Anaximenes of Miletus
Anaximenes of Miletus a Greek philosopher of the late sixth century B.C. He belonged to the Milesian school, which was founded by Thales and continued by Anaximander. Anaximenes believed air to be the primary substance of nature. The air, he said, is in eternal motion, and forms the universe by two opposite processes: condensation and expansion. As air is condensed it grows cold and forms—by successive degrees—wind, clouds, water, earth, and stones. As air is expanded it grows hot and becomes fire, which is borne upward to form the heavenly bodies. His theory explained everything in the universe by a material cause and by a physical law.