Can second order peano arithmetic prove that first order peano arithmetic is sound?
Note that I'm not just talking about its axioms, but also its theorems.
Can second order peano arithmetic prove that first order peano arithmetic is sound?
Note that I'm not just talking about its axioms, but also its theorems.
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Yes. Second-order Peano arithmetic can create the truth predicate for sentences of first-order Peano arithmetic, and verify that every theorem of first-order PA is true under that predicate. In this way, second-order arithmetic proves the consistency of first-order PA, and proving the consistency of a theory is how we normally look at proving a theory to be sound.