 Tarski showed that the liar paradox is formalisable in any formal theory containing his schema T, and thus any such theory must be inconsistent. This result is often referred to as Tarski’s theorem on the undefinability of truth. The result is basically a formalisation of the liar paradox within first-order arithmetic extended with the \(T\)-schema. In order to construct such a formalisation it is necessary to be able to formulate self-referential sentences (like the liar sentence) within first-order arithmetic. This ability is provided by the diagonal lemma. 