Answer:
The basic answer to your question is that we have to start somewhere.
The essence of mathematics (in the sense the Greeks introduced to the
world) is to take a small set of fundamental "facts," called
postulates or axioms, and build up from them a full understanding of
the objects you are dealing with (whether numbers, shapes, or
something else entirely) using only logical reasoning such that if
anyone accepts the postulates, then they must agree with you on the
rest.
It can sometimes be proven.