discuss whether it is by intelligence or by some other faculty that
these creatures work,spiders, ants, and the like. By gradual advance
in this direction we come to see clearly that in plants too that is
produced which is conducive to the end-leaves, e.g. grow to provide
shade for the fruit. If then it is both by nature and for an end
that the swallow makes its nest and the spider its web, and plants
grow leaves for the sake of the fruit and send their roots down (not
up) for the sake of nourishment, it is plain that this kind of cause
is operative in things which come to be and are by nature. And since
'nature' means two things, the matter and the form, of which the
latter is the end, and since all the rest is for the sake of the
end, the form must be the cause in the sense of 'that for the sake
of which'.
Now mistakes come to pass even in the operations of art: the
grammarian makes a mistake in writing and the doctor pours out the
wrong dose. Hence clearly mistakes are possible in the operations of
nature also. If then in art there are cases in which what is rightly
produced serves a purpose, and if where mistakes occur there was a
purpose in what was attempted, only it was not attained, so must it be
also in natural products, and monstrosities will be failures in the
purposive effort. Thus in the original combinations the 'ox-progeny'
if they failed to reach a determinate end must have arisen through the
corruption of some principle corresponding to what is now the seed.
Further, seed must have come into being first, and not straightway
the animals: the words 'whole-natured first...' must have meant seed.
Again, in plants too we find the relation of means to end, though
the degree of organization is less. Were there then in plants also
'olive-headed vine-progeny', like the 'man-headed ox-progeny', or not?
An absurd suggestion; yet there must have been, if there were such
things among animals.
Moreover, among the seeds anything must have come to be at random.
But the person who asserts this entirely does away with 'nature' and
what exists 'by nature'. For those things are natural which, by a
continuous movement originated from an internal principle, arrive at
some completion: the same completion is not reached from every
principle; nor any chance completion, but always the tendency in
each is towards the same end, if there is no impediment.
The end and the means towards it may come about by chance. We say,
for instance, that a stranger has come by chance, paid the ransom, and
gone away, when he does so as if he had come for that purpose,
though it was not for that that he came. This is incidental, for
chance is an incidental cause, as I remarked before. But when an event
takes place always or for the most part, it is not incidental or by
chance. In natural products the sequence is invariable, if there is no
impediment.
It is absurd to suppose that purpose is not present because we do
not observe the agent deliberating. Art does not deliberate. If the
ship-building art were in the wood, it would produce the same
results by nature. If, therefore, purpose is present in art, it is
present also in nature. The best illustration is a doctor doctoring
himself: nature is like that.
It is plain then that nature is a cause, a cause that operates for a
purpose.
9
As regards what is 'of necessity', we must ask whether the necessity
is 'hypothetical', or 'simple' as well. The current view places what
is of necessity in the process of production, just as if one were to
suppose that the wall of a house necessarily comes to be because
what is heavy is naturally carried downwards and what is light to
the top, wherefore the stones and foundations take the lowest place,
with earth above because it is lighter, and wood at the top of all
as being the lightest. Whereas, though the wall does not come to be
without these, it is not due to these, except as its material cause:
it comes to be for the sake of sheltering and guarding certain things.
Similarly in all other things which involve production for an end; the
product cannot come to be without things which have a necessary
nature, but it is not due to these (except as its material); it
comes to be for an end. For instance, why is a saw such as it is? To
effect so-and-so and for the sake of so-and-so. This end, however,
cannot be realized unless the saw is made of iron. It is, therefore,
necessary for it to be of iron, it we are to have a saw and perform
the operation of sawing. What is necessary then, is necessary on a
hypothesis; it is not a result necessarily determined by
antecedents. Necessity is in the matter, while 'that for the sake of
which' is in the definition.
Necessity in mathematics is in a way similar to necessity in
things which come to be through the operation of nature. Since a
straight line is what it is, it is necessary that the angles of a
triangle should equal two right angles. But not conversely; though
if the angles are not equal to two right angles, then the straight
line is not what it is either. But in things which come to be for an
end, the reverse is true. If the end is to exist or does exist, that
also which precedes it will exist or does exist; otherwise just as
there, if-the conclusion is not true, the premiss will not be true, so
here the end or 'that for the sake of which' will not exist. For
this too is itself a starting-point, but of the reasoning, not of
the action; while in mathematics the starting-point is the
starting-point of the reasoning only, as there is no action. If then
there is to be a house, such-and-such things must be made or be
there already or exist, or generally the matter relative to the end,
bricks and stones if it is a house. But the end is not due to these
except as the matter, nor will it come to exist because of them. Yet
if they do not exist at all, neither will the house, or the saw-the
former in the absence of stones, the latter in the absence of
iron-just as in the other case the premisses will not be true, if
the angles of the triangle are not equal to two right angles.
The necessary in nature, then, is plainly what we call by the name
of matter, and the changes in it. Both causes must be stated by the
physicist, but especially the end; for that is the cause of the
matter, not vice versa; and the end is 'that for the sake of which',
and the beginning starts from the definition or essence; as in
artificial products, since a house is of such-and-such a kind, certain
things must necessarily come to be or be there already, or since
health is this, these things must necessarily come to be or be there
already. Similarly if man is this, then these; if these, then those.
Perhaps the necessary is present also in the definition. For if one
defines the operation of sawing as being a certain kind of dividing,
then this cannot come about unless the saw has teeth of a certain
kind; and these cannot be unless it is o
f iron. For in the
definition too there are some parts that are, as it were, its matter.
Book III
1
NATURE has been defined as a 'principle of motion and change', and
it is the subject of our inquiry. We must therefore see that we
understand the meaning of 'motion'; for if it were unknown, the
meaning of 'nature' too would be unknown.
When we have determined the nature of motion, our next task will
be to attack in the same way the terms which are involved in it. Now
motion is supposed to belong to the class of things which are
continuous; and the infinite presents itself first in the
continuous-that is how it comes about that 'infinite' is often used in
definitions of the continuous ('what is infinitely divisible is
continuous'). Besides these, place, void, and time are thought to be
necessary conditions of motion.
Clearly, then, for these reasons and also because the attributes
mentioned are common to, and coextensive with, all the objects of
our science, we must first take each of them in hand and discuss it.
For the investigation of special attributes comes after that of the
common attributes.
To begin then, as we said, with motion.
We may start by distinguishing (1) what exists in a state of
fulfilment only, (2) what exists as potential, (3) what exists as
potential and also in fulfilment-one being a 'this', another 'so
much', a third 'such', and similarly in each of the other modes of the
predication of being.
Further, the word 'relative' is used with reference to (1) excess
and defect, (2) agent and patient and generally what can move and what
can be moved. For 'what can cause movement' is relative to 'what can
be moved', and vice versa.
Again, there is no such thing as motion over and above the things.
It is always with respect to substance or to quantity or to quality or
to place that what changes changes. But it is impossible, as we
assert, to find anything common to these which is neither 'this' nor
quantum nor quale nor any of the other predicates. Hence neither
will motion and change have reference to something over and above
the things mentioned, for there is nothing over and above them.
Now each of these belongs to all its subjects in either of two ways:
namely (1) substance-the one is positive form, the other privation;
(2) in quality, white and black; (3) in quantity, complete and
incomplete; (4) in respect of locomotion, upwards and downwards or
light and heavy. Hence there are as many types of motion or change
as there are meanings of the word 'is'.
We have now before us the distinctions in the various classes of
being between what is full real and what is potential.
Def. The fulfilment of what exists potentially, in so far as it
exists potentially, is motion-namely, of what is alterable qua
alterable, alteration: of what can be increased and its opposite
what can be decreased (there is no common name), increase and
decrease: of what can come to be and can pass away, coming to he and
passing away: of what can be carried along, locomotion.
Examples will elucidate this definition of motion. When the
buildable, in so far as it is just that, is fully real, it is being
built, and this is building. Similarly, learning, doctoring,
rolling, leaping, ripening, ageing.
The same thing, if it is of a certain kind, can be both potential
and fully real, not indeed at the same time or not in the same
respect, but e.g. potentially hot and actually cold. Hence at once
such things will act and be acted on by one another in many ways: each
of them will be capable at the same time of causing alteration and
of being altered. Hence, too, what effects motion as a physical
agent can be moved: when a thing of this kind causes motion, it is
itself also moved. This, indeed, has led some people to suppose that
every mover is moved. But this question depends on another set of
arguments, and 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
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.
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 po
tential 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.
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