I think the question should go even deeper. There are so many fundamental axioms that must be accepted on faith alone. The question I usually start with is "Can anyone prove that numbers exist outside of our imagination?" I not talking simply about perception. Even I believe that if I perceive that I am hit with a brick then the brick exists. We have no senses that can detect numbers. When I asked this question to any of the several mathematicians that I know, the answer has always been ~ Yeah, good question ~ and then they move on.