# Anaximenes

Anaximenes of Miletus (c. 586 – 526 BCE) was the third and last of the Milesian
natural philosophers, the pupil and successor of Anaximander. Where his teacher
had named an indefinite *apeiron* as the source of all things, Anaximenes
returned to a familiar substance and chose **air**. The choice looks like a step
backward toward Thales and his water, but the argument he attached to it was
new, and it is the argument that earned him his place.

## Air as the first principle

Air, he said, is the one thing that surrounds and sustains everything else.
It is invisible when evenly spread, and becomes visible only when it changes
state. The soul, which is breath, holds a person together; in the same way air
holds the whole cosmos together. The earth itself is a flat leaf riding on air,
and the heavenly bodies are leaves of fire carried around by the same current.

## Rarefaction and condensation

His lasting contribution is a mechanism. Air becomes other things by being
thinned or thickened:

1. Rarefied, air becomes fire.
2. Condensed a little, it becomes wind, then cloud.
3. Condensed further, it becomes water, then earth, then stone.

Every difference in kind is reduced to a difference in degree of one quantity.
He offered an observation in support: breathe on your hand with the mouth open
and the breath is warm; breathe with the lips pressed together and it is cold.
Compressed air is cold, loose air is hot, so temperature too is a matter of
density.

> Just as our soul, being air, holds us together, so breath and air surround
> the whole world.

## Why it mattered

The point is not that air is the right answer. It is that Anaximenes gave a
single, testable process by which one stuff takes on many forms, and he tied
the process to something anyone could feel on the back of a hand. Later Greek
physics kept the habit of explaining quality by quantity, and the atomists,
a century on, still owed him the idea.

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*Stoa Press · Philosophy Essays · Markdown edition*