IT remains to consider the following question. Was there ever a
becoming of motion before which it had no being, and is it perishing
again so as to leave nothing in motion? Or are we to say that it never
had any becoming and is not perishing, but always was and always
will be? Is it in fact an immortal never-failing property of things
that are, a sort of life as it were to all naturally constituted
things?

    Now the existence of motion is asserted by all who have anything
to say about nature, because they all concern themselves with the
construction of the world and study the question of becoming and
perishing, which processes could not come about without the
existence of motion. But those who say that there is an infinite
number of worlds, some of which are in process of becoming while
others are in process of perishing, assert that there is always motion
(for these processes of becoming and perishing of the worlds
necessarily involve motion), whereas those who hold that there is only
one world, whether everlasting or not, make corresponding
assumptions in regard to motion. If then it is possible that at any
time nothing should be in motion, this must come about in one of two
ways: either in the manner described by Anaxagoras, who says that
all things were together and at rest for an infinite period of time,
and that then Mind introduced motion and separated them; or in the
manner described by Empedocles, according to whom the universe is
alternately in motion and at rest-in motion, when Love is making the
one out of many, or Strife is making many out of one, and at rest in
the intermediate periods of time-his account being as follows:

    "Since One hath learned to spring from Manifold,

    And One disjoined makes manifold arise,

    Thus they Become, nor stable is their life:

    But since their motion must alternate be,

    Thus have they ever Rest upon their round":
for we must suppose that he means by this that they alternate from the
one motion to the other. We must consider, then, how this matter
stands, for the discovery of the truth about it is of importance,
not only for the study of nature, but also for the investigation of
the First Principle.

    Let us take our start from what we have already laid down in our
course on Physics. Motion, we say, is the fulfilment of the movable in
so far as it is movable. Each kind of motion, therefore, necessarily
involves the presence of the things that are capable of that motion.
In fact, even apart from the definition of motion, every one would
admit that in each kind of motion it is that which is capable of
that motion that is in motion: thus it is that which is capable of
alteration that is altered, and that which is capable of local
change that is in locomotion: and so there must be something capable
of being burned before there can be a process of being burned, and
something capable of burning before there can be a process of burning.
Moreover, these things also must either have a beginning before
which they had no being, or they must be eternal. Now if there was a
becoming of every movable thing, it follows that before the motion
in question another change or motion must have taken place in which
that which was capable of being moved or of causing motion had its
becoming. To suppose, on the other hand, that these things were in
being throughout all previous time without there being any motion
appears unreasonable on a moment's thought, and still more
unreasonable, we shall find, on further consideration. For if we are
to say that, while there are on the one hand things that are
movable, and on the other hand things that are motive, there is a time
when there is a first movent and a first moved, and another time
when there is no such thing but only something that is at rest, then
this thing that is at rest must previously have been in process of
change: for there must have been some cause of its rest, rest being
the privation of motion. Therefore, before this first change there
will be a previous change. For some things cause motion in only one
way, while others can produce either of two contrary motions: thus
fire causes heating but not cooling, whereas it would seem that
knowledge may be directed to two contrary ends while remaining one and
the same. Even in the former class, however, there seems to be
something similar, for a cold thing in a sense causes heating by
turning away and retiring, just as one possessed of knowledge
voluntarily makes an error when he uses his knowledge in the reverse
way. But at any rate all things that are capable respectively of
affecting and being affected, or of causing motion and being moved,
are capable of it not under all conditions, but only when they are
in a particular condition and approach one another: so it is on the
approach of one thing to another that the one causes motion and the
other is moved, and when they are present under such conditions as
rendered the one motive and the other movable. So if the motion was
not always in process, it is clear that they must have been in a
condition not such as to render them capable respectively of being
moved and of causing motion, and one or other of them must have been
in process of change: for in what is relative this is a necessary
consequence: e.g. if one thing is double another when before it was
not so, one or other of them, if not both, must have been in process
of change. It follows then, that there will be a process of change
previous to the first.

    (Further, how can there be any "before" and "after" without the
existence of time? Or how can there be any time without the
existence of motion? If, then, time is the number of motion or
itself a kind of motion, it follows that, if there is always time,
motion must also be eternal. But so far as time is concerned we see
that all with one exception are in agreement in saying that it is
uncreated: in fact, it is just this that enables Democritus to show
that all things cannot have had a becoming: for time, he says, is
uncreated. Plato alone asserts the creation of time, saying that it
had a becoming together with the universe, the universe according to
him having had a becoming. Now since time cannot exist and is
unthinkable apart from the moment, and the moment a kind of
middle-point, uniting as it does in itself both a beginning and an
end, a beginning of future time and an end of past time, it follows
that there must always be time: for the extremity of the last period
of time that we take must be found in some moment, since time contains
no point of contact for us except the moment. Therefore, since the
moment is both a beginning and an end, there must always be time on
both sides of it. But if this is true of time, it is evident that it
must also be true of motion, time being a kind of affection of
motion.)

    The same reasoning will also serve to show the imperishability of
motion: just as a becoming of motion would involve, as we saw, the
existence of a process of change previous to the first, in the same
way a perishing of motion would involve the existence of a process
of change subsequent to the last: for when a thing ceases to be moved,
it does not therefore at the same time cease to be movable-e.g. the
cessation of the process of being burned does not involve the
cessation of the capacity of being burned, since a thing may be
capable of being burned without being in process of being
burned-nor, when a thing ceases to be movent, does it therefore at the
same time cease to a be motive. Again, the destructive agent will have
to be destroyed, after what it destroys has been destroyed, and then
that which has the capacity of destroying it will have to be destroyed
afterwards, (so that there will be a process of change subsequent to
the last,) for being destroyed also is a kind of change. If, then,
view which we are criticizing involves these impossible
consequences, it is clear that motion is eternal and cannot have
existed at one time and not at another: in fact such a view can hardly
be described as anythling else than fantastic.

    And much the same may be said of the view that such is the ordinance
of nature and that this must be regarded as a principle, as would seem
to be the view of Empedocles when he says that the constitution of the
world is of necessity such that Love and Strife alternately
predominate and cause motion, while in the intermediate period of time
there is a state of rest. Probably also those who like like
Anaxagoras, assert a single principle (of motion) would hold this
view. But that which is produced or directed by nature can never be
anything disorderly: for nature is everywhere the cause of order.
Moreover, there is no ratio in the relation of the infinite to the
infinite, whereas order always means ratio. But if we say that there
is first a state of rest for an infinite time, and then motion is
started at some moment, and that the fact that it is this rather
than a previous moment is of no importance, and involves no order,
then we can no longer say that it is nature's work: for if anything is
of a certain character naturally, it either is so invariably and is
not sometimes of this and sometimes of another character (e.g. fire,
which travels upwards naturally, does not sometimes do so and
sometimes not) or there is a ratio in the variation. It would be
better, therefore, to say with Empedocles and any one else who may
have maintained such a theory as his that the universe is
alternately at rest and in motion: for in a system of this kind we
have at once a certain order. But even here the holder of the theory
ought not only to assert the fact: he ought to explain the cause of
it: i.e. he should not make any mere assumption or lay down any
gratuitous axiom, but should employ either inductive or
demonstrative reasoning. The Love and Strife postulated by
Empedocles are not in themselves causes of the fact in question, nor
is it of the essence of either that it should be so, the essential
function of the former being to unite, of the latter to separate. If
he is to go on to explain this alternate predominance, he should
adduce cases where such a state of things exists, as he points to
the fact that among mankind we have something that unites men,
namely Love, while on the other hand enemies avoid one another: thus
from the observed fact that this occurs in certain cases comes the
assumption that it occurs also in the universe. Then, again, some
argument is needed to explain why the predominance of each of the
two forces lasts for an equal period of time. But it is a wrong
assumption to suppose universally that we have an adequate first
principle in virtue of the fact that something always is so or
always happens so. Thus Democritus reduces the causes that explain
nature to the fact that things happened in the past in the same way as
they happen now: but he does not think fit to seek for a first
principle to explain this "always": so, while his theory is right in
so far as it is applied to certain individual cases, he is wrong in
making it of universal application. Thus, a triangle always has its
angles equal to two right angles, but there is nevertheless an
ulterior cause of the eternity of this truth, whereas first principles
are eternal and have no ulterior cause. Let this conclude what we have
to say in support of our contention that there never was a time when
there was not motion, and never will be a time when there will not
be motion.