Keeping in mind that Tegmark's idea is 2,500 years old and the basis of Western civilization. Pythagoras was the biggest philosophical influence on Plato. And, according to Aristotle (150 years later), Pythagoras believed that "all is number".

Thales (the "first" Greek philosopher) believed that the underlying principle of the world was water, a kind of flowing force. Anaximenes, his student, believed the underlying principle was air, a sort of bouncing around set of particles.

Pythagoras believed that underneath everything, the most basic underlying principle was math. And not just that: he believed that the natural harmonies in mathematics led to the natural harmonies in the kosmos (a term he is credited with introducing, along with the term "philosopher").