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 ex-
plained 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. “Be-
ing,” 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 theplenumand
thevacuum.
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 ori-
gin 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 heav-
ier 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 vor-
tex 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 Democri-
tus, 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 verysafething to say, because it means nothing in par-
ticular. 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