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. Sec. 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 opp