ation must be introduced _arbitrarily_, or by some _external cause_.
And inasmuch as Epicurus admits of no causes "but space and matter," and
rejects all divine or supernatural interposition, the _new_ movement
must be purely arbitrary. They deviate _spontaneously,_ and of their own
accord. "The system of nature immediately appears _as a free agent_,
released from tyrant masters, to do every thing of itself spontaneously,
without the help of the gods."[799] The manner in which Lucretius proves
this doctrine is a good example of the petitio principii. He assumes, in
opposition to the whole spirit and tendency of the Epicurean philosophy,
that man has "a free will," and then argues that if man who is nothing
but an aggregation of atoms, can "turn aside and alter his own
movements," the primary elements, of which his soul is composed, must
have some original spontaneity. "If all motion is connected and
dependent, and a new movement perpetually arises from a former one in a
certain order, and if the primary elements do not produce any
commencement of motion by deviating from the straight line to break the
laws of fate, so that cause may not follow cause in infinite succession,
_whence comes this freedom of will_ to all animals in the world? whence,
I say, is this liberty of action wrested from the fates, by means of
which we go wheresoever inclination leads each of us? whence is it that
we ourselves turn aside, and alter our motions, not at any fixed time,
nor in any fixed part of space, but just as our own minds prompt?....
Wherefore we must necessarily confess that the same is the case with the
seeds of matter, and there is some other cause besides strokes and
weight [resistance and density] from which this power [of free movement]
is innate in them, since we see that _nothing is produced from
nothing_."[800] Besides form, extension, and density, Epicurus has found
another inherent or essential quality of matter or atoms, namely,
"_spontaneous" motion._

[Footnote 798: "At some time, though at no fixed and determinate time,
and at some point, though at no fixed and determinate point, they turn
aside from the right line, but only so far as you can call the least
possible deviation."--Lucretius, "On the Nature of Things," bk. ii. l.
216-222.]

[Footnote 799: Lucretius, "On the Nature of Things" bk. ii. 1.
1092-1096.]

[Footnote 800: Id., ib., bk. ii. l. 250-290.]

By a slight "voluntary" deflection from the straight lin