History of Western Philosophy - Bertrand Russell [45]
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, 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