Now of things that cause motion or suffer motion, to some the motion
is accidental, to others essential: thus it is accidental to what
merely belongs to or contains as a part a thing that causes motion
or suffers motion, essential to a thing that causes motion or
suffers motion not merely by belonging to such a thing or containing
it as a part.

    Of things to which the motion is essential some derive their
motion from themselves, others from something else: and in some
cases their motion is natural, in others violent and unnatural. Thus
in things that derive their motion from themselves, e.g. all
animals, the motion is natural (for when an animal is in motion its
motion is derived from itself): and whenever the source of the
motion of a thing is in the thing itself we say that the motion of
that thing is natural. Therefore the animal as a whole moves itself
naturally: but the body of the animal may be in motion unnaturally
as well as naturally: it depends upon the kind of motion that it may
chance to be suffering and the kind of element of which it is
composed. And the motion of things that derive their motion from
something else is in some cases natural, in other unnatural: e.g.
upward motion of earthy things and downward motion of fire are
unnatural. Moreover the parts of animals are often in motion in an
unnatural way, their positions and the character of the motion being
abnormal. The fact that a thing that is in motion derives its motion
from something is most evident in things that are in motion
unnaturally, because in such cases it is clear that the motion is
derived from something other than the thing itself. Next to things
that are in motion unnaturally those whose motion while natural is
derived from themselves-e.g. animals-make this fact clear: for here
the uncertainty is not as to whether the motion is derived from
something but as to how we ought to distinguish in the thing between
the movent and the moved. It would seem that in animals, just as in
ships and things not naturally organized, that which causes motion
is separate from that which suffers motion, and that it is only in
this sense that the animal as a whole causes its own motion.

    The greatest difficulty, however, is presented by the remaining case
of those that we last distinguished. Where things derive their
motion from something else we distinguished the cases in which the
motion is unnatural: we are left with those that are to be
contrasted with the others by reason of the fact that the motion is
natural. It is in these cases that difficulty would be experienced
in deciding whence the motion is derived, e.g. in the case of light
and heavy things. When these things are in motion to positions the
reverse of those they would properly occupy, their motion is
violent: when they are in motion to their proper positions-the light
thing up and the heavy thing down-their motion is natural; but in this
latter case it is no longer evident, as it is when the motion is
unnatural, whence their motion is derived. It is impossible to say
that their motion is derived from themselves: this is a characteristic
of life and peculiar to living things. Further, if it were, it would
have been in their power to stop themselves (I mean that if e.g. a
thing can cause itself to walk it can also cause itself not to
walk), and so, since on this supposition fire itself possesses the
power of upward locomotion, it is clear that it should also possess
the power of downward locomotion. Moreover if things move
themselves, it would be unreasonable to suppose that in only one
kind of motion is their motion derived from themselves. Again, how can
anything of continuous and naturally connected substance move
itself? In so far as a thing is one and continuous not merely in
virtue of contact, it is impassive: it is only in so far as a thing is
divided that one part of it is by nature active and another passive.
Therefore none of the things that we are now considering move
themselves (for they are of naturally connected substance), nor does
anything else that is continuous: in each case the movent must be
separate from the moved, as we see to be the case with inanimate
things when an animate thing moves them. It is the fact that these
things also always derive their motion from something: what it is
would become evident if we were to distinguish the different kinds
of cause.

    The above-mentioned distinctions can also be made in the case of
things that cause motion: some of them are capable of causing motion
unnaturally (e.g. the lever is not naturally capable of moving the
weight), others naturally (e.g. what is actually hot is naturally
capable of moving what is potentially hot): and similarly in the
case of all other things of this kind.

    In the same way, too, what is potentially of a certain quality or of
a certain quantity in a certain place is naturally movable when it
contains the corresponding principle in itself and not accidentally
(for the same thing may be both of a certain quality and of a
certain quantity, but the one is an accidental, not an essential
property of the other). So when fire or earth is moved by something
the motion is violent when it is unnatural, and natural when it brings
to actuality the proper activities that they potentially possess.
But the fact that the term "potentially" is used in more than one
sense is the reason why it is not evident whence such motions as the
upward motion of fire and the downward motion of earth are derived.
One who is learning a science potentially knows it in a different
sense from one who while already possessing the knowledge is not
actually exercising it. Wherever we have something capable of acting
and something capable of being correspondingly acted on, in the
event of any such pair being in contact what is potential becomes at
times actual: e.g. the learner becomes from one potential something
another potential something: for one who possesses knowledge of a
science but is not actually exercising it knows the science
potentially in a sense, though not in the same sense as he knew it
potentially before he learnt it. And when he is in this condition,
if something does not prevent him, he actively exercises his
knowledge: otherwise he would be in the contradictory state of not
knowing. In regard to natural bodies also the case is similar. Thus
what is cold is potentially hot: then a change takes place and it is
fire, and it burns, unless something prevents and hinders it. So, too,
with heavy and light: light is generated from heavy, e.g. air from
water (for water is the first thing that is potentially light), and
air is actually light, and will at once realize its proper activity as
such unless something prevents it. The activity of lightness
consists in the light thing being in a certain situation, namely
high up: when it is in the contrary situation, it is being prevented
from rising. The case is similar also in regard to quantity and
quality. But, be it noted, this is the question we are trying to
answer-how can we account for the motion of light things and heavy
things to their proper situations? The reason for it is that they have
a natural tendency respectively towards a certain position: and this
constitutes the essence of lightness and heaviness, the former being
determined by an upward, the latter by a downward, tendency. As we
have said, a thing may be potentially light or heavy in more senses
than one. Thus not only when a thing is water is it in a sense
potentially light, but when it has become air it may be still
potentially light: for it may be that through some hindrance it does
not occupy an upper position, whereas, if what hinders it is
removed, it realizes its activity and continues to rise higher. The
process whereby what is of a certain quality changes to a condition of
active existence is similar: thus the exercise of knowledge follows at
once upon the possession of it unless something prevents it. So,
too, what is of a certain quantity extends itself over a certain space
unless something prevents it. The thing in a sense is and in a sense
is not moved by one who moves what is obstructing and preventing its
motion (e.g. one who pulls away a pillar from under a roof or one
who removes a stone from a wineskin in the water is the accidental
cause of motion): and in the same way the real cause of the motion
of a ball rebounding from a wall is not the wall but the thrower. So
it is clear that in all these cases the thing does not move itself,
but it contains within itself the source of motion-not of moving
something or of causing motion, but of suffering it.

    If then the motion of all things that are in motion is either
natural or unnatural and violent, and all things whose motion is
violent and unnatural are moved by something, and something other than
themselves, and again all things whose motion is natural are moved
by something-both those that are moved by themselves and those that
are not moved by themselves (e.g. light things and heavy things, which
are moved either by that which brought the thing into existence as
such and made it light and heavy, or by that which released what was
hindering and preventing it); then all things that are in motion
must be moved by something.