This question always comes whenever we face something that is supposed to be an ultimate limit, an ultimate truth. In mathematical language this is called an axiom.
Mathematical language is abstract, but it has a certain beauty to it. So seemingly far away from anything real, sometimes it offers us a means of penetrating into the depths of reality.
Pythagoras taught that “all is number”. Musical frequencies, waves of light by which we perceive the world. They are expressed in numbers.
All things need to stand on something. Just as the Earth needed to rest on four elephants who in their turn needed to stand on a giant turtle, we need to ground our understanding of things on something solid.
An axiom is the giant turtle, it is a unit of truth. But the Turtle stands on nothingness, and so does an axiom. It is unprovable by logic, we just agree on it.
We agree on it until the bitterness of doubt becomes unbearable. So it happened with parallel lines. An axiom says that they do not intersect. And for 2000 years no one questioned it.
Euclid who discovered this axiom built an entire geometry with an infinite amount of most surprising theorems, both for thought and the eye.
For example, mysteriously enough for ANY triangle, a single unique circle passes exactly through nine specific points.
And many more of the sort, each time with weird coincidences and a particular symbolism of numbers.