Parmenides, of Elea, flourished towards the beginning of the fifth century B.C. We know very little about his personal history. According to Plato, he visited Athens late in life, and there made the acquaintance of Socrates, at that time a very young man. But an unsupported statement of Plato’s must always be received with extreme caution; and this particular story is probably not less fictitious than the dialogue which it serves to introduce. Parmenides embodied his theory of the world in a poem, the most important passages of which have been preserved. They show that, while continuing the physical studies of his predecessors, he proceeded on an entirely different method. Their object was to deduce every variety of natural phenomena from a fundamental unity of substance. He declared that all variety and change were a delusion, and that nothing existed but one indivisible, unalterable, absolute reality; just as Descartes’ antithesis of thought and extension disappeared in the infinite substance of Spinoza, or as the Kantian dualism of object and subject was eliminated in Hegel’s absolute idealism. Again, Parmenides does not dogmatise to the same extent as his predecessors; he attempts to demonstrate his theory by the inevitable necessities of being and thought. Existence, he tells us over and over again, _is_, and non-existence is not, cannot even be imagined or thought of as existing, for thought is the same as being. This is not an anticipation of Hegel’s identification of being with thought; it only amounts to the very innocent proposition that a thought is something and about something—enters, therefore, into the general undiscriminated mass of being. He next proceeds to prove that what is can neither come into being nor pass out of it again. It cannot come out of the non-existent, for that is inconceivable; nor out of the existent, for nothing exists but being itself; and the same argument proves that it cannot cease to exist. Here we find the indestructibility of matter, a truth which Anaximander had not yet grasped, virtually affirmed for the first time in history. We find also that our philosopher is carried away by the enthusiasm of a new discovery, and covers more ground than he can defend in maintaining the permanence of all existence whatever. The reason is that to him, as to every other thinker of the pre-Socratic period, all existence was material, or, rather, all reality was confounded under one vague conception, of which visible resisting extension supplied the most familiar type. To proceed: Being cannot be divided from being, nor is it capable of condensation or expansion (as the Ionians had taught); there is nothing by which it can be separated or held apart; nor is it ever more or less existent, but all is full of being. Parmenides goes on in his grand style:—