 If the language under discussion (the object language) is \(L\), then the definition should be given in another language known as the metalanguage, call it \(M\). The metalanguage should contain a copy of the object language (so that anything one can say in \(L\) can be said in \(M\) too), and \(M\) should also be able to talk about the sentences of \(L\) and their syntax. Finally Tarski allowed \(M\) to contain notions from set theory, and a 1-ary predicate symbol True with the intended reading ‘is a true sentence of \(L\)’. The main purpose of the metalanguage was to formalise what was being said about the object language, and so Tarski also required that the metalanguage should carry with it a set of axioms expressing everything that one needs to assume for purposes of defining and justifying the truth definition. The truth definition itself was to be a definition of True in terms of the other expressions of the metalanguage. So the definition was to be in terms of syntax, set theory and the notions expressible in \(L\), but not semantic notions like ‘denote’ or ‘mean’ (unless the object language happened to contain these notions).