Anaximenes
c. 586–526 BC
The third Milesian, who proposed air as the origin of everything and supplied a mechanism for how it becomes other things.
Anaximenes was the third of the Milesian philosophers, after Thales and Anaximander, and the one whose method proved most durable even though his answer was wrong.
Air
Thales had proposed water as the origin; Anaximander rejected any familiar substance for the boundless apeiron. Anaximenes returned to something observable and chose air.
The choice looks like a step backward from Anaximander's abstraction. It was not, because of what he added.
Rarefaction and condensation
Anaximander had said that everything comes from the boundless and had not explained how. Anaximenes supplied a mechanism: air becomes other things by thickening and thinning.
Rarefied, air becomes fire. Condensed, it becomes wind, then cloud, then water, then earth, then stone. The differences between substances are differences of density in a single underlying stuff.
He supported it with an observation anyone can repeat: breath blown through pursed lips is cold, breath exhaled from an open mouth is warm. Compression and expansion change what something is like.
Why it matters
This is the first reductive quantitative explanation in Western thought. The apparent variety of substances is reduced to one substance plus a variable, and the variable is a matter of degree rather than kind.
The specific claim did not survive. The form of the claim is the one physics has made ever since — a single underlying constituent, differences explained by a measurable parameter, and a proposed mechanism rather than an assertion that change occurs.
Aristotle treated the Milesians as the beginning of philosophy for exactly this reason: they looked for a material principle and tried to say how it operates. See substance and pre-Socratic philosophy.
Questions
Who was Anaximenes?
The third Milesian philosopher (c. 586–526 BC), who held that air is the origin of everything and that it becomes other substances through condensation and rarefaction.
Why is Anaximenes important if his answer was wrong?
Because he supplied a mechanism. Reducing the variety of substances to one underlying stuff plus a measurable variable is the form of explanation physics has used ever since.
How did he differ from Anaximander?
Anaximander chose the boundless, an abstraction, and did not explain how things arise from it. Anaximenes chose something observable and gave a process — thickening and thinning — that could be tested against experience.