The Complete Aristotle (eng.)

actuality required But building is a kind of motion, and
the same account will apply to the other kinds also.
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    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 the change, e.g. the full-formed
man begets man from what is potentially man.
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    The solution of the difficulty that is raised about the
motion-whether it is in the movable-is plain. It is the fulfilment
of this potentiality, and by the action of that which has the power
of causing motion; and the actuality of that which has the power of
causing motion is not other than the actuality of the movable, for
it must be the fulfilment of both. A thing is capable of causing
motion because it can do this, it is a mover because it actually
does it. But it is on the movable that it is capable of acting.
Hence there is a single actuality of both alike, just as one to two
and two to one are the same interval, and the steep ascent and the
steep descent are one-for these are one and the same, although they
can be described in different ways. So it is with the mover and the
moved.
    This view has a dialectical difficulty. Perhaps it is necessary
that the actuality of the agent and that of the patient should not
be the same. The one is ‘agency’ and the other ‘patiency’; and the
outcome and completion of the one is an ‘action’, that of the other
a ‘passion’. Since then they are both motions, we may ask: in what
are they, if they are different? Either (a) both are in what is
acted on and moved, or (b) the agency is in the agent and the
patiency in the patient. (If we ought to call the latter also
‘agency’, the word would be used in two senses.)
    Now, in alternative (b), the motion will be in the mover, for
the same statement will hold of ‘mover’ and ‘moved’. Hence either
every mover will be moved, or, though having motion, it will not be
moved.
    If on the other hand (a) both are in what is moved and acted
on-both the agency and the patiency (e.g. both teaching and
learning, though they are two, in the learner), then, first, the
actuality of each will not be present in each, and, a second
absurdity, a thing will have two motions at the same time. How will
there be two alterations of quality in one subject towards one
definite