The Complete Aristotle (eng.)

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