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.