The formalist thesis comprises two claims.
1 Pure mathematics can be expressed as uninterpreted formal systems, in which the truths of mathematics are represented by formal theorems.
2 The safety of these formal systems can be demonstrated in terms of their freedom from inconsistency, by means of meta-mathematics.
Kurt Godel’s Incompleteness Theorems (Godel,1931) showed that the programme could not be fulfilled. His first theorem showed that not even all the truths of arithmetic can be derived from Peano’s Axioms (or any larger recursive axiom set).