 It is, nonetheless, this formula which Gödel uses to construct a sentence which is undecidable in \(\mathsf{P}\). This can be accomplished by the application of the so-called Diagonal Lemma  (see Gödel’s incompleteness theorems)  which states that for every formula \(\varphi(x)\) of \(\mathsf{P}\), there exists a sentence \(\psi_{\varphi}\) such that 