Anaximenes

Life

Anaximenes was born around 586 BCE in Miletus. Ancient sources identify him as a student or associate of Anaximander, making him the third generation of the Milesian school. Very little is known of his personal life. He reportedly wrote a single prose treatise in a "simple and concise Ionian style," according to Diogenes Laërtius, though only testimonial fragments survive.

Air as Archē

Anaximenes selected air (aēr) as the fundamental substance. At first glance this appears to be a retreat from Anaximander's abstract apeiron back to a concrete element, similar to Thales' water. But Anaximenes' choice was more sophisticated than it appears. Air was associated with breath (pneuma) and soul (psychē) in Greek thought, and Anaximenes explicitly drew the analogy: "Just as our soul, being air, holds us together and controls us, so breath and air encompass the whole world." Air thus served as both material substrate and animating principle — a single substance that could account for both the physical and the vital.

Rarefaction and Condensation

Anaximenes' truly revolutionary contribution was his mechanism of change. Where Thales and Anaximander left the process by which the archē generates the plurality of things unexplained or vague, Anaximenes proposed a clear, quantitative mechanism: rarefaction (manōsis) and condensation (pyknōsis). When air rarefies — becomes thinner and more diffuse — it heats up and becomes fire. When it condenses — becomes denser and more compressed — it cools and becomes progressively wind, cloud, water, earth, and stone.

This theory is remarkable for several reasons. First, it reduces qualitative differences (the difference between fire and stone) to quantitative differences (degrees of density) — a move that anticipates modern materialist and reductionist thinking. Second, it provides a single, continuous mechanism that can generate all observed substances from one starting point. Third, it is in principle testable: one can observe that compressed air feels cooler (as when blowing with pursed lips) and that exhaled air with an open mouth feels warmer.

Cosmology

Anaximenes held that the earth is flat and broad, floating on air like a leaf. The heavenly bodies, likewise, ride on air. He explained the movement of the stars not as circular orbits below the earth but as lateral motions around it, "like a cap turning around the head." He held that the sun is hidden at night not by going under the earth but by being obscured by higher parts of the terrain — a theory that subsequent thinkers rejected but which shows creative engagement with observational problems.

Legacy

Anaximenes' influence extended well beyond the Milesian school. Diogenes of Apollonia revived air-philosophy in the fifth century. More importantly, the mechanism of rarefaction and condensation — the idea that qualitative change reduces to quantitative variation of a single substance — became a template for later physical theories. The atomists Leucippus and Democritus similarly explained qualitative diversity through quantitative differences in the arrangement and density of atoms. In this sense, Anaximenes' greatest contribution was not the specific choice of air but the introduction of a specifiable mechanism of transformation.

He died around 526 BCE.