A question, however, arises about the movement
of the atoms. The only case of apparently uncaused
movement is the fall of unsupported bodies to the
earth. That fall takes place in a straight line.
Neither the circular movement, which Aristotle holds.
to be the natural and perfect movement of the celes-
tial bodies, nor the upward movement which, in his
opinion, characterizes fire, correspond in the judg-
ment of Epicurus with the observed facts of terres-
trial change. Upward movement is a mistake. As
for circular movement, it is explained by subsequent
science as a conjunction of two rectilineal move-
ments acting at right angles to each other. We can-
not, therefore, assume an initial tendency of atoms
to attract or repel each other, or to revolve round
each other. But, in apparent conformity with expe-
rience, we may assume that the atoms fall down-
wards. Of course, an up and a down in a vacant
world can only be arbitrarily fixed, ¹ and a modern
would object that every fall presupposed attraction.
But Epicurus is content with the phenomenon of fall
as experienced in daily life: he asks for no cause of
the movement so denominated, but regards it as
natural and primary. Thus, in the primeval void,
all atoms are perpetually falling. To assume more
would be to affix active properties to the atoms; but
such properties the atoms, however erratic, as we

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