Parmenides of Elea, the most original and important philosopher before Socrates, was born c. 515 BCE. He changed the course of Greek cosmology and had an even more important effect upon metaphysics and epistemology. He was the first to focus attention on the central problem of Greek metaphysics—What is the nature of real being?—and he established a frame of reference within which the discussion was to be conducted. The closely related problem of knowledge, which to a great extent dominated philosophy in the fifth and fourth centuries, was raised at once by his contrast between the Way of Truth and the Way of Seeming. His influence can be found in Empedocles, Anaxagoras, and the atomists; it is strong in most of Plato's work, particularly in the vitally important dialogues Parmenides, Theaetetus, and Sophist.

Plato in his dialogue...