| > When most mathematicians say something like "X is true", what they mean is "X can be proved from the axioms of set theory". I'm just one mathematician, but I certainly don't mean that. Before we can prove anything about sets, we need to pick some axioms. Zermelo set theory (Z) would be enough for most of ordinary mathematics. If we need something stronger, there's Zermelo–Fraenkel set theory with the axiom of choice (ZFC). Or if I need something even stronger, there's, for example, Tarski–Grothendieck set theory (TG). What I mean by "X is true" is technically difficult to define. The statements (1) X is provable in Z. (2) X is provable in ZFC. (3) X is provable in TG. are all increasingly accurate characterizations of "X is true", but none of them capture everything about it. And that's kind of the point. There is no proof system P such that "X is provable in P" would work as a faithful definition of "X is true". So the best we can get is this tower of increasingly sophisticated axioms that still always fail to capture the full meaning of "truth". There is a convention among mathematicians: Anything up to ZFC you can assume without explicitly mentioning it, but if you go beyond it, it's good to state what axioms you have used. ZFC is not a bad choice for this role. It is quite high in the tower. In most cases ZFC is strong enough, or in fact, overkill. But still, it is not at the top of the tower (there is no top!), so sometimes you need stronger axioms. The fact that ZFC has been singled out like this is ultimately a bit arbitrary - a social convention. "X is provable in ZFC" may be the most common justification for "X is true", but that doesn't make it the definition of "X is true". |