The theory of Ideas is not a something sprung suddenly upon the world out of Plato's brain. It has its roots in the past. It is, as Aristotle showed, the outcome of Eleatic, Heracleitean, and Socratic determinations. Fundamentally, however, it grows out of the distinction between sense and reason, which had been the common property of Greek thinkers since the time of Parmenides. Parmenides was the first to emphasize this distinction, and to teach that the truth is to be found by reason, the world of sense being illusory. Heracleitus, and even Democritus, were pronounced adherents of reason, as against sense. The crisis came with the Sophists, who attempted to obliterate the distinction altogether, and to find all knowledge in sensation, thus calling forth the opposition of Socrates and Plato. As against them Socrates pointed out that all knowledge is through concepts, reason: and Plato added to this that the concept is not a mere rule of thought but a metaphysical reality. This was the substance of the theory of Ideas. {235} Every philosophy which makes a systematic attempt to solve the riddle of the universe necessarily begins with a theory of the nature of that absolute and ultimate reality from which the universe is derived. This absolute reality we will call simply the Absolute. Plato's theory is that the Absolute consists of concepts. To say that the Absolute is reason, is thought, is concepts, is the universal--these are merely four different expressions of the same theory. Now this proposition, that the Absolute is reason, is the fundamental thesis of all idealism. Since Plato's time there have been several great idealistic systems of philosophy. That the Absolute is reason is the central teaching of them all. Plato, therefore, is the founder and initiator of all idealism. It is this that gives him his great place in the history of philosophy. That the Absolute is universal thought, this is what Plato has contributed to the philosophical speculation of the world. This is his crowning merit.