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. *
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 any given part filled with matter. Aristotle says ( Physics, 208 b): «The theory that the void exists involves the existence of place: for
____________________
* Bailey (op. cit. p. 75) maintains, on the contrary, that Leucippus had an answer, which was «extremely subtle.» It consisted essentially in admitting the existence of something (the void) which was not corporeal. Similarly Burnet says: «It is a curious fact that the Atomists, who are commonly regarded as the great materialists of antiquity, were actually the first to say distinctly that a thing might be real without being a body.»
-69-
one would define void as place bereft of body.» This view is set forth with the utmost explicitness by Newton, who asserts the existence of absolute space, and accordingly distinguishes absolute from relative motion. In the Copernican controversy, both sides (however little they may have realized it) were committed to this view, since they thought there was a difference between saying «the heavens revolve from east to west» and saying «the earth rotates from west to east.» If all motion is relative, these two statements are merely different ways of saying the same thing, like «John is the father of James» and «James is the son of John.» But if all motion is relative, and space is not substantial, we are left with the Parmenidean arguments against the void on our hands.
Descartes, whose arguments are of just the same sort as those of early Greek philosophers, said that extension is the essence of matter, and therefore there is matter everywhere. For him, extension is an adjective, not a substantive; its substantive is matter, and without its substantive it cannot exist. Empty space, to him, is as absurd as happiness without a sentient being who is happy. Leibniz, on somewhat different grounds, also believed in the plenum, but he maintained that space is merely a system of relations. On this subject there was a famous controversy between him and Newton, the latter represented by Clarke. The controversy remained undecided until the time of Einstein, whose theory conclusively gave the victory to Leibniz.
The modern physicist, while he still believes that matter is in some sense atomic, does not believe in empty space. Where there is not matter, there is still something, notably light-waves. Matter no longer has the lofty status that it acquired in philosophy through the arguments of Parmenides. It is not unchanging substance, but merely a way of grouping events. Some events belong to groups that can be regarded as material things; others, such as light-waves, do not. It is the events that are the stuff of the world, and each of them is of brief duration. In this respect, modern physics is on the side of Heraclitus as against Parmenides. But it was on the side of Parmenides until Einstein and quantum theory.
As regards space, the modern view is that it is neither a substance, as Newton maintained, and as Leucippus and Democritus ought to have said, nor an adjective of extended bodies, as Descartes thought,
-70-
but a system of relations, as Leibniz held. It is not by any means clear whether this view is compatible with the existence of the void. Perhaps, as a matter of abstract logic, it can be reconciled with the void. We might say that, between any two things, there is a certain greater or smaller distance, and that distance does not imply the existence of intermediate things. Such a point of view, however, would be impossible to utilize in modern physics. Since Einstein, distance is between events, not between things, and involves time as well as space. It is essentially a causal conception, and in modern physics there is no action at a distance. All this, however, is based upon empirical rather than logical grounds. Moreover the modern view cannot be stated except in terms of differential equations, and would therefore be unintelligible to the philosophers of antiquity.
It would seem, accordingly, that the logical development of the views of the atomists is the Newtonian theory of absolute space, which meets the difficulty of attributing reality to not-being. To this theory there are no logical objections. The chief objection is that absolute space is absolutely unknowable, and cannot therefore be a necessary hypothesis in an empirical science. The more practical objection is that physics can get on without it. But the world of the atomists remains logically possible, and is more akin to the actual world than is the world of any other of the ancient philosophers.
Democritus worked out his theories in considerable detail, and some of the working out is interesting. Each atom, he said, was impenetrable and indivisible because it contained no void. When you use a knife to cut an apple, the knife has to find empty places where it can penetrate; if the apple contained no void, it would be infinitely hard and therefore physically indivisible. Each atom is internally unchanging, and in fact a Parmenidean One. The only things that atoms do are to move and hit each other, and sometimes to combine when they happen to have shapes that are capable of interlocking. They are of all sorts of shapes; fire is composed of small spherical atoms, and so is the soul. Atoms, by collision, produce vortices, which generate bodies and ultimately worlds. * There are many worlds, some growing, some decaying; some may have no sun or moon, some several. Every world has a beginning and an end. A world may be
____________________
* On the way in which this was supposed to happen, see Bailey, op. cit., p. 138 ff.
-71-
destroyed by collision with a larger world. This cosmology may be summarized in Shelley’s words:
Worlds on worlds are rolling ever From creation to decay, Like the bubbles on a river Sparkling, bursting, borne away.
Life developed out of the primeval slime. There is some fire everywhere in a living body, but most in the brain or in the breast. (On this, authorities differ.) Thought is a kind of motion, and is thus able to cause motion elsewhere. Perception and thought are physical processes. Perception is of two sorts, one of the senses, one of the understanding. Perceptions of the latter sort depend only on the things perceived, while those of the former sort depend also on our senses, and are therefore apt to be deceptive. Like Locke, Democritus held that such qualities as warmth, taste, and colour are not really in the object, but are due to our sense-organs, while such qualities as weight, density, and hardness are really in the object.
Democritus was a thorough-going materialist; for him, as we have seen, the soul was composed of atoms, and thought was a physical process. There was no purpose in the universe; there were only atoms governed by mechanical laws. He disbelieved in popular religion, and he argued against the nous of Anaxagoras. In ethics he considered cheerfulness the goal of life, and regarded moderation and culture as the best means to it. He disliked everything violent and passionate; he disapproved of sex, because, he said, it involved the overwhelming of consciousness by pleasure. He valued friendship, but thought ill of women, and did not desire children, because their education interferes with philosophy. In all this, he was very like Jeremy
Bentham; he was equally so in his love of what the Greeks called democracy. *
Democritus—such, at least, is my opinion-is the last of the Greek philosophers to be free from a certain fault which vitiated all later ancient and medieval thought. All the philosophers we have been considering so far were engaged in a disinterested effort to understand the world. They thought it easier to understand than it is, but without this optimism they would not have had the courage to make
