Anaximenes (circa 570-500 bc), Greek philosopher of nature, the last member of the Ionian school founded by the philosopher Thales. Born at Miletus, Ionia, in Asia Minor, he held that air is the primary element to which everything else can be reduced. To explain how solid objects are formed from air, he introduced the notions of condensation and rarefaction. These processes, he claimed, make air, in itself invisible, visible as water, fire, and solid matter. He thought that air becomes warmer and turns to fire when it is rarefied and that it becomes colder and turns solid when it is condensed. His importance lies not in his cosmology but in his attempt to discover the ultimate nature of reality.