Anaximenes

Anaximenes of Miletus, the third of the Milesian philosophers, named air (aēr) as the first principle of all things. His lasting contribution was to explain how one stuff becomes many: air, he taught, turns into everything else by condensation and rarefaction — thinning into fire, thickening into wind, cloud, water, earth, and stone.

This was a real advance, supplying a mechanism for change that his predecessors had lacked. As the breath or air that, he held, holds the cosmos together as our breath-soul holds us, his principle also linked the life of the world to the life of the soul.