In 1933 the Polish logician Alfred Tarski published a paper in which he discussed the criteria that a definition of ‘true sentence’ should meet, and gave examples of several such definitions for particular formal languages.
Sometimes we write or speak a sentence S that expresses nothing either true or false, because some crucial information is missing about what the words mean. If we go on to add this information, so that S comes to express a true or false statement, we are said to interpret S, and the added information is called an interpretation of S.
A sentence S divides all its possible interpretations into two classes, those that are models of it and those that are not. In this way it defines a class, namely the class of all its models, written Mod(S). To take a legal example, the sentence
The development of proof theory can be naturally divided into: the prehistory of the notion of proof in ancient logic and mathematics; the discovery by Frege that mathematical proofs, and not only the propositions of mathematics, can (and should) be represented in a logical system; Hilbert's old axiomatic proof theory; Failure o