Anaximenes of Miletus

Anaximenes of Miletus was a Greek philosopher from Miletus, in the Ionian region of present-day Turkey, who lived in the sixth century BC. He was the last of the three thinkers of the Milesian School, after Thales and Anaximander. Historians regard the three as the first philosophers of the Western world, since they tried to explain the origin of things through natural causes rather than divine ones.

Little is known about his life. Theophrastus and Diogenes Laertius place him in Miletus, name his father as Eurystratus, and describe him as a follower and companion of Anaximander, probably much younger than him. Pliny the Elder credits him with the first study of shadows to measure the parts of the day, and with designing a sundial for that purpose. None of his writings survive. He is credited with a single work, On Nature, written in a simple and concise Ionic style that we know only through later quotations and summaries.

His central claim is that air is the source of everything. Where Thales chose water, and where Anaximander proposed the indefinite apeiron, Anaximenes picked a concrete and observable substance, one we notice every time we breathe. He linked air to the soul, which he thought held the body together in the same way that air holds the cosmos together.

His explanation of change rests on rarefaction and condensation. When air condenses it becomes wind, clouds, water, earth and finally stone; when it rarefies it becomes fire. To account for heat and cold, he noticed that breath feels warm when the mouth is open and cold when the lips squeeze the air, and concluded that heat and cold are effects of rarefaction and condensation rather than their causes. His cosmology imagines a flat Earth floating on air, with flat, fiery heavenly bodies that circle it like a felt cap turning on the head.

His influence reached later thinkers such as Heraclitus, Anaxagoras, Diogenes of Apollonia and Xenophanes, and he offered early natural explanations of earthquakes, lightning and the rainbow. Today he is less famous than his Milesian colleagues, but a lunar crater and an asteroid bear his name and keep his memory alive.