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