Physics   

the change, e.g. the full-formed man begets man from what is
potentially man.
Part 3
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 quality? The thing is impossible:
the actualization will be one.
But (some one will say) it is contrary to reason to suppose that there
should be one identical actualization of two things which are
different in kind. Yet there will be, if teaching and learning are the
same, and agency and patiency. To teach will be the same as to learn,
and to act the same as to be acted on-the teacher will necessarily be
learning everything that he teaches, and the agent will be acted on.
One may reply:
(1) It is not absurd that the actualization of one thing should be in
another. Teaching is the activity of a person who can teach, yet the
operation is performed on some patient-it is not cut adrift from a
subject, but is of A on B.
(2) There is nothing to prevent two things having one and the same
actualization, provided the actualizations are not described in the
same way, but are related as what can act to what is acting.
(3) Nor is it necessary that the teacher should learn, even if to act
and to be acted on are one and the same, provided they are not the
same in definition (as 'raiment' and 'dress'), but are the same merely
in the sense in which the road from Thebes to Athens and the road from
Athens to Thebes are the same, as has been explained above. For it is
not things which are in a way the same that have all their attributes
the same, but only such as have the same definition. But indeed it by
no means follows from the fact that teaching is the same as learning,
that to learn is the same as to teach, any more than it follows from
the fact that there is one distance between two things which are at a
distance from each other, that the two vectors AB and Ba, are one and
the same. To generalize, teaching is not the same as learning, or
agency as patiency, in the full sense, though they belong to the same
subject, the motion; for the 'actualization of X in Y' and the
'actualization of Y through the action of X' differ in definition.
What then Motion is, has been stated both generally and particularly.