 In the early 1930s Alfred Tarski proposed a definition of truth. His definition consisted of a necessary and sufficient condition for a sentence in the language of a typical formal theory to be true; his necessary and sufficient condition used only notions from syntax and set theory, together with the primitive notions of the formal theory in question. In fact Tarski defined the more general relation ‘formula \(\phi(x_1 ,\ldots ,x_n)\) is true of the elements \(a_1 ,\ldots ,a_n\)’; truth of a sentence is the special case where \(n = 0\). For example the question whether 