They repaired to the oracles, and watched for the will of the gods in signs and omens. In his philosophy Anaxagoras held that there was a portion of everything in everything, and that things are variously mixed in infinite numbers of seeds, each after its kind. From these, through the action of an external cause, called NOUS, which also is material, although the "thinnest of all things and the purest," and "has power over all things," there arose plants and animals. It is probable, as Professor Burnet remarks, "that Anaxagoras substituted NOUS, still conceived as a body, for the LOVE and STRIFE of Empedocles simply because he wished to retain the old Ionic doctrine of a substance that 'knows' all things, and to identify this with the new theory of a substance that 'moves' all things." Thus far speculation has run largely on the origin of life forms, but now we find revival of speculation about the nature of things generally, and the formulation of a theory which links Greek cosmology with early nineteenth-century science with Dalton's ATOMIC THEORY. Democritus of Abdera, who was born about 460 B. C., has the credit of having elaborated an atomic theory, but probably he only further developed what Leucippus had taught before him. Of this last-named philosopher nothing whatever is known; indeed, his existence has been doubted, but it counts for something that Aristotle gives him the credit of the discovery, and that Theophrastus, in the first book of his Opinions, wrote of Leucippus as follows: "He assumed innumerable and ever-moving elements, namely, the atoms. And he made their forms infinite in number, since there was no reason why they should be of one kind rather than another, and because he saw that there was unceasing becoming and change in things. He held, further, that _what is_ is no more real than _what is not_, and that both are alike causes of the things that come into being; for he laid down that the substance of the atoms was compact and full, and he called them _what is_, while they moved in the void which he called _what is not_, but affirmed to be just as real as _what is_." Thus did "he answer the question that Thales had been the first to ask."