=2. His Philosophical Sources.= Plato had received a careful education that made him familiar with all the scientific theories of current interest to the Athenians. The elements of the earlier philosophies, that were fundamental to the mechanical atomism of Democritus, were recombined in a different way by Plato under the influence of Socrates’ ethical principle. Even Plato’s political and artistic ideals are subordinate to his entire absorption in the personality and teaching of Socrates. Heracleitus, Protagoras, Parmenides, and, later, Anaxagoras and the Pythagoreans, furnished him with his philosophical materials. We may point out three of the preceding philosophies that had an especially powerful influence upon him: those of (1) Socrates; (2) Parmenides; and (3) the Pythagoreans. His revered master, Socrates, furnished Plato throughout with the conceptual principle, by which he worked over all his material into his daring system. The influence of Parmenides upon him was also very great. He speaks of the Eleatic as “Parmenides, my father.” Plato betook himself to the Eleatic school at Megara upon the death of Socrates, and this shows that he must already have been hospitable to the philosophy which taught the conception of an absolute and eternal essence of things known by the human reason. The influence of the Pythagoreans was felt by Plato on his first visit to Italy. This influence grew with him, and seems to dominate the dialogue of his old age, the _Laws_. The Eleatic Oneness was a single, immutable block. In the Pythagorean doctrine of numbers he found the conceptual divisions of that Oneness, and he also found that such conceptions would give a content to Socrates’ conception of the Good. Indeed, the numbers seemed to be the conceptual models for which Socrates was searching. Mathematical truths are independent of perception. They are innate ideas. They are eternal and immutable Forms. They were the weapons needed against the Protagorean doctrine of perception. While Plato agreed with Heracleitus that the visible world is a changing world, and with Protagoras that our sense-perceptions of that world can yield only relative truth, he developed his philosophy almost entirely on its conceptual side; and this is due to the influence first of Socrates, second of Parmenides, and third of the Pythagoreans. Plato’s completed philosophy was the theory of Ideas, worked over in his mind a half-century or more, and is in itself a history of the development of pure concepts.