Anaxagoras (c. 500-428 bc), Greek philosopher who introduced the notion of nous (Greek, “mind” or “reason”) into the philosophy of origins; previous philosophers had studied the elements (earth, air, fire, water) as ultimate reality.

Born in Clazomenae (near modern İzmir, Turkey), Anaxagoras was the first philosopher to settle (c. 480) in Athens, later a flourishing centre of philosophy. His pupils included the Greek statesman Pericles, the Greek dramatist Euripides, and probably Socrates. Anaxagoras had taught in Athens for about 30 years when he was imprisoned for impiety for suggesting that the Sun was a hot stone and the Moon made of earth. He later went to Ionia (in Asia Minor) and settled at Lampsacus, a colony of Miletus, where he died.

Anaxagoras explained his philosophy in Peri Physeos (On Nature), but only fragments of the books have survived. He held that all matter had existed originally as atoms, or molecules; that these atoms, infinitely numerous and infinitesimally small, had existed from all eternity; and that order was first produced out of this infinite chaos of minute atoms through the influence and operation of an eternal intelligence (nous). He also believed that all bodies are simply aggregations of atoms; for example, that a bar of gold, iron, or copper is composed of inconceivably minute particles of the same material.

Anaxagoras marks a great turning point in the history of Greek philosophy: his doctrine of the nous was adopted by Aristotle, and his doctrine of atoms prepared the way for the atomic theory of the philosopher Democritus.