Anaximenes

Definition: Anaximenes (d. c. 528 B.C.) was a Pre-Socratic philosopher, together with Anaximander and Thales members of what we call the Milesian School because all three were from Miletus and may have studied with one another. Where Anaximander believed the universe was composed of an indefinite substance he called apeiron, Anaximenes believed the underlying substance of the universe was air because air is neutral but can take on various properties.The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.).

Anaximenes is on the list of Most Important People to Know in Ancient History.