Anaximenes of Miletus was a 6th‑century BCE thinker associated with the Milesian school of early Greek philosophy. He is commonly regarded as a younger contemporary or follower of Anaximander and as part of a tradition seeking natural rather than mythic explanations for the world. Much of what is known about him survives only as brief reports and fragments preserved by later writers.

Main doctrine and terminology

Anaximenes is best known for identifying air (Greek: a?r, often rendered "aer") as the arche, or first principle, from which all things arise. Rather than appeal to an indefinite substance, he argued that qualitative differences in matter result from processes he called rarefaction and condensation. Through rarefaction air becomes fire; by condensation it becomes wind, then cloud, water, earth and stone.

He also extended the idea of air to animate life and mind, describing the soul as a form of airy breath whose proper balance produces consciousness and motion. His explanations show an early attempt to unify physical, biological and meteorological phenomena under a single natural principle.

Cosmology, method and surviving evidence

In surviving reports Anaximenes is represented as offering concrete pictures: the earth rests on air, and celestial bodies are related to heated or compressed air. He favored mechanical and continuous transformations over supernatural interventions. Our knowledge of his views comes mostly through later doxographers and commentators; only fragments and summaries survive, so reconstructions require caution.

  • Primary substance: air (aer) as material and active principle.
  • Key processes: rarefaction (making lighter/warmer) and condensation (making denser/cooler).
  • Scope: cosmology, meteorology, biology, and psychology treated as connected.

Anaximenes has been important historically as a representative of early Greek naturalism and monism. He stands between Thales, who is often credited with proposing a single originating substance, and later philosophers who developed atomistic or mathematical accounts. Modern readers value him for applying observation and simple physical ideas to a wide range of phenomena.

For introductory summaries and selections of fragments see general resources on Presocratic thought: biographical notes, comparative overviews at Milesian school, fragment collections referenced in many introductions (fragment editions) and modern discussions of early cosmology (further reading).