Anaximenes of Miletus (c. 585–c. 525 BC)
Anaximenes was a Greek philosopher and probable student or associate of Anaximander, who taught that air is the fundamental material of the universe; this becoming, for instance, fire on rarefaction, water, and finally earth on condensation. He was also the first to draw a clear distinction between planets and stars. Anaximenes believed the Sun to be hot because of its quick motion around the Earth, that the stars were too remote to send us detectable heat, and that the stars were fastened to a crystalline sphere. As did Anaximander and Thales, Anaximenes hailed from the Greek city of Miletus in what is modern-day Turkey.