This proof-theoretic result has since been exemplified in mathematics by Paris and Harrington, whose version of Ramsey’s Theorem is true but not provable in Peano Arithmetic (Barwise, 1977). The second Incompleteness Theorem showed that in the desired cases consistency proofs eequire a meta-mathematics more powerful than the consistency of Peano Arithmetic requires all the axioms of that system and further assumptions, such as the principle of transfinite induction over countable ordinals (Gentzen, 1936).