Anaximenes and the Milesian achievement

Anaximenes of Miletus (c. 585–525 BCE), the third and last of the great Milesians, proposed that the archē is aēr (aēr = air, mist, breath). In choosing a definite substance he seems to step back from Anaximander’s boundless apeiron — but he keeps something of it, for air is invisible, unlimited in extent, and everywhere.

His real breakthrough is not the choice of air but the mechanism of change he attaches to it. All other substances arise from air through two opposite processes: rarefaction (manōsis, thinning out) and condensation (pyknōsis, packing together). Crucially, this is a single, quantitative account — differences of stuff are reduced to differences of density of one stuff.

Follow the recipe. Air rarefied (thinned) becomes fire. Air condensed (packed) becomes, in stages, wind, then cloud, then water, then earth, and finally stone — the densest thing of all. One substance, one dial marked “more or less packed together,” and the whole ladder of materials falls out along it.

Step back and take in what these three men achieved together. They created rational cosmology: a general account of the origin and structure of the world argued from natural principles. They were monists — each held that reality is fundamentally one kind of stuff (water, the boundless, air), unity beneath multiplicity.

Above all, the Milesians set philosophy’s agenda. Two questions they framed would drive the next two centuries of Greek thought: What is reality ultimately made of? and How does change happen at all? Parmenides, Heraclitus, the atomists, Plato and Aristotle are all, in part, answering the Milesians.