If you want an example of genuinely conflicting axiomatic systems, look no further than Euclidean geometry vs. hyperbolic geometry. If you adopt the view that there is some “absolute truth” floating out there somewhere, then one of them must be “right” and the other “wrong”.
But you cannot decide which is right or wrong because you can’t compare two axiomatic systems with different axioms. There’s no way to decide which axioms are right. If you could decide such a thing, you could prove those axioms.
Some other examples of mathematical ideas that are counterintuitive or contentious:
- Pretty much any expansion of the real number system was fought every step of the way, such as the complex number system and the hyperreal number system. So if you ask something like “Does x^2+1=0 have a solution?” the answer will depend on the axioms you’ve chosen.
- Some mathematicians to this day still dislike the inclusion of the Axiom of Choice in set theory.
Some arithmetical examples:
- You may have heard of floating point arithmetic, which deals with formal ways of approximating tedious calculations. It is a common joke that 2+2 could equal 5 in floating point arithmetic because of rounding errors.
- Some versions of arithmetic are uninterested in equality and look at weaker relations, such as congruence. Modular arithmetic lets you choose a modulus and regard two numbers as equivalent provided that their difference is a multiple of the modulus.
- I’ve already given the example of the zero ring, where 1+1=0. You can dismiss this as trivial, but it’s simply meant to illustrate that there are other consistent ways of defining addition and multiplication. I’m sure we could dream up more interesting examples. If we couldn’t, abstract algebra would hardly be worth the time as a subject, no? Another example: In fields of characteristic 2, we get the nice formula (x+y)^2=x^2+y^2
I should have known that I would have to quote the theorem for you since you can’t be bothered to look it up. Here is Gödel’s second incompleteness theorem: “For any formal effectively generated theory T including basic arithmetical truths and also certain truths about formal provability, if T includes a statement of its own consistency then T is inconsistent.”
In plain English, sufficiently complicated axiomatic systems cannot be used to demonstrate their own consistency. You cite Peano arithmetic as an example, but that isn’t sufficiently complicated. Peano arithmetic is only a first-order theory. For heavy-duty mathematics to get off the ground, that isn’t enough. For example, you can’t prove that the natural numbers are countably infinite using only first-order logic. Basically, you can only talk about individuals of sets with first-order logic, not the sets themselves.