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were wholly empirical. The atomic theory was revived in modern times to explain the facts of chemistry, but these facts were not known to the Greeks. There was no very sharp distinction, in ancient times, between empirical observation and logical argument. Parmenides, it is true, treated observed facts with contempt, but Empedocles and Anaxagoras would combine much of their metaphysics with observations on water-clocks and whirling buckets. Until the Sophists, no philosopher seems to have doubted that a complete metaphysic and cosmology could be established by a combination of much reasoning and some observation. By good luck, the atomists hit on a hypothesis for which, more than two thousand years later, some evidence was found, but their belief, in their day, was none the less destitute of any solid foundation.7

Like the other philosophers of his time, Leucippus was concerned to find a way of reconciling the arguments of Parmenides with the obvious fact of motion and change. As Aristotle says:8

'Although these opinions [those of Parmenides] appear to follow logically in a dialectical discussion, yet to believe them seems next door to madness when one considers the facts. For indeed no lunatic seems to be so far out of

his senses as to suppose that fire and ice are "one"; it is only between what is right and what seems right from habit that some people are mad enough to see no difference.'

Leucippus, however, thought he had a theory which harmonized with sense-perception and would not abolish either coming-to-be and passing-away or motion and the multiplicity of things. He made these concessions to the facts of perception: on the other hand, he conceded to the Monists that there could be no motion without a void. The result is a theory which he states as follows: 'The void is a not-being, and no part of what is is a not-being; for what is in the strict sense of the term is an absolute plenum. This plenum, however, is not one; on the contrary, it is a many infinite in number and invisible owing to the minuteness of their bulk. The many move in the void (for there is a void): and by coming together they produce coming-to-be, while by separating they produce passing-away. Moreover, they act and suffer action whenever they chance to be in contact (for there they are not one), and they generate by being put together and become intertwined. From the genuinely one, on the other hand, there could never have come to be a multiplicity, nor from the genuinely many a one: that is impossible.'

It will be seen that there was one point on which everybody so far was agreed, namely that there could be no motion in a plenum. In this, all alike were mistaken. There can be cyclic motion in a plenum, provided it has always existed. The idea was that a thing could only move into an empty place, and that, in a plenum, there are no empty places. It might be contended, perhaps validly, that motion could never begin in a plenum. but it cannot be validly maintained that it could not occur at all. To the Greeks, however, it seemed that one must either acquiesce in the unchanging world of Parmenides, or admit the void.

Now the arguments of Parmenides against not-being seemed logically irrefutable against the void, and they were reinforced by the discovery that where there seems to be nothing there is air. (This is an example of the confused mixture of logic and observation that was common.) We may put the Parmenidean position in this way: 'You say there is the void; therefore the void is not nothing; therefore it is not the void.' It cannot be said that the atomists answered this argument; they merely proclaimed that they proposed to ignore it, on the ground that motion is a fact of experience, and therefore there must be a void, however difficult it may be to conceive.9

Let us consider the subsequent history of this problem. The first and most obvious way of avoiding the logical difficulty is to distinguish between matter and space. According to this view, space is not nothing, but is of the nature of a receptacle, which may or may not have