This matter will be made clearer, however, if we start afresh from
another point. We must consider whether it is or is not possible
that there should be a continuous motion, and, if it is possible,
which this motion is, and which is the primary motion: for it is plain
that if there must always be motion, and a particular motion is
primary and continuous, then it is this motion that is imparted by the
first movent, and so it is necessarily one and the same and continuous
and primary.

    Now of the three kinds of motion that there are-motion in respect of
magnitude, motion in respect of affection, and motion in respect of
place-it is this last, which we call locomotion, that must be primary.
This may be shown as follows. It is impossible that there should be
increase without the previous occurrence of alteration: for that which
is increased, although in a sense it is increased by what is like
itself, is in a sense increased by what is unlike itself: thus it is
said that contrary is nourishment to contrary: but growth is
effected only by things becoming like to like. There must be
alteration, then, in that there is this change from contrary to
contrary. But the fact that a thing is altered requires that there
should be something that alters it, something e.g. that makes the
potentially hot into the actually hot: so it is plain that the
movent does not maintain a uniform relation to it but is at one time
nearer to and at another farther from that which is altered: and we
cannot have this without locomotion. If, therefore, there must
always be motion, there must also always be locomotion as the
primary motion, and, if there is a primary as distinguished from a
secondary form of locomotion, it must be the primary form. Again,
all affections have their origin in condensation and rarefaction: thus
heavy and light, soft and hard, hot and cold, are considered to be
forms of density and rarity. But condensation and rarefaction are
nothing more than combination and separation, processes in
accordance with which substances are said to become and perish: and in
being combined and separated things must change in respect of place.
And further, when a thing is increased or decreased its magnitude
changes in respect of place.

    Again, there is another point of view from which it will be
clearly seen that locomotion is primary. As in the case of other
things so too in the case of motion the word "primary" may be used
in several senses. A thing is said to be prior to other things when,
if it does not exist, the others will not exist, whereas it can
exist without the others: and there is also priority in time and
priority in perfection of existence. Let us begin, then, with the
first sense. Now there must be motion continuously, and there may be
continuously either continuous motion or successive motion, the
former, however, in a higher degree than the latter: moreover it is
better that it should be continuous rather than successive motion, and
we always assume the presence in nature of the better, if it be
possible: since, then, continuous motion is possible (this will be
proved later: for the present let us take it for granted), and no
other motion can be continuous except locomotion, locomotion must be
primary. For there is no necessity for the subject of locomotion to be
the subject either of increase or of alteration, nor need it become or
perish: on the other hand there cannot be any one of these processes
without the existence of the continuous motion imparted by the first
movent.

    Secondly, locomotion must be primary in time: for this is the only
motion possible for things. It is true indeed that, in the case of any
individual thing that has a becoming, locomotion must be the last of
its motions: for after its becoming it first experiences alteration
and increase, and locomotion is a motion that belongs to such things
only when they are perfected. But there must previously be something
else that is in process of locomotion to be the cause even of the
becoming of things that become, without itself being in process of
becoming, as e.g. the begotten is preceded by what begot it: otherwise
becoming might be thought to be the primary motion on the ground
that the thing must first become. But though this is so in the case of
any individual thing that becomes, nevertheless before anything
becomes, something else must be in motion, not itself becoming but
being, and before this there must again be something else. And since
becoming cannot be primary-for, if it were, everything that is in
motion would be perishable-it is plain that no one of the motions next
in order can be prior to locomotion. By the motions next in order I
mean increase and then alteration, decrease, and perishing. All
these are posterior to becoming: consequently, if not even becoming is
prior to locomotion, then no one of the other processes of change is
so either.

    Thirdly, that which is in process of becoming appears universally as
something imperfect and proceeding to a first principle: and so what
is posterior in the order of becoming is prior in the order of nature.
Now all things that go through the process of becoming acquire
locomotion last. It is this that accounts for the fact that some
living things, e.g. plants and many kinds of animals, owing to lack of
the requisite organ, are entirely without motion, whereas others
acquire it in the course of their being perfected. Therefore, if the
degree in which things possess locomotion corresponds to the degree in
which they have realized their natural development, then this motion
must be prior to all others in respect of perfection of existence: and
not only for this reason but also because a thing that is in motion
loses its essential character less in the process of locomotion than
in any other kind of motion: it is the only motion that does not
involve a change of being in the sense in which there is a change in
quality when a thing is altered and a change in quantity when a
thing is increased or decreased. Above all it is plain that this
motion, motion in respect of place, is what is in the strictest
sense produced by that which moves itself; but it is the self-movent
that we declare to be the first principle of things that are moved and
impart motion and the primary source to which things that are in
motion are to be referred.

    It is clear, then, from the foregoing arguments that locomotion is
the primary motion. We have now to show which kind of locomotion is
primary. The same process of reasoning will also make clear at the
same time the truth of the assumption we have made both now and at a
previous stage that it is possible that there should be a motion
that is continuous and eternal. Now it is clear from the following
considerations that no other than locomotion can be continuous.
Every other motion and change is from an opposite to an opposite: thus
for the processes of becoming and perishing the limits are the
existent and the non-existent, for alteration the various pairs of
contrary affections, and for increase and decrease either greatness
and smallness or perfection and imperfection of magnitude: and changes
to the respective contraries are contrary changes. Now a thing that is
undergoing any particular kind of motion, but though previously
existent has not always undergone it, must previously have been at
rest so far as that motion is concerned. It is clear, then, that for
the changing thing the contraries will be states of rest. And we
have a similar result in the case of changes that are not motions: for
becoming and perishing, whether regarded simply as such without
qualification or as affecting something in particular, are
opposites: therefore provided it is impossible for a thing to
undergo opposite changes at the same time, the change will not be
continuous, but a period of time will intervene between the opposite
processes. The question whether these contradictory changes are
contraries or not makes no difference, provided only it is
impossible for them both to be present to the same thing at the same
time: the point is of no importance to the argument. Nor does it
matter if the thing need not rest in the contradictory state, or if
there is no state of rest as a contrary to the process of change: it
may be true that the non-existent is not at rest, and that perishing
is a process to the non-existent. All that matters is the intervention
of a time: it is this that prevents the change from being
continuous: so, too, in our previous instances the important thing was
not the relation of contrariety but the impossibility of the two
processes being present to a thing at the same time. And there is no
need to be disturbed by the fact that on this showing there may be
more than one contrary to the same thing, that a particular motion
will be contrary both to rest and to motion in the contrary direction.
We have only to grasp the fact that a particular motion is in a
sense the opposite both of a state of rest and of the contrary motion,
in the same way as that which is of equal or standard measure is the
opposite both of that which surpasses it and of that which it
surpasses, and that it is impossible for the opposite motions or
changes to be present to a thing at the same time. Furthermore, in the
case of becoming and perishing it would seem to be an utterly absurd
thing if as soon as anything has become it must necessarily perish and
cannot continue to exist for any time: and, if this is true of
becoming and perishing, we have fair grounds for inferring the same to
be true of the other kinds of change, since it would be in the natural
order of things that they should be uniform in this respect.