Anaxagoras c. 500 BC – c. 428 BC

Anaxagoras of Clazomenae was an ancient Greek Pre-Socratic philosopher of the 5th century BC, born in Ionia and active for many years in Athens, where he was a friend and reportedly a teacher of Pericles. He held that the cosmos is composed of an infinite number of qualitatively distinct elementary particles, each containing a portion of all things, and that these particles were originally ordered into the perceptible cosmos by a cosmic Mind (nous). The introduction of nous as a principle of cosmic order was widely regarded in antiquity as a major step in the development of Greek philosophy. He was eventually charged in Athens with impiety for his naturalistic explanation of the sun and moon as physical bodies rather than divinities. His writings survive only in fragments.

Anaxagoras was born around 500 BC at Clazomenae in Ionia. Around 480 BC, perhaps in the wake of the Persian retreat from Greece, he came to Athens, where for three decades he was a familiar figure in the circle of the statesman Pericles. Plutarch reports that Pericles attended his lectures and that the close intellectual association of the two men was used by Pericles's enemies to bring Anaxagoras to trial.

His prose treatise On Nature, of which substantial fragments survive in Simplicius and others, taught that the original cosmos was a homogeneous mixture of infinitely many qualitatively different substances and that an immaterial intelligence (nous) set this mixture in motion to produce a vortex that separated the heavenly bodies and the four regions of the cosmos. He explained the heavenly bodies as red-hot stones, the moon as earthy and reflective of the sun's light, and eclipses by the interposition of bodies.

Charged with impiety for these doctrines around 450 BC, Anaxagoras was forced into exile and retired to Lampsacus on the Hellespont, where he opened a school and was honored after his death around 428 BC by the dedication of an annual school holiday. His introduction of mind as a cosmic principle was praised by Plato and Aristotle, both of whom criticized him for not making sufficient use of it once introduced.