Since everything that changes changes from something to something,
that which has changed must at the moment when it has first changed be
in that to which it has changed. For that which changes retires from
or leaves that from which it changes: and leaving, if not identical
with changing, is at any rate a consequence of it. And if leaving is a
consequence of changing, having left is a consequence of having
changed: for there is a like relation between the two in each case.

    One kind of change, then, being change in a relation of
contradiction, where a thing has changed from not-being to being it
has left not-being. Therefore it will be in being: for everything must
either be or not be. It is evident, then, that in contradictory change
that which has changed must be in that to which it has changed. And if
this is true in this kind of change, it will be true in all other
kinds as well: for in this matter what holds good in the case of one
will hold good likewise in the case of the rest.

    Moreover, if we take each kind of change separately, the truth of
our conclusion will be equally evident, on the ground that that that
which has changed must be somewhere or in something. For, since it has
left that from which it has changed and must be somewhere, it must
be either in that to which it has changed or in something else. If,
then, that which has changed to B is in something other than B, say G,
it must again be changing from G to B: for it cannot be assumed that
there is no interval between G and B, since change is continuous. Thus
we have the result that the thing that has changed, at the moment when
it has changed, is changing to that to which it has changed, which
is impossible: that which has changed, therefore, must be in that to
which it has changed. So it is evident likewise that that that which
has come to be, at the moment when it has come to be, will be, and
that which has ceased to be will not-be: for what we have said applies
universally to every kind of change, and its truth is most obvious
in the case of contradictory change. It is clear, then, that that
which has changed, at the moment when it has first changed, is in that
to which it has changed.

    We will now show that the "primary when" in which that which has
changed effected the completion of its change must be indivisible,
where by "primary" I mean possessing the characteristics in question
of itself and not in virtue of the possession of them by something
else belonging to it. For let AG be divisible, and let it be divided
at B. If then the completion of change has been effected in AB or
again in BG, AG cannot be the primary thing in which the completion of
change has been effected. If, on the other hand, it has been
changing in both AB and BG (for it must either have changed or be
changing in each of them), it must have been changing in the whole AG:
but our assumption was that AG contains only the completion of the
change. It is equally impossible to suppose that one part of AG
contains the process and the other the completion of the change: for
then we shall have something prior to what is primary. So that in
which the completion of change has been effected must be
indivisible. It is also evident, therefore, that that that in which
that which has ceased to be has ceased to be and that in which that
which has come to be has come to be are indivisible.

    But there are two senses of the expression "the primary when in
which something has changed". On the one hand it may mean the
primary when containing the completion of the process of change- the
moment when it is correct to say "it has changed": on the other hand
it may mean the primary when containing the beginning of the process
of change. Now the primary when that has reference to the end of the
change is something really existent: for a change may really be
completed, and there is such a thing as an end of change, which we
have in fact shown to be indivisible because it is a limit. But that
which has reference to the beginning is not existent at all: for there
is no such thing as a beginning of a process of change, and the time
occupied by the change does not contain any primary when in which
the change began. For suppose that AD is such a primary when. Then
it cannot be indivisible: for, if it were, the moment immediately
preceding the change and the moment in which the change begins would
be consecutive (and moments cannot be consecutive). Again, if the
changing thing is at rest in the whole preceding time GA (for we may
suppose that it is at rest), it is at rest in A also: so if AD is
without parts, it will simultaneously be at rest and have changed: for
it is at rest in A and has changed in D. Since then AD is not
without parts, it must be divisible, and the changing thing must
have changed in every part of it (for if it has changed in neither
of the two parts into which AD is divided, it has not changed in the
whole either: if, on the other hand, it is in process of change in
both parts, it is likewise in process of change in the whole: and
if, again, it has changed in one of the two parts, the whole is not
the primary when in which it has changed: it must therefore have
changed in every part). It is evident, then, that with reference to
the beginning of change there is no primary when in which change has
been effected: for the divisions are infinite.

    So, too, of that which has changed there is no primary part that has
changed. For suppose that of AE the primary part that has changed is
AZ (everything that changes having been shown to be divisible): and
let OI be the time in which DZ has changed. If, then, in the whole
time DZ has changed, in half the time there will be a part that has
changed, less than and therefore prior to DZ: and again there will
be another part prior to this, and yet another, and so on to infinity.
Thus of that which changes there cannot be any primary part that has
changed. It is evident, then, from what has been said, that neither of
that which changes nor of the time in which it changes is there any
primary part.

    With regard, however, to the actual subject of change-that is to say
that in respect of which a thing changes-there is a difference to be
observed. For in a process of change we may distinguish three
terms-that which changes, that in which it changes, and the actual
subject of change, e.g. the man, the time, and the fair complexion. Of
these the man and the time are divisible: but with the fair complexion
it is otherwise (though they are all divisible accidentally, for
that in which the fair complexion or any other quality is an
accident is divisible). For of actual subjects of change it will be
seen that those which are classed as essentially, not accidentally,
divisible have no primary part. Take the case of magnitudes: let AB be
a magnitude, and suppose that it has moved from B to a primary "where"
G. Then if BG is taken to be indivisible, two things without parts
will have to be contiguous (which is impossible): if on the other hand
it is taken to be divisible, there will be something prior to G to
which the magnitude has changed, and something else again prior to
that, and so on to infinity, because the process of division may be
continued without end. Thus there can be no primary "where" to which a
thing has changed. And if we take the case of quantitative change,
we shall get a like result, for here too the change is in something
continuous. It is evident, then, that only in qualitative motion can
there be anything essentially indivisible.