His successor, Democritus, adopted his theory of the origin of knowledge, and by applying it to the problem of the One and the Many, produced the most striking of ancient anticipations of modern science. He regarded the world as something made up of invisible particles, each absolutely similar to the other; these formed the essential unity which could be grasped only by the reason, but by their various combinations and arrangements they brought about the apparent multiplicity of objects which the senses perceived. Such was the foundation of the atomic theory of Democritus. He conceived the atom as a centre of force, and not as a particle having weight and material qualities. As, however, his hypothesis was purely a metaphysical one, it did not lead to any of the discoveries which have followed on the establishment of the modern scientific theory, which was arrived at in a different way, and has a different signification. Democritus also threw out in vague outline the idea of gravitation. But this was not science: it was guess-work; it afforded no ground on which the fabric of verified knowledge could be erected, and no sure method of obtaining this knowledge. _II.--The School of Socrates_ It was against the vain and premature hypotheses of the physiologists of his day that the greatest and noblest intellect in Greece revolted. Socrates was the knight-errant of philosophy. It was his confessed aim and purpose to withdraw the mind from the contemplation of the phenomena of nature, and fix it on its own phenomena. "I have not leisure for physical speculations," he said, with characteristic irony, "and I will tell you why: I am not yet able, according to the Delphic inscription, to _know myself_, and it seems to me very ridiculous, while ignorant of myself, to inquire into what I am not concerned in." Weary of disputes about the origin of the universe, he turned to the one field in which the current method of abstract reasoning could be fruitfully applied--the field of ethics. Living in an age of wild sophistry, he endeavoured to steady and enlighten the conscience of men by establishing right principles of conduct. His method of proceeding by definitions and analogy has been misapplied, but in his hands it was a powerful instrument in discovering and marking out a new field of inquiry. His religious genius, the ideal character of his ethics, and the heroic character of his life, have been his great titles to fame, but it is his method which gives him his high position in the history of philosophy.