All knowledge is conjectural. It's our best understanding at any given ime, you're right to say that the axioms might be incorrect. How do we know that an axiom is incorrect? Traditionally, the answer has been because it's clearly and obviously the case. So even in mathematics, where we try and be as precise as possible, it's possible for people to make errors. And by the way, this particular example of aclidian geometry, because geometry was traditionally always done in two dimensions on a piece of paper, led to geometry in curved space which led to einstein coming up with the general theory of relativity.