First published Wed Aug 22, 2007; substantive revision Thu Oct 1, 2015

Anaxagoras of Clazomenae (a major Greek city of Ionian Asia Minor), a Greek philosopher of the 5 th century B.C.E. (born ca. 500–480), was the first of the Presocratic philosophers to live in Athens. He propounded a physical theory of "everything-in-everything," and claimed that nous (intellect or mind) was the motive cause of the cosmos. He was the first to give a correct explanation of eclipses, and was both famous and notorious for his scientific theories, including the claims that the sun is a mass of red-hot metal, that the moon is earthy, and that the stars are fiery stones. Anaxagoras maintained that the original state of the cosmos was a mixture of all its ingredients (the basic realities of his system). The ingredients are thoroughly mixed, so that no individual ingredient as such is evident, but the mixture is not entirely uniform or homogeneous. Although every ingredient is ubiquitous, some ingredients are present in higher concentrations than others, and these proportions may also vary from place to place (even if they do not do so in the original state of the cosmos). The mixture is unlimited in extent, and at some point in time it is set into motion by the action of nous (intellect). The mixture begins to rotate around some small point within it, and as the whirling motion proceeds and expands through the mass, the ingredients in the mixture are shifted and separated out (in terms of relative density) and remixed with each other, ultimately producing the cosmos of apparently separate material masses and material objects, with differential properties, that we perceive.

Anaxagoras, son of Hegesibulus (or Eubulus), was a native of Clazomenae, on the west coast of what is now Turkey. According to Diogenes Laertius (see the article on Doxography of Ancient Philosophy) (Diels-Kranz [DK] 59 A1), Anaxagoras came from an aristocratic and landed family, but abandoned his inheritance to study philosophy. (We do not know how he acquired his philosophical learning). He came to Athens (perhaps about the middle of the 5th century, perhaps earlier), and was a friend and protégé of Pericles, the Athenian general and political leader. There is controversy about his time in Athens; Diogenes Laertius says that he came to Athens to study philosophy as a young man. Some scholars claim that his arrival was as early as the Persian invasion of 480 (O'Brien 1968, Woodbury 1981, Graham 2006), others argue for a later date of about 456 (Mansfeld 1979, see also Mansfeld 2011 on Aristotle's evidence; there is a good discussion of the problems about dating Anaxagoras' life in Sider 2005). It is clear from their dramas that his work was known to Sophocles, Euripides, and perhaps Aeschylus (Seneca suggests in his Natural Questions 4a.2.17 that they shared Anaxagoras' view about the source of the Nile); it was certainly familiar to the comic playwright Aristophanes (Curd 2007, Sider 2005; see Mansfeld 1979). He was a resident in the city for at least twenty years; he was said to have been charged with impiety (and perhaps with Medism, political sympathy with the Persians), and banished from the city (437/6?). The charges against Anaxagoras may have been as much political as religious, because of his close association with Pericles. He retreated to Lampsacus (in the eastern Hellespont) where he died; ancient reports say that he was much honored there before and after his death. Democritus, a younger contemporary, dated his own life in relation to Anaxagoras', saying that he was young in the old age of Anaxagoras (DK59A5); he reportedly accused Anaxagoras of plagiarism. Although Anaxagoras lived in Athens when Socrates was a youth and young adult, there are no reports that Anaxagoras and Socrates ever met. In Plato's Phaedo, Socrates says that he heard someone reading from Anaxagoras' book (probably Archelaus, who was, according to Diogenes Laertius, pupil of Anaxagoras and teacher of Socrates). Anaxagoras is included in the ancient lists of those who wrote only one book: in the Apology, Socrates reports that it could be bought for a drachma.

As with all the Presocratics, Anaxagoras' work survives only in fragments quoted by later philosophers and commentators; we also have testimonia about his views in many ancient sources. The standard collection of Presocratic texts (both fragments and testimonia) is H. Diels and W. Kranz, Die