 The language of arithmetic does not contain the unary predicate symbol \(T\), so let \(\mathcal{L}_T\) be the language of arithmetic augmented by the new unary predicate symbol \(T\) for truth. If \(\phi\) is a sentence of \(\mathcal{L}_T, \ulcorner \phi \urcorner\) is a name for \(\phi\) in the language \(\mathcal{L}_T\); formally speaking, it is the numeral of the Gödel number of \(\phi\). In general, Greek letters like \(\phi\) and \(\psi\) are variables of the metalanguage, that is, the language used for talking about theories of truth and the language in which this entry is written (i.e., English enriched by some symbols). \(\phi\) and \(\psi\) range over formulas of the formal language \(\mathcal{L}_T\).