The Peano Axioms are consistent. If not, mathematics (and the science resting on it) is overthrown. Moreover, it is absurd to suppose that they are merely contingently consistent: that in some other possible world a contradiction follows logically from them, but in the actual world no contradiction follows from them. So the Peano Axioms are necessarily consistent. But they aren't logically necessarily consistent: the consistency of the Peano Axioms cannot be proved (according to Goedel's second incompleteness theorem, not even if one helps oneself to the Peano Axioms in the proof, at least assuming they really are consistent). So we must suppose a necessity that isn't logical necessity, but is nonetheless very, very strong. We call it metaphysical necessity.