Anaxagoras was born at Clazomenae, in Ionia, about the year 500 B.C. He spent about thirty years of his life in Athens around 462 B.C to 432 B.C. He was probably induced to come by Pericles and was the first one to introduce philosophy to Athenians. As Pericles grew week his men, including Anaxagoras, were troubled by his enemies. What happened actually is not certain, except that Anaxagoras had to leave Athens. He returned to Ionia, where he founded a school.
Anaxagoras carried out on the scientific, rationalist tradition of Ionia. He was from the school of Anaximene. He was first to suggest mind as primary cause of physical change. Anaxagoras held that everything is infinitely divisible, and that even the smallest portion of matter contains some of each elements. Things appear to be that of which they continue most. For e.g. everything contains some fire but what we know as fire is one in which fire is preponderant.
Like Empedocles, he agrees against the void. His greatest achievement is that he differed from his predecessors in regarding mind (nous) as a substance that enters into the composition of living things and distinguishes them from dead matter. In everything, he says, there is portion of everything except mind. There are certain things which contains mind also. In such things mind has power over all things that have life; it is infinite and self ruled and is mixed with nothing. 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 center and the heaviest to fall towards 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.
Aristotle and Socrates complain that Anaxagoras, after introducing mind, makes very little use of it. Aristotle points out that Anaxagoras introduces mind as a cause when he know no other. Wherever he can, he gives mechanical explanation.
In cosmology he had great achievements. If we discount Parmenides cryptic suggestion, Anaxagoras was the first to explain that moon shines by reflected light. He gave the correct theory of eclipse. The sun and stars, he said, are fiery stones, but we do not feel the heat of stars as they are distant. The sun is larger than Peloponnesus, the moon has mountains and inhabitants.
Bertrand Russell summarizes Anaxagoras contribution as:
“Anaxagoras kept alive the rationalist and the scientific tradition of Ionians. One does not find in him the ethical and religious preoccupations which, passing from Pythagoras to Socrates and from Socrates to Plato, brought an obscurantist bias in Greek Philosophy. He is not quite in the first rank, but he is important as the first to bring philosophy to Athens, and one of the influences that helped to form Socrates.”