Contrary to nouns and verbs that either do not include a co-positing of parts, including nouns and some verbs, or if they are, their parts do not significate separately, a sentence (λόγος) is a ‘significant portion of speech by co-positing, its parts signify something separately, though not as a positive or negative judgment but as utterance.’ (OI ., I, 4, 16b26-28). Therefore, every utterance in language that i) includes parts, ii) its signification is based on the co-positing of its parts, iii) each of its parts is separately significant but iv) the signification of its parts is the signification of an utterance and not a judgment, is a sentence. This definition distinguishes a sentence from:
a) Nouns: because they do not have parts;
b) Verbs: because they either do not have parts or if they have, their parts do not significate separately;
c) Those co-positings of words whose significance is not based on their co-positing. Thus a co-positing like ‘wall tree house’ is not a sentence because it has no significance on the basis of its co-positing;
d) Those co-positings that the signification of at least some of their parts is the signification of judgments. Thus, ‘Socrates is from Athens and is a philosopher’ is not a sentence because at least one of its parts, e.g. ‘Socrates is from Athens’ signifies a judgment.
Those sentences that either of truth or falsity belongs to them are propositions (ἀπόφανσις or λόγος ἀπόφαντικὸς). (OI., I, 4, 17a2-3) It is this and only this kind of sentence that is the subject of study in philosophy. (OI., I, 4, 17a5-7) The primary forms of a proposition are, firstly, an affirmation (κατάφασις) and then a negation (ἀπόφασις). (OI., I, 4, 17a8-9) To have an affirmation or a denial and, thus, a proposition, what firstly is necessary is a verb or the tense of a verb (OI, I, 4, 17a9-12; I, 10, 19b12-13; OI, I, 4, 16b28-30) and a noun (OI, I, 10, 19b10-12).
Aristotle draws a contrast between single and plural proposition:
a) Single proposition: a proposition that ‘indicate a single fact, or the conjunction of the parts of which results in unity.’ (OI., I, 5, 17a15-16)
b) Plural propositions: a proposition that ‘are separate and many in number, which indicate many facts, or whose parts have no conjunction.’ (OI., I, 5, 17a16-17)
He also differentiates between simple and composite propositions (OI, I, 5, 17a20-22):
a) Simple proposition: ‘that which asserts or denies something of something’;
b) Composite propositions: ‘that which is compounded of simple propositions.’
A simple (ἁπλῆ) proposition is a statement about the belonging or not belonging of something (OI, I, 5, 17a22-24). Affirmation and denial are indeed kinds of simple proposition, one affirming the belonging and the other not belonging. An affirmation or denial may be single or non-single. A single affirmation or denial ‘signifies some one fact about some one subject. This singularity is destroyed neither by the universality of subject nor by the universal character of the statement. The only thing that destroys this singularity is that one part signifies more than one thing, which makes the proposition more than one proposition (OI, I, 8, 18a13-27). In other words, neither ‘predication of one thing of many subjects’ nor ‘many things of the same subject’ results in a unitary proposition. (OI., II, 11, 20b13-16) There is also another cause that might destroy the unity of predicate and thus the unity of proposition, namely when one of the parts of the predicate is implicit in the other. Thus, e.g. animal-man does not form a unity because the notion ‘animal’ is implicit in ‘man.’ (OI., II, 11, 21a16-18)
The following are characteristics of propositions:
1. That about which an affirmation signifies something is a noun, whether definite or indefinite. (OI., I, 10, 19b5-12) Moreover, there can be no affirmation or denial without a verb. (OI., I, 10, 19b12-19) Therefore, a proposition must involve at least a noun and a verb.
2. Contrary propositions: since both belonging and not belonging of something to something is possible, we are able to make a negative proposition out of an affirmative one and vice versa. (OI., I, 5, 17a26-31) Therefore, ‘every affirmation has an opposite denial’ and vice versa. (OI., I, 6, 17a31-33) The contradictory of each proposition is made by changing the positivity or negativity of the verb ‘to be’ to its opposite but not by changing the subject to its opposite. Thus, the contradictory of ‘man is white’ is ‘man is not white’ and not ‘not-man is white.’ (OI., II, 11, 21a38-b5) What happens in the mentioned wrong form of contradictory propositions is that they change the subject while the denial must be the contrary of the affirmation about the same subject. (cf. OI., II, 14, 24b1-6) This is made plain in propositions involving a verb other than ‘to be’. What must be changed in these propositions is the verb and not the noun or the subject. Thus, the contradictory of ‘Socrates runs’ is ‘Socrates does not run’ and not ‘not-Socrates runs.’
3. Conversion possible: ‘Out of conversion (μετατιθεμένου) of the name and the verb, the same affirmation and denial is generated. (OI., I, 10, 20b10-12) In other words, the two affirmations or denials ‘signify the same thing.’ (OI, I, 10, 20b1-2)