First, the Atomists developed the theory of particles. According to Leucippus and Democritus, if matter were divided far enough, we should ultimately come to indivisible units. These indivisible units are called atoms, and atoms are therefore the ultimate constituents of matter. They are infinite in number, and are too small to be perceptible to the senses. Empedocles had assumed four different kinds of matter. But, for the Atomists, there is only one kind. All the atoms are composed of exactly the same kind of matter. With certain exceptions, which I will mention in a moment, they possess no quality. They are entirely non-qualitative, the only differences between them being differences of quantity. They differ in size, some being larger, some smaller. And they likewise differ in shape. Since the ultimate particles of things thus possess no quality, all the actual qualities of objects must be due to the [{89}] arrangement and position of the atoms. This is the logical development of the tentative mechanism of Empedocles.
I said that the atoms possess no qualities. They must, however, be admitted to possess the quality of solidity, or impenetrability, since they are defined as being indivisible. Moreover it is a question whether the atoms of Democritus and Leucippus were thought to possess weight, or whether the weight of objects is to be explained, like other qualities, by the position and movement of the atoms. There is no doubt that the Epicureans of a later date considered the atoms to have weight. The Epicureans took over the atomism of Democritus and Leucippus, with few modifications, and made it the basis of their own teaching. They ascribed weight to the atoms, and the only question is whether this was a modification introduced by them, or whether it was part of the original doctrine of Democritus and Leucippus.
The atoms are bounded, and separated off from each other. Therefore, they must be separated by something, and this something can only be empty space. Moreover, since all becoming and all qualitativeness of things are to be explained by the mixing and unmixing of atoms, and since this involves movement of the atoms, for this reason also empty space must be assumed to exist, for nothing can move unless it has empty space to move in. Hence there are two ultimate realities, atoms and empty space. These correspond respectively to the Being and not-being of the Eleatics. But whereas the latter denied any reality to not-being, the Atomists affirm that not-being, that is, empty space, is just as real as being. Not-being also exists. "Being," said [{90}] Democritus, "is by nothing more real than nothing." The atoms being non-qualitative, they differ in no respect from empty space, except that they are "full." Hence atoms and the void are also called the plenum and the vacuum.
How, now, is the movement of the atoms brought about? Since all becoming is due to the separation and aggregation of atoms, a moving force is required. What is this moving force? This depends upon the question whether atoms have weight. If we assume that they have weight, then the origin of the world, and the motion of atoms, becomes clear. In the system of the Epicureans the original movement of the atoms is due to their weight, which causes them to fall perpetually downwards through infinite space. Of course the Atomists had no true ideas of gravitation, nor did they understand that there is no absolute up and down. The large atoms are heavier than the smaller. The matter of which they are composed is always the same. Therefore, volume for volume, they weigh the same. Their weight is thus proportional to their size, and if one atom is twice as large as another, it will also be twice as heavy. Here the Atomists made another mistake, in supposing that heavier things fall in a vacuum more quickly than light things. They fall, as a matter of fact, with the same speed. But according to the Atomists, the heavier atoms, falling faster, strike against the lighter, and push them to one side and upwards. Through this general concussion of atoms a vortex is formed, in which like atoms come together with like. From the aggregation of atoms worlds are created. As space is infinite and the atoms go on falling eternally, there must have been innumerable worlds of which our world is only one. [{91}] When the aggregated atoms fall apart again, this particular world will cease to exist. But all this depends upon the theory that the atoms have weight. According to Professor Burnet, however, the weight of atoms is a later addition of the Epicureans. If that is so, it is very difficult to say how the early Atomists, Leucippus and Democritus, explained the original motion. What was their moving force, if it was not weight? If the atoms have no weight, their original movement cannot have been a fall. "It is safest to say," says Professor Burnet, "that it is simply a confused motion this way and that." [Footnote 7] Probably this is a very safe thing to say, because it means nothing in particular. Motion itself cannot be confused. It is only our ideas of motion which can be confused. If this theory is correct, then, we can only say that the Atomists had no definite solution of the problem of the origin of motion and the character of the moving force. They apparently saw no necessity for explanation, which seems unlikely in view of the fact that Empedocles had already seen the necessity of solving the problem, and given a definite, if unsatisfactory, solution, in his theory of Love and Hate. This remark would apply to Democritus, if not to Leucippus.
[Footnote 7: Early Greek Philosophy, chap. ix. § 179.]
The Atomists also spoke of all movement being under the force of "necessity." Anaxagoras was at this time teaching that all motion of things is produced by a world-intelligence, or reason. Democritus expressly opposes to this the doctrine of necessity. There is no reason or intelligence in the world. On the contrary, all phenomena and all becoming are completely determined by blind mechanical causes. In this connection there arises [{92}] among the Atomists a polemic against the popular gods and the popular religion. Belief in gods Democritus explains as being due to fear of great terrestrial and astronomical phenomena, such as volcanoes, earthquakes, comets, and meteors. But somewhat inconsistently with this, Democritus believed that the air is inhabited by beings resembling men, but larger and of longer life, and explained belief in the gods as being due to projection from these of images of themselves composed of atoms which impinge upon human senses, and produce the ideas of gods.
Different kinds of matter must be explained, in any atomic theory, by the shape, size, and position of the atoms of which they are composed. Thus the Atomists taught that fire is composed of smooth round atoms. The soul is also composed of smooth round atoms, and is an exceptionally pure and refined fire. At death the soul atoms are scattered, and hence there is, of course, no question of a future life. Democritus also put forward a theory of perception, according to which objects project into space images of themselves composed of atoms. These images strike against the senses. Like atoms are perceived by like. Thought is true when the soul is equable in temperature. The sensible qualities of things, such as smell, taste, colour, do not exist in the things themselves, but merely express the manner in which they affect our senses, and are therefore relative to us. A number of the ethical maxims of Democritus have come down to us. But they are not based in any way upon the Atomic theory, and cannot be deduced from it. Hence they have no scientific foundation but are merely detached sayings, epitomizing the experience [{93}] and worldly wisdom of Democritus. That one should enjoy oneself as much and vex oneself as little as possible seems to have been his principal idea. This, however, is not to be interpreted in any low, degraded, or sensual way. On the contrary, Democritus says that the happiness of man does not depend on material possessions, but upon the state of the soul. He praises equanimity and cheerfulness, and these are best attained, he thinks, by moderation and simplicity.
ANAXAGORAS