He was a man of vast and varied learning, and a most voluminous author, though none of his works have come down to us;—in them he carried out the theory of atoms which he had derived from Leucippus; insisting on the reality of a vacuum and of motion, which he held was the eternal and necessary consequence of the original variety of atoms in this vacuum. These atoms, according to this theory, being in constant motion and impenetrable, offer resistance to one another, and so create a whirling motion which gives birth to worlds. Moreover, from this arise combinations of distinct atoms which become real things and beings. The first cause of all existence he called _chance_ (τύχη), in opposition to the νοῦς of Anaxagoras. But Democritus went further; for he directed his investigations especially to the discovery of causes. Besides the infinite number of atoms, he likewise supposed the existence of an infinite number of worlds, each being kept together by a sort of shell or skin. He derived the four elements from the form, quality, and proportionate magnitude of the atoms predominating in each; and in deriving individual things from atoms, he mainly considered the qualities of warm and cold; the soul he considered as derived from fire atoms; and he did not consider mind as anything peculiar, or as a power distinct from the soul or sensuous perception; but he considered knowledge derived from reason to be a sensuous perception. In his ethical philosophy, he considered (as we may see from the _de Finibus_) the acquisition of peace of mind as the end and ultimate object of all our actions, and as the last and best fruit of philosophical inquiry. Temperance and moderation in prosperity and adversity were, in his eyes, the principal means of acquiring this peace of mind. And he called those men alone pious and beloved by the Gods who hate whatever is wrong. _Empedocles_ was a Sicilian, who flourished about the time when Thrasydæus, the son of Theron, was expelled from Agrigentum, to the tyranny of which he had succeeded; in which revolution he took an active part: it is even said that the sovereignty of his native city was offered to and declined by him.