Physics   

the truth will be made clear later. is possible for a thing to cause
motion, though it is itself incapable of being moved.
It is the fulfilment of what is potential when it is already fully
real and operates not as itself but as movable, that is motion. What I
mean by 'as' is this: Bronze is potentially a statue. But it is not
the fulfilment of bronze as bronze which is motion. For 'to be bronze'
and 'to be a certain potentiality' are not the same.
If they were identical without qualification, i.e. in definition, the
fulfilment of bronze as bronze would have been motion. But they are
not the same, as has been said. (This is obvious in contraries. 'To be
capable of health' and 'to be capable of illness' are not the same,
for if they were there would be no difference between being ill and
being well. Yet the subject both of health and of sickness-whether it
is humour or blood-is one and the same.)
We can distinguish, then, between the two-just as, to give another
example, 'colour' and visible' are different-and clearly it is the
fulfilment of what is potential as potential that is motion. So this,
precisely, is motion.
Further it is evident that motion is an attribute of a thing just when
it is fully real in this way, and neither before nor after. For each
thing of this kind is capable of being at one time actual, at another
not. Take for instance the buildable as buildable. The actuality of
the buildable as buildable is the process of building. For the
actuality of the buildable must be either this or the house. But when
there is a house, the buildable is no longer buildable. On the other
hand, it is the buildable which is being built. The process then of
being built must be the kind of actuality required But building is a
kind of motion, and the same account will apply to the other kinds
also.
Part 2
The soundness of this definition is evident both when we consider the
accounts of motion that the others have given, and also from the
difficulty of defining it otherwise.
One could not easily put motion and change in another genus-this is
plain if we consider where some people put it; they identify motion
with or 'inequality' or 'not being'; but such things are not
necessarily moved, whether they are 'different' or 'unequal' or
'non-existent'; Nor is change either to or from these rather than to
or from their opposites.
The reason why they put motion into these genera is that it is thought
to be something indefinite, and the principles in the second column
are indefinite because they are privative: none of them is either
'this' or 'such' or comes under any of the other modes of predication.
The reason in turn why motion is thought to be indefinite is that it
cannot be classed simply as a potentiality or as an actuality-a thing
that is merely capable of having a certain size is not undergoing
change, nor yet a thing that is actually of a certain size, and motion
is thought to be a sort of actuality, but incomplete, the reason for
this view being that the potential whose actuality it is is
incomplete. This is why it is hard to grasp what motion is. It is
necessary to class it with privation or with potentiality or with
sheer actuality, yet none of these seems possible. There remains then
the suggested mode of definition, namely that it is a sort of
actuality, or actuality of the kind described, hard to grasp, but not
incapable of existing.
The mover too is moved, as has been said-every mover, that is, which
is capable of motion, and whose immobility is rest-when a thing is
subject to motion its immobility is rest. For to act on the movable as
such is just to move it. But this it does by contact, so that at the
same time it is also acted on. Hence we can define motion as the
fulfilment of the movable qua movable, the cause of the attribute
being contact with what can move so that the mover is also acted on.
The mover or agent will always be the vehicle of a form, either a
'this' or 'such', which, when it acts, will be the source and cause of