Written circa 350 B.C.
Translated by J. L. Stocks
Book III.2
The necessity that each of the simple bodies should have a
naturalmovement may be shown as follows. They manifestly move, and if they have
no proper movement they must move by constraint: and theconstrained is the same
as the unnatural. Now an unnatural movement presupposes a natural movement
which it contravenes, and which,
however many the unnatural movements, is always one. For naturally a
thing moves in one way, while its unnatural movements are manifold.
The same may be shown, from the fact of rest. Rest, also, must
either be constrained or natural, constrained in a place to which
movement was constrained, natural in a place movement to which was
natural. Now manifestly there is a body which is at rest at the
centre. If then this rest is natural to it, clearly motion to this
place is natural to it. If, on the other hand, its rest is
constrained, what is hindering its motion? Something, which is at
rest: but if so, we shall simply repeat the same argument; and
either we shall come to an ultimate something to which rest where it
is or we shall have an infinite process, which is impossible. The
hindrance to its movement, then, we will suppose, is a moving thing-as
Empedocles says that it is the vortex which keeps the earth still-:
but in that case we ask, where would it have moved to but for the
vortex? It could not move infinitely; for to traverse an infinite is
impossible, and impossibilities do not happen. So the moving thing
must stop somewhere, and there rest not by constraint but naturally.
But a natural rest proves a natural movement to the place of rest.
Hence Leucippus and Democritus, who say that the primary bodies are in
perpetual movement in the void or infinite, may be asked to explain
the manner of their motion and the kind of movement which is natural
to them. For if the various elements are constrained by one another to
move as they do, each must still have a natural movement which the
constrained contravenes, and the prime mover must cause motion not
by constraint but naturally. If there is no ultimate natural cause
of movement and each preceding term in the series is always moved by
constraint, we shall have an infinite process. The same difficulty
is involved even if it is supposed, as we read in the Timaeus, that
before the ordered world was made the elements moved without order.
Their movement must have been due either to constraint or to their
nature. And if their movement was natural, a moment's consideration
shows that there was already an ordered world. For the prime mover
must cause motion in virtue of its own natural movement, and the other
bodies, moving without constraint, as they came to rest in their
proper places, would fall into the order in which they now stand,
the heavy bodies moving towards the centre and the light bodies away
from it. But that is the order of their distribution in our world.
There is a further question, too, which might be asked. Is it possible
or impossible that bodies in unordered movement should combine in some
cases into combinations like those of which bodies of nature's
composing are composed, such, I mean, as bones and flesh? Yet this
is what Empedocles asserts to have occurred under Love. 'Many a head',
says he, 'came to birth without a neck.' The answer to the view that
there are infinite bodies moving in an infinite is that, if the
cause of movement is single, they must move with a single motion,
and therefore not without order; and if, on the other hand, the causes
are of infinite variety, their motions too must be infinitely
varied. For a finite number of causes would produce a kind of order,
since absence of order is not proved by diversity of direction in
motions: indeed, in the world we know, not all bodies, but only bodies
of the same kind, have a common goal of movement. Again, disorderly
movement means in reality unnatural movement, since the order proper
to perceptible things is their nature. And there is also absurdity and
impossibility in the notion that the disorderly movement is infinitely
continued. For the nature of things is the nature which most of them
possess for most of the time. Thus their view brings them into the
contrary position that disorder is natural, and order or system
unnatural. But no natural fact can originate in chance. This is a
point which Anaxagoras seems to have thoroughly grasped; for he starts
his cosmogony from unmoved things. The others, it is true, make things
collect together somehow before they try to produce motion and
separation. But there is no sense in starting generation from an
original state in which bodies are separated and in movement. Hence
Empedocles begins after the process ruled by Love: for he could not
have constructed the heaven by building it up out of bodies in
separation, making them to combine by the power of Love, since our
world has its constituent elements in separation, and therefore
presupposes a previous state of unity and combination.