 The definition should be ‘materially adequate’ (trafny – a better translation would be ‘accurate’). This means that the objects satisfying \(\phi\) should be exactly the objects that we would intuitively count as being true sentences of \(L\), and that this fact should be provable from the axioms of the metalanguage. At first sight this is a paradoxical requirement: if we can prove what Tarski asks for, just from the axioms of the metalanguage, then we must already have a materially adequate formalisation of ‘true sentence of \(L\)’ within the metalanguage, suggesting an infinite regress. In fact Tarski escapes the paradox by using (in general) infinitely many sentences of \(M\) to express truth, namely all the sentences of the form