His challenge was now taken up by the Atomists, who boldly affirmed that if non-being meant empty space, it was just as conceivable and just as necessary as being. A further stimulus may have been received from the Pythagorean school, whose doctrines had, just at this time, been systematised and committed to writing by Philolaus, its most eminent disciple. The hard saying that all things were made out of number might be explained and confirmed if the integers were interpreted as material atoms. It will have been observed that, so far, the merit of originating atomism has been attributed to Leucippus, instead of to the more celebrated Democritus, with whose name it is usually associated. The two were fast friends, and seem always to have worked together in perfect harmony. But Leucippus, although next to nothing is known of his life, was apparently the older man, and from him, so far as we can make out, emanated the great idea, which his brilliant coadjutor carried into every department of enquiry, and set forth in works which are a loss to literature as well as to science, for the poetic splendour of their style was not less remarkable than the encyclopaedic range of their contents. Democritus was born at Abdêra, a Thracian city, 470 B.C., a year before Socrates, and lived to a very advanced age—more than a hundred, according to some accounts. However this may be, he was probably, like most of his great countrymen, possessed of immense vitality. His early manhood was spent in Eastern travel, and he was not a little proud of the numerous countries which he had visited, and the learned men with whom he had conversed. His time was mostly occupied in observing Nature, and in studying mathematics; the sages of Asia and Egypt may have acquainted him with many useful scientific facts, but we have seen that his philosophy was derived from purely Hellenic sources. A few fragments of his numerous writings still survive—the relics of an intellectual Ozymandias. In them are briefly shadowed forth the conceptions which Lucretius, or at least his modern English interpreters, have made familiar to all educated men and women. Everything is the result of mechanical causation. Infinite worlds are formed by the collision of infinite atoms falling for ever downward through infinite space. No place is left for supernatural agency; nor are the unaided operations of Nature disguised under Olympian appellations. Democritus goes even further than Epicurus in his rejection of the popular mythology. His system provides no interstellar refuge for abdicated gods. He attributed a kind of objective existence to the apparitions seen in sleep, and even a considerable influence for good or for evil, but denied that they were immortal. The old belief in a Divine Power had arisen from their activity and from meteorological phenomena of an alarming kind, but was destitute of any stronger foundation. For his own part, he looked on the fiery spherical atoms as