Anaximenes (c. 580–500 B.C.E.), who followed Anaximander in the Ionian school founded by Thales, believed that the primary substance of the universe was air. Air could itself change from hot to cold and back, so with air as the primary substance it was no longer necessary to explain how the primary substance caused the separate perceptible substances. Air could either expand or contract: expanded air became fire; contracted air became the denser materials of wind, cloud, water, earth, and stone. In many religious traditions, including Hindu yoga, life itself is breath. The ancient Greeks strongly held this association, going back to the eighth century B.C.E., but Anaximenes was the first to give it formal expression.
Most people think of Pythagoras in terms of his contributions to mathematics, but few realize that his work has also been important to philosophy (iStock).