Various Works Page 8

  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

  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.

  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