Aristotle, who was born in 384 B.C., was Plato's pupil. He, however, completely broke away from his master's theory. He maintained that individual objects alone exist. But if only individual objects exist, only by the senses can they be known; and if we have only sense-knowledge, how can we arrive at the general truths on which both philosophy and science are founded? This was the problem which had led Plato to claim for ideas, or types of general truths, a higher origin than the intermittent and varying data of the senses. Aristotle held that it could be solved in a natural way without the conception of an ideal world. In his view, ideas were obtained by induction. Sensation is the basis of all knowledge. But we have another faculty besides that of sensation; we have memory. Having perceived many objects, we remember our perceptions, and this enables us to discern wherein things differ and wherein they agree. Then, by means of the art of induction, we arrive at ideas. Aristotle's theory of induction is clearly explained by him: "Experience furnishes the principles of every science. Thus astronomy is grounded on observation. For if we were to observe properly the phenomena of the heavens, we might demonstrate the laws which regulate them. The same applies to other sciences." Had he always held before his eyes this conception of science, he would have anticipated Bacon--he would have been the Father of Positive Science. But he could not confine himself to experience, as there was not sufficient experience accumulated in his age from which to generalise with any effect. So he turned to logic as an instrument for investigating the mystery of existence, and by bringing physics and metaphysics together again, he paved the way for a new era--the era of scepticism. All the wisdom of the ancient world was powerless against the sceptics. Faith in truth was extinct; faith in human nature was gone; philosophy was impossible. And, though the influence of Socrates continued to be felt in the field of ethics, the ethics of the Greeks were at best narrow and egotistical. What a light was poured upon all questions of morality by that one divine axiom, "Love your enemy." No Greek ever attained the sublimity of such a point of view. Still, the progress made by the Greeks was immense, and they must ever occupy in the history of humanity an honourable place. _III.--Philosophy and Science_ Francis Bacon is the father of experimental philosophy. He owes his title to his method. Many philosophers, ancient and modern, had cursorily referred to observation and experiment as furnishing the materials of physical knowledge; but no one before him had attempted to systematise the true method of discovery.