 Alfred Tarski's great accomplishment was to show how to give — contra this scepticism — a formal definition of truth for a wide class of formalized languages. Tarski did not, however, show how to give a definition of truth for languages (such as English) that contain their own truth predicates. He thought that this could not be done, precisely because of the liar's paradox. More generally, Tarski reckoned that any language with its own truth predicate would be inconsistent, as long as it obeyed the rules of standard classical logic, and had the ability to refer to its own sentences. As we will see in our remarks on Theorem 2.1 in Section 2.3, Tarski was not quite right: there are consistent classical interpreted languages that refer to their own sentences and have their own truth predicates. (This point originates in Gupta 1982 and is strengthened in Gupta and Belnap 1993.)