Various Works

  an impulse towards or away from an object. Further, if it were the

  nutritive faculty, even plants would have been capable of

  originating such movement and would have possessed the organs

  necessary to carry it out. Similarly it cannot be the sensitive

  faculty either; for there are many animals which have sensibility

  but remain fast and immovable throughout their lives.

  If then Nature never makes anything without a purpose and never

  leaves out what is necessary (except in the case of mutilated or

  imperfect growths; and that here we have neither mutilation nor

  imperfection may be argued from the facts that such animals (a) can

  reproduce their species and (b) rise to completeness of nature and

  decay to an end), it follows that, had they been capable of

  originating forward movement, they would have possessed the organs

  necessary for that purpose. Further, neither can the calculative

  faculty or what is called 'mind' be the cause of such movement; for

  mind as speculative never thinks what is practicable, it never says

  anything about an object to be avoided or pursued, while this movement

  is always in something which is avoiding or pursuing an object. No,

  not even when it is aware of such an object does it at once enjoin

  pursuit or avoidance of it; e.g. the mind often thinks of something

  terrifying or pleasant without enjoining the emotion of fear. It is

  the heart that is moved (or in the case of a pleasant object some

  other part). Further, even when the mind does command and thought bids

  us pursue or avoid something, sometimes no movement is produced; we

  act in accordance with desire, as in the case of moral weakness.

  And, generally, we observe that the possessor of medical knowledge

  is not necessarily healing, which shows that something else is

  required to produce action in accordance with knowledge; the knowledge

  alone is not the cause. Lastly, appetite too is incompetent to account

  fully for movement; for those who successfully resist temptation

  have appetite and desire and yet follow mind and refuse to enact

  that for which they have appetite.

  10

  These two at all events appear to be sources of movement: appetite

  and mind (if one may venture to regard imagination as a kind of

  thinking; for many men follow their imaginations contrary to

  knowledge, and in all animals other than man there is no thinking or

  calculation but only imagination).

  Both of these then are capable of originating local movement, mind

  and appetite: (1) mind, that is, which calculates means to an end,

  i.e. mind practical (it differs from mind speculative in the character

  of its end); while (2) appetite is in every form of it relative to

  an end: for that which is the object of appetite is the stimulant of

  mind practical; and that which is last in the process of thinking is

  the beginning of the action. It follows that there is a

  justification for regarding these two as the sources of movement, i.e.

  appetite and practical thought; for the object of appetite starts a

  movement and as a result of that thought gives rise to movement, the

  object of appetite being it a source of stimulation. So too when

  imagination originates movement, it necessarily involves appetite.

  That which moves therefore is a single faculty and the faculty of

  appetite; for if there had been two sources of movement-mind and

  appetite-they would have produced movement in virtue of some common

  character. As it is, mind is never found producing movement without

  appetite (for wish is a form of appetite; and when movement is

  produced according to calculation it is also according to wish), but

  appetite can originate movement contrary to calculation, for desire is

  a form of appetite. Now mind is always right, but appetite and

  imagination may be either right or wrong. That is why, though in any

  case it is the object of appetite which originates movement, this

  object may be either the real or the apparent good. To produce

  movement the object must be more than this: it must be good that can

  be brought into being by action; and only what can be otherwise than

  as it is can thus be brought into being. That then such a power in the

  soul as has been described, i.e. that called appetite, originates

  movement is clear. Those who distinguish parts in the soul, if they

  distinguish and divide in accordance with differences of power, find

  themselves with a very large number of parts, a nutritive, a

  sensitive, an intellective, a deliberative, and now an appetitive

  part; for these are more different from one another than the faculties

  of desire and passion.

  Since appetites run counter to one another, which happens when a

  principle of reason and a desire are contrary and is possible only

  in beings with a sense of time (for while mind bids us hold back

  because of what is future, desire is influenced by what is just at

  hand: a pleasant object which is just at hand presents itself as

  both pleasant and good, without condition in either case, because of

  want of foresight into what is farther away in time), it follows

  that while that which originates movement must be specifically one,

  viz. the faculty of appetite as such (or rather farthest back of all

  the object of that faculty; for it is it that itself remaining unmoved

  originates the movement by being apprehended in thought or

  imagination), the things that originate movement are numerically many.

  All movement involves three factors, (1) that which originates the

  movement, (2) that by means of which it originates it, and (3) that

  which is moved. The expression 'that which originates the movement' is

  ambiguous: it may mean either (a) something which itself is unmoved or

  (b) that which at once moves and is moved. Here that which moves

  without itself being moved is the realizable good, that which at

  once moves and is moved is the faculty of appetite (for that which

  is influenced by appetite so far as it is actually so influenced is

  set in movement, and appetite in the sense of actual appetite is a

  kind of movement), while that which is in motion is the animal. The

  instrument which appetite employs to produce movement is no longer

  psychical but bodily: hence the examination of it falls within the

  province of the functions common to body and soul. To state the matter

  summarily at present, that which is the instrument in the production

  of movement is to be found where a beginning and an end coincide as

  e.g. in a ball and socket joint; for there the convex and the

  concave sides are respectively an end and a beginning (that is why

  while the one remains at rest, the other is moved): they are

  separate in definition but not separable spatially. For everything

  is moved by pushing and pulling. Hence just as in the case of a wheel,

  so here there must be a point which remains at rest, and from that

  point the movement must originate.

  To sum up, then, and repeat what I have said, inasmuch as an

  animal is capable of appetite it is capable of self-movement; it is

  not capable of appetite
without possessing imagination; and all

  imagination is either (1) calculative or (2) sensitive. In the

  latter an animals, and not only man, partake.

  11

  We must consider also in the case of imperfect animals, sc. those

  which have no sense but touch, what it is that in them originates

  movement. Can they have imagination or not? or desire? Clearly they

  have feelings of pleasure and pain, and if they have these they must

  have desire. But how can they have imagination? Must not we say

  that, as their movements are indefinite, they have imagination and

  desire, but indefinitely?

  Sensitive imagination, as we have said, is found in all animals,

  deliberative imagination only in those that are calculative: for

  whether this or that shall be enacted is already a task requiring

  calculation; and there must be a single standard to measure by, for

  that is pursued which is greater. It follows that what acts in this

  way must be able to make a unity out of several images.

  This is the reason why imagination is held not to involve opinion,

  in that it does not involve opinion based on inference, though opinion

  involves imagination. Hence appetite contains no deliberative element.

  Sometimes it overpowers wish and sets it in movement: at times wish

  acts thus upon appetite, like one sphere imparting its movement to

  another, or appetite acts thus upon appetite, i.e. in the condition of

  moral weakness (though by nature the higher faculty is always more

  authoritative and gives rise to movement). Thus three modes of

  movement are possible.

  The faculty of knowing is never moved but remains at rest. Since the

  one premiss or judgement is universal and the other deals with the

  particular (for the first tells us that such and such a kind of man

  should do such and such a kind of act, and the second that this is

  an act of the kind meant, and I a person of the type intended), it

  is the latter opinion that really originates movement, not the

  universal; or rather it is both, but the one does so while it

  remains in a state more like rest, while the other partakes in

  movement.

  12

  The nutritive soul then must be possessed by everything that is

  alive, and every such thing is endowed with soul from its birth to its

  death. For what has been born must grow, reach maturity, and decay-all

  of which are impossible without nutrition. Therefore the nutritive

  faculty must be found in everything that grows and decays.

  But sensation need not be found in all things that live. For it is

  impossible for touch to belong either (1) to those whose body is

  uncompounded or (2) to those which are incapable of taking in the

  forms without their matter.

  But animals must be endowed with sensation, since Nature does

  nothing in vain. For all things that exist by Nature are means to an

  end, or will be concomitants of means to an end. Every body capable of

  forward movement would, if unendowed with sensation, perish and fail

  to reach its end, which is the aim of Nature; for how could it

  obtain nutriment? Stationary living things, it is true, have as

  their nutriment that from which they have arisen; but it is not

  possible that a body which is not stationary but produced by

  generation should have a soul and a discerning mind without also

  having sensation. (Nor yet even if it were not produced by generation.

  Why should it not have sensation? Because it were better so either for

  the body or for the soul? But clearly it would not be better for

  either: the absence of sensation will not enable the one to think

  better or the other to exist better.) Therefore no body which is not

  stationary has soul without sensation.

  But if a body has sensation, it must be either simple or compound.

  And simple it cannot be; for then it could not have touch, which is

  indispensable. This is clear from what follows. An animal is a body

  with soul in it: every body is tangible, i.e. perceptible by touch;

  hence necessarily, if an animal is to survive, its body must have

  tactual sensation. All the other senses, e.g. smell, sight, hearing,

  apprehend through media; but where there is immediate contact the

  animal, if it has no sensation, will be unable to avoid some things

  and take others, and so will find it impossible to survive. That is

  why taste also is a sort of touch; it is relative to nutriment,

  which is just tangible body; whereas sound, colour, and odour are

  innutritious, and further neither grow nor decay. Hence it is that

  taste also must be a sort of touch, because it is the sense for what

  is tangible and nutritious.

  Both these senses, then, are indispensable to the animal, and it

  is clear that without touch it is impossible for an animal to be.

  All the other senses subserve well-being and for that very reason

  belong not to any and every kind of animal, but only to some, e.g.

  those capable of forward movement must have them; for, if they are

  to survive, they must perceive not only by immediate contact but

  also at a distance from the object. This will be possible if they

  can perceive through a medium, the medium being affected and moved

  by the perceptible object, and the animal by the medium. just as

  that which produces local movement causes a change extending to a

  certain point, and that which gave an impulse causes another to

  produce a new impulse so that the movement traverses a medium the

  first mover impelling without being impelled, the last moved being

  impelled without impelling, while the medium (or media, for there

  are many) is both-so is it also in the case of alteration, except that

  the agent produces produces it without the patient's changing its

  place. Thus if an object is dipped into wax, the movement goes on

  until submersion has taken place, and in stone it goes no distance

  at all, while in water the disturbance goes far beyond the object

  dipped: in air the disturbance is propagated farthest of all, the

  air acting and being acted upon, so long as it maintains an unbroken

  unity. That is why in the case of reflection it is better, instead

  of saying that the sight issues from the eye and is reflected, to

  say that the air, so long as it remains one, is affected by the

  shape and colour. On a smooth surface the air possesses unity; hence

  it is that it in turn sets the sight in motion, just as if the

  impression on the wax were transmitted as far as the wax extends.

  13

  It is clear that the body of an animal cannot be simple, i.e.

  consist of one element such as fire or air. For without touch it is

  impossible to have any other sense; for every body that has soul in it

  must, as we have said, be capable of touch. All the other elements

  with the exception of earth can constitute organs of sense, but all of

  them bring about perception only through something else, viz.

  through the media. Touch takes place by direct contact with its

  objects, whence also its name. All the other organs of sense, no

  doubt, perceive by contact, only the contact is mediate: touch alone

&nbs
p; perceives by immediate contact. Consequently no animal body can

  consist of these other elements.

  Nor can it consist solely of earth. For touch is as it were a mean

  between all tangible qualities, and its organ is capable of

  receiving not only all the specific qualities which characterize

  earth, but also the hot and the cold and all other tangible

  qualities whatsoever. That is why we have no sensation by means of

  bones, hair, c., because they consist of earth. So too plants,

  because they consist of earth, have no sensation. Without touch

  there can be no other sense, and the organ of touch cannot consist

  of earth or of any other single element.

  It is evident, therefore, that the loss of this one sense alone must

  bring about the death of an animal. For as on the one hand nothing

  which is not an animal can have this sense, so on the other it is

  the only one which is indispensably necessary to what is an animal.

  This explains, further, the following difference between the other

  senses and touch. In the case of all the others excess of intensity in

  the qualities which they apprehend, i.e. excess of intensity in

  colour, sound, and smell, destroys not the but only the organs of

  the sense (except incidentally, as when the sound is accompanied by an

  impact or shock, or where through the objects of sight or of smell

  certain other things are set in motion, which destroy by contact);

  flavour also destroys only in so far as it is at the same time

  tangible. But excess of intensity in tangible qualities, e.g. heat,

  cold, or hardness, destroys the animal itself. As in the case of every

  sensible quality excess destroys the organ, so here what is tangible

  destroys touch, which is the essential mark of life; for it has been

  shown that without touch it is impossible for an animal to be. That is

  why excess in intensity of tangible qualities destroys not merely

  the organ, but the animal itself, because this is the only sense which

  it must have.

  All the other senses are necessary to animals, as we have said,