The philosopher Anaxagoras, though not the equal of Pythagoras, Heraclitus, or Parmenides, has nevertheless a considerable historical importance. He was an Ionian, and carried on the scientific, rationalist tradition of Ionia. He was the first to introduce philosophy to the Athenians, and the first to suggest mind as the primary cause of physical changes. He was born at Clazomenae, in Ionia, about the year 500 B.C., but he spent about thirty years of his life in Athens, approximately from 462 to 432 B.C.

He was probably induced to come by Pericles, who was bent on civilizing his fellow-townsmen. Perhaps Aspasia, who came from Miletus, introduced him to Pericles. Plato, in the Phaedrus, says:

Pericles 'fell in, it seems with Anaxagoras, who was a scientific man; and satiating himself with the theory of things on high, and having attained to a knowledge of the true nature of intellect and folly, which were just what the discourses of Anaxagoras were mainly about, he drew from that source whatever was of a nature to further him in the art of speech.'

It is said that Anaxagoras also influenced Euripides, but this is more doubtful.

The citizens of Athens, like those of other cities in other ages and continents, showed a certain hostility to those who attempted to introduce a higher level of culture than that to which they were accustomed. When Pericles was growing old, his opponents began a campaign against him by attacking his friends. They accused Pheidias of embezzling some of the gold that was to be employed on his statues. They passed a law permitting impeachment of those who did not practise religion and taught theories about 'the things on high'. Under this law, they prosecuted Anaxagoras, who was accused of teaching that the sun was a red-hot stone and the moon was earth. (The same accusation was repeated by the prosecutors of Socrates, who made fun of them for being out of date.) What happened is not certain, except that Anaxagoras had to leave Athens. It seems probable that Pericles got him out of prison and managed to get him away. He returned to Ionia, where he founded a school. In accordance with his will, the anniversary of his death was kept as a schoolchildren's holiday.

Anaxagoras held that everything is infinitely divisible, and that even the smallest portion of matter contains some of each element. Things appear to be that of which they contain most. Thus, for example, everything contains some fire, but we only call it fire if that element preponderates. Like Empedocles, he argues against the void, saying that the clepsydra or an inflated skin shows that there is air where there seems to be nothing.

He differed from his predecessors in regarding mind (nous) as a substance which enters into the composition of living things, and distinguishes them from dead matter. In everything, he says, there is a portion of everything except mind, and some things contain mind also. Mind has power over all things that have life; it is infinite and self-ruled, and is mixed with nothing. Except as regards mind, everything, however small, contains portions of all opposites, such as hot and cold, white and black. He maintained that snow is black (in part).

Mind is the source of all motion. It causes a rotation, which is gradually spreading throughout the world, and is causing the lightest things to go to the circumference, and the heaviest to fall towards the centre. Mind is uniform, and is just as good in animals as in man. Man's apparent superiority is due to the fact that he has hands; all seeming differences of intelligence are really due to bodily differences.

Both Aristotle and the Platonic Socrates complain that Anaxagoras, after introducing mind, makes very little use of it. Aristotle points out that he only introduces mind as a cause when he knows no other. Whenever he can, he gives a mechanical explanation. He rejected necessity and chance as giving the origins of things; nevertheless, there was no 'Providence' in his cosmology. He does not seem to have thought much about ethics or religion; probably he was an atheist, as his prosecutors maintained. All his predecessors influenced him, except Pythagoras. The influence of Parmenides was the same in his case as in that of Empedocles.

In science he had great merit. It was he who first explained that the moon shines by reflected light, though there is a cryptic fragment in Parmenides suggesting that he also knew this. Anaxagoras gave the correct theory of eclipses, and knew that the moon is below the sun. The sun and stars, he said, are fiery stones, but we do not feel the heat of the stars because they are too distant. The sun is larger than the Peloponnesus. The moon has mountains, and (he thought) inhabitants.

Anaxagoras is said to have been of the school of Anaximenes; certainly he kept alive the rationalist and scientific tradition of the Ionians. One does not find in him the ethical and religious preoccupations which, passing from the Pythagoreans to Socrates and from Socrates to Plato, brought an obscurantist bias into Greek philosophy. He is not quite in the first rank, but he is important as the first to bring philosophy to Athens, and as one of the influences that helped to form Socrates.

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