B: Or, truths of arithmetic have been coded into your instincts by evolution, but truths of Maxwell's equations aren't.

A: But dude! How the fuck do you know they're truths?

B: Regarding arithmetic, I know they're truths because I've defined the underlying territory. I - collectively speaking - decided on axioms from which the whole tree of lemmata and corollaries spawns. Granted, I cannot fully track all the branches of the tree due to Goedel, there will be true statements that follow from my axioms that I'll never be able to trace back to the roots (which would be the proof), but the statements that I do prove I can rely upon to be true.

Why? Because it is I who made up the whole system, I don't need to match it to any external system of unknowns. Unlike my model of physics.