Anaximenes 586–526 BCE

On a clear morning in Miletus, the air feels almost weightless. A faint breeze moves between the houses, stirring dust in the streets, filling sails in the harbor, rising as a shimmer over the tiled roofs as the day warms. Most people barely notice it unless it fails; only when the wind drops and the city lies still in heavy heat do they realize how much their comfort depends on something they cannot see. For Anaximenes, the third of the old Milesian inquirers, that invisible element was not an afterthought but the key to everything. Where Thales had said that all is water and Anaximander had spoken of the indefinable apeiron, Anaximenes answered in a single, ordinary Greek word: air.

He did not mean air in the trivial, background sense of “whatever happens to be between things.” He meant air as the continuous, unseen substance that pervade ...