Anaxag'oras, one of the most eminent Ionic philosophers, was born at Clazomenæ, in Ionia, 500 B.C. He belonged to a wealthy family, but devoted himself exclusively to intellectual pursuits; and, still young, moved to Athens, where, in the course of a thirty years' sojourn, he acquired a high reputation, and had many illustrious pupils, among whom were Pericles, Euripides, and possibly Socrates. But at last, his explanations of physical phenomena by natural causes rendered him suspicious to the polytheists; he opposed divination and astrology; and, being accused of impiety towards the gods, he was condemned to death. His sentence, however, was commuted into banishment for life, through the eloquence of Pericles. He withdrew to Lampsacus, on the Hellespont, where he died in 428.
It is not easy to ascertain what were the opinions of Anaxagoras in philosophy. Fragments merely of his works have been preserved, and even these are sometimes contradictory. Of one thing we are certain, that he had a deeper knowledge of physical laws than any of his predecessors or contemporaries. He also arrived at some tolerably accurate conclusions regarding the cause of the moon's light, of eclipses, earthquakes, meteors, of the rainbow, of wind, and of sound. His great contribution to ancient philosophy, however, was his doctrine as to the origin of all things. He held that all matter existed originally not in the form of the so-called elements, but in the condition of atoms; 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 minutiae through the influence and operation of an eternal intelligence (Gr. nous). He also maintained that all bodies were simply aggregations of these atoms, and that a bar of gold, or iron, or copper was composed of inconceivably minute particles of the same material; but he did not allow that objects had taken their shape through accident or blind fate, but through the agency of this 'shaping spirit' or Nous, which he described as infinite, self-potent, and unmixed with anything else. 'Nous,' he again says, 'is the most pure and subtle of all things, and has all knowledge about all things, and infinite power.' His theory of the Nous was vague, but makes a great advance in the direction of theism, though personality is not attributed to the Nous. Anaxagoras marks a great turning-point in the history of speculation: while, on the one hand, his doctrine of the Nous passed to Aristotle, his doctrine of atoms prepared the way for Democritus (q.v.) and the Atomie School. With Anaxagoras, too, philosophy left its old home in Ionia, and established itself in its most famous seat—in Athens. His most notable work, On Nature, has survived only in fragments, edited by Schaubach (1827), and by Schorn (1829).