In respect of Substance there is no motion, because Substance has no
contrary among things that are. Nor is there motion in respect of
Relation: for it may happen that when one correlative changes, the
other, although this does not itself change, is no longer
applicable, so that in these cases the motion is accidental. Nor is
there motion in respect of Agent and Patient-in fact there can never
be motion of mover and moved, because there cannot be motion of motion
or becoming of becoming or in general change of change.

    For in the first place there are two senses in which motion of
motion is conceivable. (1) The motion of which there is motion might
be conceived as subject; e.g. a man is in motion because he changes
from fair to dark. Can it be that in this sense motion grows hot or
cold, or changes place, or increases or decreases? Impossible: for
change is not a subject. Or (2) can there be motion of motion in the
sense that some other subject changes from a change to another mode of
being, as e.g. a man changes from falling ill to getting well? Even
this is possible only in an accidental sense. For, whatever the
subject may be, movement is change from one form to another. (And
the same holds good of becoming and perishing, except that in these
processes we have a change to a particular kind of opposite, while the
other, motion, is a change to a different kind.) So, if there is to be
motion of motion, that which is changing from health to sickness
must simultaneously be changing from this very change to another. It
is clear, then, that by the time that it has become sick, it must also
have changed to whatever may be the other change concerned (for that
it should be at rest, though logically possible, is excluded by the
theory). Moreover this other can never be any casual change, but
must be a change from something definite to some other definite thing.
So in this case it must be the opposite change, viz. convalescence. It
is only accidentally that there can be change of change, e.g. there is
a change from remembering to forgetting only because the subject of
this change changes at one time to knowledge, at another to ignorance.

    In the second place, if there is to be change of change and becoming
of becoming, we shall have an infinite regress. Thus if one of a
series of changes is to be a change of change, the preceding change
must also be so: e.g. if simple becoming was ever in process of
becoming, then that which was becoming simple becoming was also in
process of becoming, so that we should not yet have arrived at what
was in process of simple becoming but only at what was already in
process of becoming in process of becoming. And this again was
sometime in process of becoming, so that even then we should not
have arrived at what was in process of simple becoming. And since in
an infinite series there is no first term, here there will be no first
stage and therefore no following stage either. On this hypothesis,
then, nothing can become or be moved or change.

    Thirdly, if a thing is capable of any particular motion, it is
also capable of the corresponding contrary motion or the corresponding
coming to rest, and a thing that is capable of becoming is also
capable of perishing: consequently, if there be becoming of
becoming, that which is in process of becoming is in process of
perishing at the very moment when it has reached the stage of
becoming: since it cannot be in process of perishing when it is just
beginning to become or after it has ceased to become: for that which
is in process of perishing must be in existence.

    Fourthly, there must be a substrate underlying all processes of
becoming and changing. What can this be in the present case? It is
either the body or the soul that undergoes alteration: what is it that
correspondingly becomes motion or becoming? And again what is the goal
of their motion? It must be the motion or becoming of something from
something to something else. But in what sense can this be so? For the
becoming of learning cannot be learning: so neither can the becoming
of becoming be becoming, nor can the becoming of any process be that
process.

    Finally, since there are three kinds of motion, the substratum and
the goal of motion must be one or other of these, e.g. locomotion will
have to be altered or to be locally moved.

    To sum up, then, since everything that is moved is moved in one of
three ways, either accidentally, or partially, or essentially,
change can change only accidentally, as e.g. when a man who is being
restored to health runs or learns: and accidental change we have
long ago decided to leave out of account.

    Since, then, motion can belong neither to Being nor to Relation
nor to Agent and Patient, it remains that there can be motion only
in respect of Quality, Quantity, and Place: for with each of these
we have a pair of contraries. Motion in respect of Quality let us call
alteration, a general designation that is used to include both
contraries: and by Quality I do not here mean a property of
substance (in that sense that which constitutes a specific distinction
is a quality) but a passive quality in virtue of which a thing is said
to be acted on or to be incapable of being acted on. Motion in respect
of Quantity has no name that includes both contraries, but it is
called increase or decrease according as one or the other is
designated: that is to say motion in the direction of complete
magnitude is increase, motion in the contrary direction is decrease.
Motion in respect of Place has no name either general or particular:
but we may designate it by the general name of locomotion, though
strictly the term "locomotion" is applicable to things that change
their place only when they have not the power to come to a stand,
and to things that do not move themselves locally.

    Change within the same kind from a lesser to a greater or from a
greater to a lesser degree is alteration: for it is motion either from
a contrary or to a contrary, whether in an unqualified or in a
qualified sense: for change to a lesser degree of a quality will be
called change to the contrary of that quality, and change to a greater
degree of a quality will be regarded as change from the contrary of
that quality to the quality itself. It makes no difference whether the
change be qualified or unqualified, except that in the former case the
contraries will have to be contrary to one another only in a qualified
sense: and a thing's possessing a quality in a greater or in a
lesser degree means the presence or absence in it of more or less of
the opposite quality. It is now clear, then, that there are only these
three kinds of motion.

    The term "immovable" we apply in the first place to that which is
absolutely incapable of being moved (just as we correspondingly
apply the term invisible to sound); in the second place to that
which is moved with difficulty after a long time or whose movement
is slow at the start-in fact, what we describe as hard to move; and in
the third place to that which is naturally designed for and capable of
motion, but is not in motion when, where, and as it naturally would be
so. This last is the only kind of immovable thing of which I use the
term "being at rest": for rest is contrary to motion, so that rest
will be negation of motion in that which is capable of admitting
motion.

    The foregoing remarks are sufficient to explain the essential nature
of motion and rest, the number of kinds of change, and the different
varieties of motion.