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Anaxagoras (c. 500 - 428 B.C.) was an early Pre-Socratic Greek philosopher from Ionia, although he was one of the first philosophers to move to Athens as a base.
He is sometimes considered to be part of the poorly-defined school of Pluralism, and some of his ideas also influenced the later development of Atomism. Many of his ideas in the physical sciences were quite revolutionary in their day, and quite insightful in retrospect.
Anaxagoras (pronounced an-ax-AG-or-as) was born around 500 B.C. to an aristocratic and landed family in the city of Clazomenae (or Klazomenai) in the Greek colony of Ionia (on the west coast of present-day Turkey). As a young man, he became the first of the major Pre-Socratic philosophers to move to Athens (which was then rapidly becoming the centre of Greek culture), where he remained for about thirty years.
During this time he became a favourite (and possibly a teacher) of the prominent and influential statesman, orator and general Pericles (c. 495 – 429 B.C.), one of the architects of Athens' primacy during the Golden Age. Although it seems that Anaxagoras and the young Socrates never actually met, one of Socrates' teachers, Archelaus, studied under Anaxagoras for some time. His work was also known to the major writers of the day, including Sophocles, Euripides, Aeschylus and Aristophanes.
In about 450 B.C., however, Anaxagoras was arrested by Pericles' political opponents on a charge of contravening the established religion by his teachings on origins of the universe, the first philosopher before Socrates to be brought to trial for impiety. With Pericles' influence he was released, but he was forced to retire from Athens to exile in Lampsacus in Ionia, where he died around the year 428 B.C.
Anaxagoras wrote at least one book of philosophy, but only fragments of the first part of this have survived in work of Simplicius of Cilicia in the 6th Century A.D.
He is best known for his cosmological theory of the origins and structure of the universe. He maintained that the original state of the cosmos was a thorough mixture of all its ingredients, although this mixture was not entirely uniform, and some ingredients are present in higher concentrations than others and varied from place to place. At some point in time, this primordial mixture was set in motion by the action of nous ("mind"), and the whirling motion shifted and separated out the ingredients, ultimately producing the cosmos of separate material objects (with differential properties) that we perceive today.
For Anaxagoras, this was a purely mechanistic and naturalistic process, with no need for gods or any theological repercussions. However, he did not elucidate on the precise nature of Mind, which he appears to consider material, but distinguished from the rest of matter in that it is finer, purer and able to act freely. It is also present in some way in everything, a kind of Dualism.
Anaxagoras developed his metaphysical theories from his cosmological theory. He accepted the ideas of Parmenides and the Eleatics that the senses cannot be trusted and that any apparent change is merely a rearrangement of the unchanging, timeless and indestructible ingredients of the universe. Not only then is it impossible for things to come into being (or to cease to be), he also held that there is a share of everything in everything, and that the original ingredients of the cosmos are effectively omnipresent (e.g. he argued that the food an animal eats turns into bone, hair, flesh, etc, so it must already contain all of those constituents within it). He denied that there is any limit to the smallness or largeness of the particles of the original cosmic ingredients, so that infinitesimally small fragments of all other ingredients can still be present within an object which appears to consist entirely of just one material (presaging to some extent the ideas of Atomism).
In the physical sciences, Anaxagoras was the first to give the correct explanation of eclipses, and was both famous and notorious for his scientific theories, including his claims that the sun is a mass of red-hot metal, that the moon is earthy, and that the stars are fiery stones.
See the additional sources and recommended reading list below, or check the philosophy books page for a full list.