denition. That is, it allows a consistent
formal truth denition that does not rely on changing the grammar of the language or restricting
the grammatically acceptable sentences. This means that it has the potential to be applied to our
understanding of truth in natural languages. However, before we discuss this potential and how
far it can be applied, it is worth backtracking a little and going over aspects of the denitions to
gain a better understanding of how they work, particularly in the light of the analysis in the rst
three chapters.
7.1 Understanding the Formal Denition
In Chapter 2, various solutions to the Liar Paradox were analysed against the nine conditions that
characterise a logical language which is aected by the Liar Paradox. It is useful to measure the
denitions in
against these conditions, to see how they t in with the other
Both of the languages
have a method of expressing untruth (Condition 1)
since they include a truth predicate and a negation (Condition 6), untruth can be predicated of
sentences or sentence/name pairs (Condition 2) via names which allow sentences to refer to other
sentences or sentence/name pairs (Conditions 3). There are, moreover, no restrictions on the way
that sentences can be constructed using these (Condition 4). As per Chapter 2, this means that
SLT 2 satisfy
all of the conditions that natural languages satisfy.
Out of the remaining conditions, a contradiction can be derived from the paradoxical sentences
(Condition 9) and the ex contradictio quodlibet holds (Condition 5), as the base logic is a classical
logic. Obviously, the T-Schema does not hold in either
so the eighth condition
does not hold. As the T-Schema, and the assumptions about truth that it reects, were identied
as a key contributor to the Liar Paradox, the T- Schema is replaced by the Strengthened T-Schema
in both systems.
dier as to whether they satisfy the seventh condition. All sentences
and all sentence/name pairs in
to express this fact for every sentence within
are either true or not true, however it is not possible
This is the problem of semantic closure for
which was discussed in Chapters 5 and 6 and which lead to the development of
is therefore similar to most of the solutions examined in Chapter 2 which also do not satisfy
condition 7, although this is not philosophically ideal for reasons already discussed.
satisfy Condition 7, since it is semantically closed.
Nevertheless, the fact that both
satisfy the rst four conditions and therefore
the conditions on a Grammar-Only language being aected by the Liar Paradox mean that they
are both candidates for a plausible account of the truth predicate in natural languages.
the signicant advantage that it is semantically closed for the truth predicate, but requires the
philosophical commitment to sentence/name pairs being primary truth bearers, rather than simply
sentences or sentence types. It is not in the scope of this thesis to decide between these alternatives,
as any choice depends on broader philosophical concerns which have not been considered.
7.1.1 Evaluability
While there is this dierence between
SLT 1 and SLT 2 , the key to these highly successful denitions
is the Strengthened T-Schema and the denition of Evaluability. However, while the consistency
of these languages depend crucially on the Strengthened T-Schema, they do not depend crucially
on the particular denition of Evaluability that was adopted.
There are potentially a range of
denitions of Evaluability that will formally suce, and any decision on the best denition of
Evaluability must depend on non-formal considerations.
The clearest way of seeing that other denitions of Evaluability may suce is to look at the
fragment of
which includes the Strengthened T-Schema but does not include the Name
Calculus. In this fragment, the
Eval predicate is not dened outside of the Strengthened T-Schema.
What is interesting about this fragment is that we can still prove that paradoxical sentences are
not evaluable, even though
Lemma.
In the fragment of
is not dened. The following Lemma is an example.
has name l), it is provable that
without the Name Calculus, for the sentence
(which
Proof. The proof simply consists of a simple derivation which we will break into two. Notice that
no rules from the Name Calculus are required.
Equivalent derivations obviously hold for other paradoxical examples, and this demonstrates
the power of the Strengthened T-Schema.
Paradoxical sentences can be judged to be not true
simply as a result of the Strengthened T-Schema's logical structure.
In fact, in this derivation,
predicate could represent any concept, not necessarily Evaluability. So long as there is a
second term on the right hand side of the Strengthened T-Schema, paradoxical sentences will turn
out to be not true. This means that even if the idea of evaluability that has been adopted here is
not correct, the basic structure of the truth denition can still hold and it can still be consistent.
The most important consequence of this fact from a formal perspective is that the Strengthened
T-Schema shifts the responsibility for the consistency of the system from the Truth Predicate to
the Evaluability Predicate (or whatever the second term on the right hand side is). If we extend
the derivation above to all the dierent paradoxical cases, we can see that all of the paradoxical
sentences will be provably not evaluable in these systems. This means that, so long as on the formal
denition of evaluability all paradoxical sentences are not evaluable, the system will be consistent.
The key to dening a consistent truth denition is therefore to dene evaluability so that all of
the paradoxical sentences are not evaluable.
It is this fact that allows
consistent, since they both include a syntactically dened evaluability predicate which satises
We can formulate this a little more precisely as a criterion on a satisfactory denition of the
Evaluability Predicate for any formal language (L) which is being extended to include a truth
denition (LT ):
Eval:
` ¬Eval(a)
(in
is satisfactory i whenever
is paradoxical it is
LT ).1
The exact denition of Evaluability used in
is simple to dene, possible to
intuitively motivate and Satisfactory on this denition. However, it is arguably more coarse grained
than would be ideal. It errs on the side of caution and potentially identies too many sentences
as not evaluable. For example, we will consider (in
unique name is
According to our denition of
to itself, and therefore that
¬Eval(q).
SLT 1 )
Ref , Ref1 (q) = {q}.
Thus, although the sentence
¬T r(q) ∨ (P ⊃ P )
is provable
within the system, and it is intuitively evaluable as we can evaluate one of the disjuncts, it can be
proven to be not evaluable and hence not true within
This suggests that we need a more nely-grained formal denition of evaluability in order to
capture our intuitions in these cases.
This may be achievable, however, there is a signicant
¬T r(r) ∨ (P ∧ ¬P )
obstacle in the way of an improved denition. Consider instead the sentence
` ¬T r(r).
Again on the denition of
in any of the dened systems,
` ¬Eval(r) and hence
In this case, however, the conclusion seems justied, since although one disjunct is
evaluable, the truth value of the sentence depends on the unevaluable disjunct. Thus although
share a very similar form, the truth values of the disjuncts appear to introduce dierences
in evaluability.
It is obviously not possible to deal with this in a purely syntactic way as was provided previously,
as there would need to be some sort of recursive denition of sentences which takes into account
both evaluability and truth value. There is also the potential for problems with circular denitions
in this case, since Truth (via the Strengthened T-Schema) depends on Evaluability which for some
sentences depends on the Truth Values of sentences. It is probably possible to devise a denition of
evaluability along these lines, and so long as it is Satisfactory on the denition above, it will work
in a formal truth denition. Even if successful, it would make the conceptual task of articulating
the relevant concept of evaluability far more dicult.
There is an alternative approach to dealing with this problem implicit in
a way around this issue. If we consider the sentence
which oers
with a name other than
this sentence/name pair is evaluable and true, as is intuitively correct. With
as the
name, the sentence/name pair is not true due to the circular reference, but we can still say that
the relevant sentence is true using a dierent name. Similarly, the sentence
with a dierent name is evaluable and true, since it is the case that
¬T r(r).
Allowing multiple
names for one sentence allows us to preserve the intuitions without having to provide a far more
complicated denition of evaluability.
1 There
is a formal denition of paradoxical as part of the proof in the Appendix.
7.1.2 Bivalence
One of the most remarkable features of this truth denition is that it can be carried out within
a classical bivalent logic. However, one of the lessons of the Liar Paradox is very often taken to
be that classical bivalence is untenable and either we must accept truth value gaps or gluts. This
raises the question, which has been studiously avoided so far, of whether the truth denition oered
here is bivalent. The key issue is what is meant by bivalence.
The most trivial point is that the underlying logic is bivalent, and that the Law of the Excluded
Middle holds. This means that for any sentence
every sentence that
is dened, and if
P , T rpP q ∨ ¬T rpP q
is true. Furthermore, for
is either provable or disprovable, then
either provable or disprovable. Thus all of the relevant sentences are either true or not true, and
this truth denition is obviously bivalent in this sense.
However, bivalence is normally taken to be the position that every sentence is either true or
false. The question of whether
understand falsity to be.
are bivalent in this sense depends on what we
There are two standard ways of understanding what it means for a
sentence to be false: either the sentence is not true, or its negation is true. These two conceptions
of falsity are however not identical in these systems.
falsity denitions within
F 1 pP q ≡ ¬T rpP q
F2 pP q ≡ T rp¬P q
To see this, we will consider two possible
SLT 1 :
On the rst denition of falsity, every sentence is either true or false. Within the language, we
know that
T rpP q ∨ ¬T rpP q
is true as it is a tautology of Classical Logic (and the language is
consistent). Moreover, within the model every sentence will be either true or false. However, there
are some unintuitive consequences if we accept this denition as a denition of falsity. We consider
whether the sentence with name
is understood here as meaning that
It was shown in Chapter 5 that
other words,
T-Schema,
F1 p¬T r(l)q
is false on the rst denition. It should be noted that
is provable.
so by denition,
is provable. However, we now consider the sentence
T rpT r(l)q ≡ T r(l) ∧ EvalpT r(l)q. T r(l)
However, by the Str. T-Schema for
the two equivalences, we get
hence we can conclude that
By the denition of
Thus we have proven that there is a sentence in
By the Str.
EvalpT r(l)q
T rpT r(l)q ≡ T r(l).
T r(l) ≡ ¬T r(l) ∧ Eval(l).
T rpT r(l)q ≡ ¬T r(l) ∧ Eval(l).
¬T rpT r(l)q.
is not self-referring, and so
true and the Str. T-Schema reduces to the normal T-Schema:
F1 pT r(l)q.
F1 (l)
Hence, by equating
is false, and
this means that we have proven
SLT 1 such that both it and its negation
are false. This seems to undermine the concept of negation.
The situation in
is more intricate but not so counter-intuitive.
is also provable
SLT 2 , however, if s is also a name for the sentence ¬T r(l), then we can prove that T r(s)and
¬F1 (s).
but the pair
To put it dierently, in
s/¬T r(l) is true.
the pair l/¬T r(l) is false (on the rst denition)
It is also the case in
T r(l) is false (on the rst denition),
but it does not follow that its negation is false. It depends on which name/sentence pair we are
looking at. Moreover, where truth values depend on the name of a sentence, it is plausible that we
should take an evaluable name/sentence pair as providing the correct truth value for the sentence.
If we adopt this principle, then
is false and
is true, as expected.
Thus if we take the rst denition of falsity, every sentence is both systems are either true
or false, however we need to be careful about how we understand falsity in certain cases as there
are some counter-intuitive consequences. In particular, in
there are cases where both the
sentence and its negation are false.
If we take the second denition of falsity, it turns out that the language is not bivalent as
there are sentences which are neither true nor false. Unsurprisingly, one such sentence is the Liar
Sentence. We have already proven that it is not true, and the proof that it is not false (on the
second denition) follows the example just outlined for the rst denition.
We have proven
and we want to show that
that means that we need to show
¬T rp¬(¬T r(l))q,
¬F2 (l).
¬¬T r(l) is not self-referring, and so Evalp¬¬T r(l)q is provable.
T-Schema reduces to the normal T-Schema:
¬F2 (l),
We can easily see that
This means that the strengthened
T rp¬¬T r(l)q ≡ ¬¬T r(l).
In both
is provable, and hence we can prove from this biconditional that
have proven that
¬T rp¬¬T r(l)q.
and the Liar Sentence is neither true nor false on this denition. That
is, there is a sentence that is not true and its negation is also not true.
It is important to note is that it is necessary to be precise about how we understand this
a/¬¬T r(l)
the name/sentence pair
is not true, and any name/sentence pair
is also not true. Thus the name/sentence pair
is plausibly neither true nor
false, on this second denition of falsity. However, this does not mean that any other sentence pair
s/¬T r(l)
is neither true nor false (they are true), and nor does it mean that any sentence
is also neither true nor false.
the possible pairs of the form
all pairs of the form
s/¬T r(l))
t/T r(l)
Any such pair
is not true in
however only one of
is also not true. Given this, it is plausible to argue that
are simply false on this denition, as the negation (pairs of the form
If we accept the second denition of falsity, then the systems are no longer bivalent in the sense
that every sentence (or name/sentence pair) is true or false. Unsurprisingly, Liar Sentences come
out as neither true nor false.
The aim of this discussion is not to decide between these two denitions of falsity.
Each of
the denitions has intuitive support and each is equally tenable in this context. The choice does
not impact the validity of the truth denition. However, what is clear is that the two denitions
are not equivalent in either of the two developed systems. There are sentences which are false on
one denition but not false (and not true) on the other denition.
This is a result of adopting
the Strengthened T-Schema, and the concept of evaluability. Furthermore, we cannot accept that
these two denitions are equivalent without accepting inconsistency, as has been shown elsewhere.
7.2 Application to Natural Language
This thesis began with the observation that, if the Liar Paradox is a serious problem, it means
that it is not possible to consistently use natural languages to assert truth of sentences within
This arose because certain properties of natural languages, combined with
intuitive principles about truth and the relation between truth and language, lead to a necessary
inconsistency. The nine conditions identied in Chapter 1 articulated the relevant properties and
The challenge in responding to the Liar Paradox was to nd a solution that does not require us
to change the grammar of natural languages, or to restrict what can be said truthfully in natural
languages. Changing the grammar is an implausible solution and imposing restrictions on what
can be said truthfully undermines any attempt to discuss the Liar Paradox and hence negates any
solution. Nevertheless, in Chapter 2, it was argued that existing solutions have not accomplished
this in a satisfactory way. Chapter 3 identied two key assumptions in standard modern formal
semantics, the denition of all semantic properties directly with respect to the model, and that
sentence types are truth bearers, which are incompatible with a satisfactory solution. The systems
were attempts to get around these limitations by following a new approach to
the denition of truth, which was mainly articulated in Chapter 4.
This new approach is built on the observation that the pattern of what sentences refer to other
sentences within a language has semantic consequences, when that reference occurs within semantic
predicates. That is, the truth value of a sentence is aected both by what sentences it refers to,
but also by what sentences refer to it. This fact requires an extra condition to be included in any
truth denition, to take into account when this occurs. We adopted the concept of Evaluability to
represent this idea, and dened the Strengthened T-Schema to capture the concept of truth.
This idea was implemented in
while in
this observation was taken further. If the
reference pattern in a language aects truth values, it is to be expected that the method we use to
identify sentences within the scope of predicates, whether that is names or denite descriptions or
by speaker, may also aect the truth value. In
we therefore took name/sentence pairs as the
primary truth bearers, rather than sentences alone. This allowed the system to be semantically
J. C. Beall and Octavio Bueno. The simple liar without bivalence? In: Analysis 62.1 (2002), pp. 2226;
and Adam Rieger. The Liar, The Strengthened Liar, and Bivalence. In: Erkenntnis 54 (2001), pp. 195203.
closed in the way we intended, which demonstrates the value of implementing this observation.
Both of these systems are highly successful with respect to responding to the philosophical
challenge, although only
is truly semantically closed and does not place any restrictions on
what can be said truthfully in the language.
Thus, we can say that the philosophical problem
posed by the Liar Paradox has been solved, and we have an understanding of truth and reasoning
in natural languages which allows us to consistently use natural languages to assert the truth of
sentences.
It is important however to be careful about what exactly has been shown in this thesis. The
approach to the denition of truth that was articulated in Chapter 4 and the start of Chapter 6
oers an understanding of truth and reasoning which is consistent with the way we use natural
languages. This fact was demonstrated by the successful denitions of
this does not mean that either
reasoning for natural languages.
oer a complete or even satisfactory account of
Each system has limitations with respect to its application to
The most obvious limitation is that neither of these systems allow quantication, which obviously exists in natural languages. However extending either
is not necessarily a straightforward task.
to predicate logic
There are questions about domains of quantication
that need to be addressed, and if complete applicability to natural languages is sort, then quantication over names and sentences needs to be introduced. This raises further questions about
dierence paradoxes which have not been addressed here. The exibility of this approach adopted,
particularly in the denition of Evaluability, suggests that a successful denition is very possible.
However, without this,
A second limitation is that both
have only limited application to natural languages.
have more regimented and limited methods
of referring to other sentences than natural languages do. In both of these, the method of reference
is by means of explicit syntactic names. As this formal method of naming is syntactic, it has the
strong advantage that it allows explicit denitions of properties of the reference relations (such
as Evaluability) within the language. Natural languages however do not have a natural class of
names for sentences and we generally do not use explicit names for sentences. The most common
devices in natural languages are demonstratives (This sentence ....), denite descriptions (The rst
sentence on the page....) and name forming devices such as quotation marks. None of these devices
are directly found within
SLT 2 , which is further evidence that these systems are limited
in their application to natural languages.
It should be noted that it is possible to translate these devices into the systems
It is easier to translate these devices into
mirrors natural languages in
allowing more than one way to refer to a single sentence. For example, each denite description
which denotes a sentence can be translated into
as a unique name. Also, the quotation name
3 Demonstratives are
of a sentence can be translated into any evaluable name of that sentence.
more dicult to translate, as they depend on their context for meaning. However, it is generally
at least in the sense of translating the same
reference pattern between the relevant sentences.
To do this, however, it is necessary to assign
possible to translate demonstratives into
names to the relevant sentences (or relevant sentence tokens) and then use those names within
in the place of the demonstratives.
Translating natural language sentences into
is more dicult, as each sentence is only
allowed one name. This means, for example, that in the case of denite descriptions, one must
identify the sentence being referred to and then translate the denite description by the name of
that sentence. Quotation names are obviously easy to translate into
as each quote name
is replaced by the canonical name of the sentence. These examples show that
have a place in the analysis of natural languages, but cannot be considered to be complete formal
accounts of natural languages.
The limitations with
however do not call into question the success of the
key principles used in the denition of the systems. The Strengthened T-Schema, the concept of
Evaluability, and taking name/sentence pairs to be primary truth bearers, are concepts that are
broader than these systems. The arguments for these principles did not depend on any features of
SLT 2 , and they therefore can be used very broadly.
The method by which these concepts
prevent the Liar Paradox and allow semantic closure in particular do not depend on anything in
particular in
These concepts therefore embody an approach to the denition of a
truth predicate which is successful in solving the Liar Paradox in a satisfactory way.
There is however one nal limitation to
which was touched on in the dis-
cussion of Tarski's denition of truth. This is a signicant limitation as it arguably applies to all
modern languages, and raises questions about the typical formulation of the T-Schema and the
Strengthened T-Schema. As noted in Chapter 2, the only sentences in formal languages that can
be legitimately stated or asserted are provable sentences. The rules that govern assertion in formal
languages are rules of derivation. In natural languages, on the other hand, sentences can be stated
regardless of their provability. We expect only justiable sentences to be asserted in many contexts,
but there are no linguistic rules which prevent unjustiable sentences from being asserted.
The formal language approach to assertability has a signicant advantage with regard to formal
truth denitions. In a formal language, all assertable sentences are provable, and therefore must
be true. This means that we can use the assertability of a sentence as a guide to its truth, and
assertability can be determined within the formal language. As a result of this, a truth denition
does not need to refer to the semantic of the language, since provability is a guarantee of truth (and
disprovability is a guarantee of untruth). In this formal context, schema such as the T-Schema or
3 Quotation
names can produce the Liar Paradox if we allow quotation names of predicates, as in the Quinean
Liar. These are not allowed in
so there are no problems in these systems.
the Strengthened T-Schema can dene a truth predicate. They tie
to the provability of
which works as provability guarantees truth.
Natural languages do not in general possess an uncontroversial concept of provability in the
same way that formal languages do. While certain fragments of natural languages, say the fragment
that deals with mathematics, plausibly have a clear concept of provability, in general we often do
not have an explicit grasp on what it means to prove that, say Snow is white. There are various
ways that one can go about justifying natural language statements, and dierent methods of
justication are considered to be proof in dierent contexts. While this does not undermine the
proposed approach to the Liar Paradox, it means that we need to be careful when using schema
such as the T-Schema or the Strengthened T-Schema in natural language contexts.
If we are using either version of the T-Schema as a schema for generating truths, we need
to think about what it means to show or prove that
proving that
The derivation that
for the Strengthened T-Schema). Proving that
however does not
mean the same thing in natural languages as it does in formal languages, and this has potential
issues for our understanding of truth in natural languages.
For example, at times we accept a
level of proof in natural languages that does not imply complete certainty. If provability implies
truth, this could in turn mean that we are accepting something as true which is not completely
certain.
Intuitions as to whether this is legitimate can go either way, which demonstrates that
some caution and further consideration is required at this point. The fact that a T-Schema can
be used to generate truths in formal languages does not necessarily mean that it explains truth in
natural languages. More should be said and there is more work to do on this issue as it goes to the
heart of how we understand the concept of truth and the relationship between natural and formal
It should be noted however that this does not meant that the Strengthened T-Schema does not
apply in natural languages.
It very plausibly and consistently describes the logical relationship
In any case, these considerations do not undermine key achievements of this thesis. By changing
the focus of analysis of the Liar Paradox onto languages, structural conditions were identied which
ensure languages which satisfy these are trivialised by the paradox. A further structural analysis
of formal semantics and truth denitions identied some key ideas that allowed consistent truth
denitions within classical sentential logic with a very high degree of semantic closure. These key
ideas, the Strengthened T-Schema, Evaluability and taking name/sentence pairs as truth values,
are applicable well beyond the formal languages of
They provide a structural un-
derstanding of truth and truth predicates which allows a clear, consistent and appealing resolution
of the Liar Paradox.
7.3 Philosophical Consequences
It was noted in Chapter 4 that the focus of this thesis was on the formal denition of the concept
of truth, not on any of the debates about the nature or correct theory of truth, or truth-bearers.
Nevertheless, it is worth noting that, if correct, the approach to the formal denition of truth
developed in this thesis has some signicant consequences for these broader philosophical debates.
These consequences can only be sketched here, and they raise many questions and areas for future
work, but are well worth noting.
Firstly, with regards to theories of truth, the success of our approach undermines the popular
theory of Deationism. Roughly speaking, Deationism about truth is that the truth predicate
does not strictly mean anything, and the
is simply another way of saying that
these are always intersubstitutable. This theory obviously relies on the universal applicability of
the Tarskian T-Schema, which has been rejected in this thesis with impressive results.
Adopting the Strengthened T-Schema requires that we accept there are two conditions on a
sentence such as
both whether
T rpP q being true, rather that the single condition of P .
is evaluable. If this is correct, then
It is necessary to identify
are obviously
not always substitutable and it cannot be the case that the truth predicate is simply a device, as J.
C. Be all as expressed it, to enable generalizations that, given our nite constraints, we couldn't
otherwise express.
Secondly, the approach adopted for
which was the most successful denition in terms
of semantic closure, has consequences for our understanding of truth bearers. The formal device
adopted in Chapter 6 was to treat name/sentence pairs as truth bearers. This exact approach may
not be the most appropriate for natural languages, particularly as sentences are not automatically
assigned names in natural languages, but if this approach is adopted, it means that primary truth
bearers must be sensitive to the way we refer to a sentence. There are various ways that this might
be realised, perhaps through a token approach, perhaps through a particular denition of context
and accommodates names or method of reference, or perhaps an account of truth bearers such
that a sentence qua one method of reference has a dierent truth value to a sentence qua another.
However, we cannot simply treat sentence tokens or sentence types as truth bearers, as examples
above showed cases where the same sentence token or type could vary in truth value depending on
who we referred to it.
There is a further consequence of adopting the approach here that aects another debate in
the philosophy of language. The denition of truth in both
non-recursive for truth values.
purely on
The truth value of sentence of the form
were shown to be
did not depend
This means, in the philosophy of language, that the successful denition of truth
4 J. C. Beall. Spandrels
5 See Chapter 6.
of Truth. Oxford University Press, 2009, p. 1
adopted here is not compositional for truth values. This means in turn, either that meaning is not
compositional, or that meaning is compositional and that truth values do not always purely depend
on meaning. Either of these approaches can be accommodated within the logical framework here,
but the traditional view that meaning is compositional, meaning imposes truth conditions and
therefore truth values are compositional cannot survive unaltered.
While the approach to the formal denition of truth developed in this thesis does not solve any
of these broader philosophical issues, it rules out certain approaches to these issues and provides
parameters that other approaches must t within. There is signicant work to be done following
through the philosophical consequences of this approach more exactly, since, as noted previously,
a complete solution to the Liar Paradox involves a complete philosophy of language, truth and
It has been argued that the Liar Paradox arises as a side eect of the patterns of reference necessary
to the denition of Truth as a predicate.
The patterns of reference allow situations where it is
not possible to follow the reference through to an evaluation of the truth value of the sentences
Once we take into account this possibility as a further condition on the denition of
the truth predicate through the Strengthened T-Schema, a consistent understanding of truth is
possible, and formally denable. The key problem identied with the Liar Paradox is that it brings
into question our ability to use natural languages consistently in any discussion which involves
The analysis and formal denition in this thesis demonstrates that there is a consistent
understanding of the truth predicate which is compatible with the way we normally want to use
To go back to the terminology from the Introduction, the approach to the
denition of the truth predicate is a plausible candidate for the building
the correct
linguistic system which includes English and the correct assumptions about reasoning and truth.
The fact that there is a consistent candidate for
English+ that resolves the Liar Paradox and allows
us to assert the truth or falsity of any sentence in the language, ensures that the Liar Paradox no
longer poses a problem for our use of natural languages.
In Chapter 5, it was pointed out that it is possible to prove the consistency of
without using
a standard soundness proof with respect to a model. This proof was not given in Chapter 5 as it
is more technically and mathematically involved that the consistency proof oered, and tends to
obscure the structure of the solution. However, for completeness, and as few proofs of this sort
exist, the alternative consistency proof is presented here. It can be adapted to a proof of
with some minor modications.
In order to make it more readable, the following proof uses a notational convention that was
mentioned in Chapter 5. The name in
lower case letter. For example,
of a sentence denoted by a capital letter will be the
is the name of
Alternative Consistency Proof
The structure of this proof of the consistency of
starts from the observation that if we replace
the Strengthened T-Schema with the ordinary T-Schema, then the language will be inconsistent.
Given that the Strengthened T-Schema licenses certain instances of the ordinary T-Schema, namely
those instances where the sentence is Evaluable,
can only be consistent if the denition of
Evaluability succeeds in preventing the instances of the ordinary T-Schema which generate the
paradox. This in turn requires some method of identifying the relevant sentences and instances of
We will therefore begin by considering the language
SLT *1
which is identical to
that it does not contain the Strengthened T-Schema. This means that
same sentences as
SLT 1 , but not all of the same theorems.
contains all of the
The general method will be to consider
the consequences of introducing various instances of the ordinary T-Schema (T r(a)
SLT *1 .
We will begin with a couple of Lemmas about
is consistent, then so is
includes all of
≡ A)
SLT *1 :
plus the rules governing the Name Calculus, i.e. the denition of
REF , and the Introduction Rules the Eval predicate.
The denitions of
simply dene sets of sentences, and hence these, by themselves cannot introduce a contradiction.
The consistency of
therefore depends on the consistency of the introduction rules for the
predicate. That is, whether the following two rules are consistent:
From these rules, we can see that the denition of the
it is possible that there is some sentence such that both
However, the denition of
predicate can only be inconsistent, if
a ∈ Refi (a)
¬(a ∈ REF (a))
REF (a) is such that Refi (a) is a subset of REF (a) for all i.
predicate is
that it is not possible that both of these hold, and therefore the denition of the
consistent. Therefore
hold.
The only theorems in
which contain the Truth Predicate are classical tautolo-
gies.
Proof. Since
does not contain any axioms or rules governing the Truth Predicate, any
(for some
only theorems which contain
can only be treated as if it is an atomic sentence. Therefore, the
are classical tautologics (e.g.
T r(a) ⊃ T r(a))
The rst of these two Lemmas demonstrates that in order to prove that
we only need to focus on the eects of introducing the Strengthened T-Schema into
second plays a key technical role in the proof, since it establishes that
theorems of the form
does not include any
This means that it is possible to isolate the eects of introducing the
Strengthened T-Schema, and therefore demonstrate that
To do this, we will begin by comparing the Strengthened T-Schema with the Ordinary TSchema, since introducing the Ordinary T-Schema into
will introduce a contradiction. It is
important to note that not every instance of the Ordinary T-Schema is inconsistent. The point
of the Strengthened T-Schema is that it is equivalent to the Ordinary T-Schema for evaluable
sentences, and otherwise it implies that the relevant sentence is not true. If we can show that all
of the `problematic' instances of the T-Schema are not evaluable, then
can be shown to be
consistent.
The rst thing to note is that it is not necessary to introduce the ordinary T-Schema as a general
axiom schema in order to generate a contradiction. Introducing particular subsets of instances of it
as axioms will suce. For example, if we introduce the single instance
T r(l) ≡ L
(for
as dened
above), then the language is immediately inconsistent. This means that for any set of instances
of the ordinary T-Schema that is inconsistent when assumed in
SLT *1 ,
there will be a subset of
this, whose instances are necessary for the derivation of a contradiction. That is, for a set
instances of the ordinary T-Schema such that
SLT *1 , Λ ` ⊥,
there will be some set
∆ ⊆ Λ,
SLT *1 , Λ \ ∆ 0 ⊥.
Λ,
A couple of points are important to note here. Firstly, the set
for example in the case that
we know that for any
Λ = {T r(l) ≡ L}.
∆.
is identifying the particular subsets of in-
stances of the T-Schema will generate a contradiction. As
which satises this property, there is a corresponding non-empty
The rst step in proving the consistency of
is consistent, and In particular,
only allows nite proofs, we only need to consider nite subsets of instances of the
normal T-Schema.
In turn, once we have identied which subsets generate a contradiction, it
is possible in turn to identify which instances of the T-Schema (and hence which sentences) are
essential to generating the contradiction, and hence are paradoxical.
Denition 28.
A sentence,
(with name
instances of the ordinary T-Schema, such that
is Paradoxical i, there is some set
T r(a) ≡ A ∈
/ Λ;
SLT *1 , T r(a) ≡ A, Λ ` ⊥
SLT *1 , Λ 0 ⊥.
That is, a sentence is Paradoxical i it is essential to the derivation of a contradiction that
arises from the T-Schema.
An immediate consequence of this is that
is Paradoxical, since
SLT *1 , T r(l) ≡ L ` ⊥, but SLT *1 0 ⊥ by Lemma 26 (assuming Sentential Logic is consistent).
point of introducing this concept is that it picks out the sentences, or more precisely the sentences
whose instances of the T-schema, that are crucial to the derivation of a contradiction in some
case. Removing the instances of the T-schema for any particular paradoxical sentence need not
guarantee the consistency of a system, as there may be other paradoxical sentences in the system.
In fact, there are systems where every sentence is paradoxical. However, the point is that if the
instances of the T-schema for all of the paradoxical sentences are removed, then the system will
be consistent by denition. This is proven for the case at hand in the following Lemma:
Π={all
the following holds:
paradoxical sentences in
SLT *1 }
Σ = {T r(a) ≡ A : all a in SLT *1 },
SLT *1 , Σ \ {T r(a) ≡ A : A ∈ Π} 0 ⊥.
Proof. We will prove this using a Reductio argument. We will therefore begin by assuming the
conclusion is false, namely that
SLT *1 , Σ\{T r(a) ≡ A : A ∈ Π} ` ⊥.
That is we are assuming that
plus all instances of the T-schema except those of paradoxical sentences is not consistent.
Given that
is consistent (and that
be some nite subset
is a nitary system), by Compactness there must
Ψ ⊆ Σ \ {T r(a) ≡ A : A ∈ Π}
(possibly the whole set). Now take
that allows the derivation of a contradiction
and remove each element in turn. At every stage, either
a contradiction will be provable from the remaining elements of
or it will be not.
stage, however, since the empty set is consistent, the removal of some element of
That element of
means this element is in
is therefore a Paradoxical sentence by Defn 10.
and therefore cannot be a member of
Thus we have a contradiction, and it follows that
At some
will produce
However, that
Ψ ⊆ Σ \ {T r(a) ≡ A : A ∈ Π}.
It remains to be now shown that the criterion of Evaluability, as dened above, suces to
remove all of the Paradoxical instances of the ordinary T-Schema.
secondly, where
a ∈ Refn (a),
is paradoxical, then
being paradoxical means that there is some set
|Λ| = 1, then when |Λ| ≥ 2.
with the same property
|Λ0 | = m.
¬T r(a)).
Λ=∅
is itself a contradiction. (For example,
Now, from the perspective of Sentential Logic, the string
might be
is treated
exactly as though it is an atomic sentence. Therefore, since there are no rules governing
SLT *1 , any sentence of the form T r(a) behaves like an atomic sentence in SLT *1 .
is eectively an atomic sentence, the most minimal condition on
is repeated somewhere as part of
Refn , a ∈ Ref1 (a),
being a contradiction
then by
as required.
The rest of the cases involves the situation where
is essential to the derivation of a
Λ 6= ∅).
We note rst that since there are
contradiction, however it itself is not contradictory (since
no derivation rules or axioms governing the Truth Predicate within
SLT *1 , T r(a)
has the status
of an atomic proposition. That means that the only provable sentences which contain
are tautologies.
In order to investigate the following cases, we assume therefore that we are given a set
instances of the T-Schema for the sentences
could not contain any conditions
which contradict the Left Hand Side of the biconditional. However, if
Λ = ∅;
Furthermore, we will assume the minimality condition
|Λ| = n, then there is no m < n such that there is a Λ0
In the rst case, if
is that
The proof will proceed by cases. Firstly, we will consider the case where
that for a givenA, if
SLT *1 , Λ1 0 ⊥.
B1 , B2 , ..., Bn ,
SLT *1 , T r(a) ≡ A, Λ1 ` ⊥,
This means that we have a list of biconditionals from which we can derive a
contradiction as follows:
T r(b1 ) ≡ B1
T r(bn ) ≡ Bn
This list of biconditionals has some useful properties.
Firstly, every term on the Left Hand
Side of a biconditional only appears once on the LHS. Secondly, since
each of the biconditionals is individually consistent with
is not itself inconsistent,
Thirdly, independently of these
biconditionals, the LHS of the biconditionals cannot be proven or disproven within
because there are no rules governing the
predicate in
and hence the only theorems which
contain them must be tautologies in
A sentence of the form
cannot be a tautology
|Λ1 | = 1.
We will now consider the rst case, where
Thus, by denition,SLT 1 , T r(a)
B1 .
≡ A, T r(b1 ) ≡ B1 ` ⊥,
Importantly, it follows from our minimality condition that
condition means that a contradiction is not derivable from
Λ1 = {T r(b1 ) ≡ B1 }
SLT *1 , T r(b1 ) ≡ B1 0 ⊥.6
is also paradoxical. The minimality
independently of
T r(b1 ) ≡
is critical to the derivation of a contradiction in the same wat that
and hence is paradoxical.
As noted, the only theorems within
that contain
T r(b1 )
are tautologies. Also,
are tautologies since it is impossible for the T-schema for
paradoxical sentence, that is one that is crucial to the derivation of a contradiction, to be a
tautology.It must be remembered that
so both
SLT *1 does not contain any rules governing the T r predicate,
behave like atomic propositions with respect to the rules of
Now, we have assumed that
SLT *1 , T r(b1 ) ≡ B1 0 ⊥
This means that from the assumption of
something that is contradictory to
cannot be a theorem of
is only possible if
T r(a) ≡ A.
SLT *1 , T r(a) ≡ A, T r(b1 ) ≡ B1 ` ⊥.
it must be possible to derive
This something must include
is included as a part of
from which it follows that
as a part, and
must be a part of
B1 ,
acts like a basic proposition within
T r(b1 ) ≡ B1 .
a ∈ Ref1 (b1 ).
This can be seen in a dierent way if we consider the possible truth values of the sentences,
which given we are working with classical sentential logic is valid. The fact that we have assumed
SLT *1 , T r(a) ≡ A, T r(b1 ) ≡ B1 ` ⊥
truth values such that both
means that there is no possible consistent assignment of
consistent assignment of truth values such that
come out true. We know that there is a
T r(b1 ) ≡ B1 is true (since SLT *1 , T r(b1 ) ≡ B1 0 ⊥).
This means that for every assignment of truth values such that
must come out as false. Given that
the only theorems involving
As noted,
be a part of
T r(a) (as
- at least in relation to the truth values of
is a part of
But that means that
A is.
can only constrain the
This means that we can use exactly
In particular, this means that
T r(b1 ) must
b1 ∈ Ref1 (a).
predicates,it follows that
The proof for the cases where
T r(b1 ) as just applied to T r(a).
By the denition of the
is also paradoxical in the same way that
the same argument for
T r(a) are tautologies), this means that the assumption of T r(b1 ) ≡ B1
would not come out as false. However,
possible truth value of
is true,
does not contrain the possibles truth value for
must constrain the possible truth values of
|Λ1 | = n > 1
a ∈ Ref2 (a),
is based on the same basic observation, but the
extra possibilities in reference structure mean that the argument is a little more involved.
The rst thing to note is that, as in the case where
|Λ1 | = 1,
the minimality condition means
that every sentence whose instance of the T-schema appears within
B1 , ..., Bn
is paradoxical. If some
is not paradoxical, then it is not crucial for a paradox and the list
A, B1 , ..., Bn
will give rise to paradox, which contradicts the minimality condition. This means that
anything we conclude about
will hold equally for every
Bi .
The minimality condition also has another important consequence in this case. We have assumed that a series of equivalences of the form
alence of the form
T r(bi ) ≡ Bi
are inconsistent with another equiv-
Moreover, the minimality condition means that there is no subset
equivalences with the same property. This means that each equivalence must provide
information that is relevant to the derivation of the contradiction. More precisely, each equivalence
must provide constraints on the possible truth values assigned to the basic propositions involved.
Given the form of the equivalences, the relevant basic propositions are
T r(bi ).
only possible that every equivalence is necessary for the contradiction if each equivalence contrains
the possible truth values for at least one of these basic propositions. It follows that every
T r(bj )
must contain
In the case above, the fact that
is paradoxical means that
must be contained in
The same argument applies in the more general case here, except that we can only conclude that
is included in at least one
be some
that refers to
To put this in terms of references, it means that there must
We will assume without loss of generality that
and may be part of others.
Our basic strategy will be to follow this pattern of reference and show that it must be the case
a ∈ Refj (a)
There are numerous cases to deal with, and we will structure it as a
reductio argument. That is, we will assume that
The same argument means that
that it be in
since then we get
must be included in some
which contradicts our assumption. Thus
must be part of some
a∈
/ Refj (a).
T r(b1 ) is part of B1 , and is part of no other Bi , we get the situation where the only constraint
T r(b1 ) is in the equivalence T r(b1 ) ≡ B1 .
on the truth values of
is paradoxical, it follow that
this equivalence must be contradictory. This however contradicts the minimality conditions. Thus
We will therefore assume without loss of generality
Our basic argument applies to
arguments as for
i 6= 1.
T r(b2 ) must be included in some Bi .
mean that it cannot be part of
cases, rstly that it is only part of
We will consider the case where
or only part of
We will consider two
and secondly that it is part of some other
Bi ,
T r(b2 ) is only part of B1 , and T r(b1 ) is only part of B2 .
case the equivalences containing
T r(b2 ).
are the only equivalences that contrain the truth values
Given each of these is paradoxical, it follows that these two equivalences
are the only ones necessary for the contradiction, which contradicts the minimality condition. If
is only part of
so that this other
argument considered for
So either
is part of some
that is a part of some
we will reorder our series
The familiar argument means
above shows that this contradicts the minimality condition.
or we must be able to reorder the series so that there is a
This, however, will be impossible to continue once
so that
T r(bi )
must be a part of some
Bi+1 .
i = n, and hence our reductio assumption cannot
|Λ1 |, so it follows that, for any paradoxical sentence
We have now dealt with all possible cases for
a, a ∈ Refn (a)
If this sets up some self-contained reference loop, the
The argument will repeat for each following
hold. Hence
and some other
and we go through this step again.
We will now consider the case where
From this, the obvious result is as follows:
Proof. If a sentence
is Paradoxical, then it is not evaluable.
is Paradoxical, by Lemma 30, it follows that
a ∈ Refn (a)
by the Eval rules, it is provable that
¬Eval(a).
Proof. Firstly, if
cal to
with the Strengthened T-Schema as an axiom, that is
SLT *1 , {T r(a) ≡ (A ∧ Eval(a)) : all a} ` P .
SLT *1 ∪ {T r(a) ≡ (A ∧ Eval(a)) : all a}
Now the
is consistent (Lemma 26).
rules are in
SLT 1 ` P
is identi-
if, and only if,
is consistent exactly when
so for all sentences, either
SLT *1 ` Eval(a)
SLT *1 `
SLT *1 ` Eval(a),
SLT *1 , T r(a) ≡ (A ∧ Eval(a)) ` T r(a) ≡ A
SLT *1 , T r(a) ≡ A ` T r(a) ≡ (A ∧ Eval(a)).7
all a} ` P
(Lemma 8) and
SLT *1 , {T r(a) ≡ (A ∧ Eval(a)) :
SLT *1 , {T r(a) ≡ A : all A s.t. ` Eval(a)}, {T r(a) ≡ (A ∧ Eval(a)) :
all a s.t. ` ¬Eval(a)} ` P .
SLT *1 ` ¬Eval(a), then it follows
SLT 1 , ¬T r(a) ` T r(a) ≡ (A ∧ Eval(a)).
shown as follows:
proof of this is trivial.
SLT *1 , T r(a) ≡ (A ∧ Eval(a)) ` ¬T r(a)
The rst of these is Lemma 9.
The second can be
T r(a) ∨ (A ∧ Eval(a))
Disj. Syll, ln 2,4
T r(a) ⊃ (A ∧ Eval(a))
Introduction, ln 3
Introduction, ln 3-5
Elim ln 7
Eval(a) ∨ T r(a)
Intro ln 8
Disj. Syll ln1,9
(A ∧ Eval(a)) ⊃ T r(a)
Intro, ln7-10
T r(a) ≡ (A ∧ Eval(a)]
Itro, ln 6,11
This means that for all
that are not evaluable in
ened T-Schema are equivalent in
SLT *1 , ¬Eval(a), ¬T r(a)
and the Strength-
It follows that assuming the Strengthened T-Schema is equivalent to assuming the ordinary
T-Schema for evaluable sentences and assuming that
SLT *1 , {T r(a) ≡ (A ∧ Eval(a)) : all a} ` P
for unevaluable sentences. That is,
SLT *1 , {T r(a) ≡ A : all A s.t. `
Eval(a)}, {¬T r(a) : all a s.t. ` ¬Eval(a)} ` P .
everything that is provable in
is provable from
SLT *1 ∪{T r(a) ≡ A : all A s.t. `
is consistent, if, and only if,
SLT *1 ∪ {T r(a) ≡ A : all A s.t. ` Eval(a)} ∪ {¬T r(a) : all a s.t. ` ¬Eval(a)}
Now, by Lemma 29,
SLT *1 ∪ {T r(a) ≡ A : all A not paradoxical}
Lemma 31, all Paradoxical Sentences are not evaluable, so
Since, by
does not introduce an inconsistency.
The addition of the set of sentences of the form
SLT *1 , {T r(a) ≡ A : all A s.t. ` Eval(a)} ` T r(b)
SLT *1 ∪{T r(a) ≡ A : all A that are evaluable}
is consistent. It therefore, is only necessary to demonstrate that adding
SLT *1 ∪ {T r(a) ≡ (A ∧ Eval(a)) : all a}.
However, as just shown, everything provable in this is provable in
Eval(a)} ∪ {¬T r(a) : all a s.t. ` ¬Eval(a)}.
is equivalent to
SLT *1 0 T r(b)
can only introduce a consistency if
for at least one
Furthermore, for any relevant
all A s.t. ` Eval(a)} ` T r(b), T r(b) ≡ B
` ¬Eval(b).
By Lemma
SLT *1 , {T r(a) ≡ A :
{T r(a) ≡ A : all A s.t. ` Eval(a)}.
This means that the question of consistency reduces to the question of whether it is possible to
derive a
T r(b) such that SLT *1 ` ¬Eval(b) from the set {T r(a) ≡ A : all A s.t. ` Eval(a)} within
Firstly, the set
{T r(a) ≡ A : all A s.t. ` Eval(a)}
cannot be proven by means of a contradiction. Secondly, if
which means that
is provable, it must at the
very least appear in some Right Hand Side of an equivalence, since all other instances of
are tautological, i.e.
However, essentially the same situation
as in the case of the proof of Lemma 30 arises here:
¬A1
A1 ,
T r(a1 ) ≡ A1
must be provable. However, given that
T r(a1 ) ≡ A1 , it follows that neither A1
means that
is a member of the set. If
T r(a1 )
This in turn is only possible if
and so on. It follows that
is a part of some
T r(an )
Refn−m (am ) ,→ am
this contradicts the denition of the set. Therefore
- which
depending on
will not be provable, unless this chain eventually
refers back to some previous element in the chain, that is
However if that happens, it follows that
is provable or disprovable independently
is part of the relevant set. Exactly the same argument applies again, with
some further
is provable, then
is not provable independently of
are provable independently of
is only provable or disprovable if
T r(a1 ) ≡ A1 .
and hence
m < n.
is not evaluable. But
is not provable.
consistent, which means that
While this proof does not bring out the structure of
and the Strengthened T-Schema as
clearly as the soundness proof, it highlights the key reason why the Strengthened T-Schema succeeds in solving the Liar Paradox.
sentences are not evaluable.
ensure that
The key to this proof is showing that all of the paradoxical
Once this is established, the Strengthened T-Schema is shown to
This is, as noted in Chapter 7, the reason that the approach
works: the new concept of Evaluability isolates the paradoxical sentence by virtue of its place in
the Strengthened T-Schema.
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Logic Worksheet #1:
Adding and Subtracting Complex Numbers
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