  Tarski’s theory of truth (the semantic conception of truth) has two aspects: philosophical and formal. Philosophically, it is a version of Aristotle’s idea that truth consists in saying that what is, is and what is not, is not (it is related to the idea of correspondence). However, the main problem was formal. Tarski had to offer a construction free of semantic paradoxes, in particular the Liar. He achieved this goal by postulating that the concept of truth must be defined for a definite, well-constructed formalized language L. However, the definition itself should be formulated in the metalanguage ML. The definition is to be formally correct, that is, it cannot lead to contradictions and has to satisfy the usual conditions of correctness (non-circularity, etc.). It should also be materially adequate. According to Tarski, the basic intuition is captured by the T-scheme: s is true if and only if P, where the letter s represents the name of a sentence and P is a translation of this sentence into the metalanguage ML. Now the condition of material adequacy (the Convention T) says that a truth definition TD is materially adequate if and only if all equivalences (that is, for all sentences in L) stemming from the T-scheme by appropriate replacements are provable from the definition. The conditions are satisfied by the following definition: