1 + 1 = 2 is a definition of 2, as the successor of 1. It is not a particularly fundamental one, as we can do arithmetic without it, just not in any base > 2.

1 + 1 = 2 is a definition of 2, as the successor of 1. It is not a particularly fundamental one, as we can do arithmetic without it, just not in any base > 2. Definitions are axioms, and they are inescapable in philosophy. Rationality cannot get you any further than what follows from your axioms.