Aristotle
384-322 B.C.E. - Wrote in Greek
On the Motion of Animals
Written 350 B.C.E
Translated by A. S. L. Farquharson
On the Motion of Animals
By Aristotle
Part 1
Elsewhere we have investigated in detail the movement of animals after
their various kinds, the differences between them, and the reasons for
their particular characters (for some animals fly, some swim, some walk,
others move in various other ways); there remains an investigation of the
common ground of any sort of animal movement whatsoever.
Now we have already determined (when we were discussing whether
eternal motion exists or not, and its definition, if it does exist) that
the origin of all other motions is that which moves itself, and that the
origin of this is the immovable, and that the prime mover must of necessity
be immovable. And we must grasp this not only generally in theory, but
also by reference to individuals in the world of sense, for with these
in view we seek general theories, and with these we believe that general
theories ought to harmonize. Now in the world of sense too it is plainly
impossible for movement to be initiated if there is nothing at rest, and
before all else in our present subject- animal life. For if one of the
parts of an animal be moved, another must be at rest, and this is the purpose
of their joints; animals use joints like a centre, and the whole member,
in which the joint is, becomes both one and two, both straight and bent,
changing potentially and actually by reason of the joint. And when it is
bending and being moved one of the points in the joint is moved and one
is at rest, just as if the points A and D of a diameter were at rest, and
B were moved, and DAC were generated. However, in the geometrical illustration,
the centre is held to be altogether indivisible (for in mathematics motion
is a fiction, as the phrase goes, no mathematical entity being really moved),
whereas in the case of joints the centres become now one potentially and
divided actually, and now one actually and divided potentially. But still
the origin of movement, qua origin, always remains at rest when the lower
part of a limb is moved; for example, the elbow joint, when the forearm
is moved, and the shoulder, when the whole arm; the knee when the tibia
is moved, and the hip when the whole leg. Accordingly it is plain that
each animal as a whole must have within itself a point at rest, whence
will be the origin of that which is moved, and supporting itself upon which
it will be moved both as a complete whole and in its
members.
Part 2
But the point of rest in the animal is still quite ineffectual
unless there be something without which is absolutely at rest and immovable.
Now it is worth while to pause and consider what has been said, for it
involves a speculation which extends beyond animals even to the motion
and march of the universe. For just as there must be something immovable
within the animal, if it is to be moved, so even more must there be without
it something immovable, by supporting itself upon which that which is moved
moves. For were that something always to give way (as it does for mice
walking in grain or persons walking in sand) advance would be impossible,
and neither would there be any walking unless the ground were to remain
still, nor any flying or swimming were not the air and the sea to resist.
And this which resists must needs be different from what is moved, the
whole of it from the whole of that, and what is thus immovable must be
no part of what is moved; otherwise there will be no movement. Evidence
of this lies in the problem why it is that a man easily moves a boat from
outside, if he push with a pole, putting it against the mast or some other
part, but if he tried to do this when in the boat itself he would never
move it, no not giant Tityus himself nor Boreas blowing from inside the
ship, if he really were blowing in the way painters represent him; for
they paint him sending the breath out from the boat. For whether one blew
gently or so stoutly as to make a very great wind, and whether what were
thrown or pushed were wind or something else, it is necessary in the first
place to be supported upon one of one's own members which is at rest and
so to push, and in the second place for this member, either itself, or
that of which it is a part, to remain at rest, fixing itself against something
external to itself. Now the man who is himself in the boat, if he pushes,
fixing himself against the boat, very naturally does not move the boat,
because what he pushes against should properly remain at rest. Now what
he is trying to move, and what he is fixing himself against is in his case
the same. If, however, he pushes or pulls from outside he does move it,
for the ground is no part of the boat.
Part 3
Here we may ask the difficult question whether if something moves
the whole heavens this mover must be immovable, and moreover be no part
of the heavens, nor in the heavens. For either it is moved itself and moves
the heavens, in which case it must touch something immovable in order to
create movement, and then this is no part of that which creates movement;
or if the mover is from the first immovable it will equally be no part
of that which is moved. In this point at least they argue correctly who
say that as the Sphere is carried round in a circle no single part remains
still, for then either the whole would necessarily stand still or its continuity
be torn asunder; but they argue less well in supposing that the poles have
a certain force, though conceived as having no magnitude, but as merely
termini or points. For besides the fact that no such things have any substantial
existence it is impossible for a single movement to be initiated by what
is twofold; and yet they make the poles two. From a review of these difficulties
we may conclude that there is something so related to the whole of Nature,
as the earth is to animals and things moved by them.
And the mythologists with their fable of Atlas setting his feet
upon the earth appear to have based the fable upon intelligent grounds.
They make Atlas a kind of diameter twirling the heavens about the poles.
Now as the earth remains still this would be reasonable enough, but their
theory involves them in the position that the earth is no part of the universe.
And further the force of that which initiates movement must be made equal
to the force of that which remains at rest. For there is a definite quantity
of force or power by dint of which that which remains at rest does so,
just as there is of force by dint of which that which initiates movement
does so; and as there is a necessary proportion between opposite motions,
so there is between absences of motion. Now equal forces are unaffected
by one another, but are overcome by a superiority of force. And so in their
theory Atlas, or whatever similar power initiates movement from within,
must exert no more force than will exactly balance the stability of the
earth- otherwise the earth will be moved out of her place in the centre
of things. For as the pusher pushes so is the pushed pushed, and with equal
force. But the prime mover moves that which is to begin with at rest, so
that the power it exerts is greater, rather than equal and like to the
power which produces absence of motion in that which is moved. And similarly
also the power of what is moved and so moves must be greater than the power
of that which is moved but does not initiate movement. Therefore the force
of the earth in its immobility will have to be as great as the force of
the whole heavens, and of that which moves the heavens. But if that is
impossible, it follows that the heavens cannot possibly be moved by any
force of this kind inside them.
Part 4
There is a further difficulty about the motions of the parts of
the heavens which, as akin to what has gone before, may be considered next.
For if one could overcome by force of motion the immobility of the earth
he would clearly move it away from the centre. And it is plain that the
power from which this force would originate will not be infinite, for the
earth is not infinite and therefore its weight is not. Now there are more
senses than one of the word 'impossible'. When we say it is impossible
to see a sound, and when we say it is impossible to see the men in the
moon, we use two senses of the word; the former is of necessity, the latter,
though their nature is to be seen, cannot as a fact be seen by us. Now
we suppose that the heavens are of necessity impossible to destroy and
to dissolve, whereas the result of the present argument would be to do
away with this necessity. For it is natural and possible for a motion to
exist greater than the force by dint of which the earth is at rest, or
than that by dint of which Fire and Aether are moved. If then there are
superior motions, these will be dissolved in succession by one another:
and if there actually are not, but might possibly be (for the earth cannot
be infinite because no body can possibly be infinite), there is a possibility
of the heavens being dissolved. For what is to prevent this coming to pass,
unless it be impossible? And it is not impossible unless the opposite is
necessary. This difficulty, however, we will discuss
elsewhere.
To resume, must there be something immovable and at rest outside
of what is moved, and no part of it, or not? And must this necessarily
be so also in the case of the universe? Perhaps it would be thought strange
were the origin of movement inside. And to those who so conceive it the
word of Homer would appear to have been well spoken:
'Nay, ye would not pull Zeus, highest of all from heaven to the
plain, no not even if ye toiled right hard; come, all ye gods and goddesses!
Set hands to the chain'; for that which is entirely immovable cannot possibly
be moved by anything. And herein lies the solution of the difficulty stated
some time back, the possibility or impossibility of dissolving the system
of the heavens, in that it depends from an original which is
immovable.
Now in the animal world there must be not only an immovable without,
but also within those things which move in place, and initiate their own
movement. For one part of an animal must be moved, and another be at rest,
and against this the part which is moved will support itself and be moved;
for example, if it move one of its parts; for one part, as it were, supports
itself against another part at rest.
But about things without life which are moved one might ask the
question whether all contain in themselves both that which is at rest and
that which initiates movement, and whether they also, for instance fire,
earth, or any other inanimate thing, must support themselves against something
outside which is at rest. Or is this impossible and must it not be looked
for rather in those primary causes by which they are set in motion? For
all things without life are moved by something other, and the origin of
all things so moved are things which move themselves. And out of these
we have spoken about animals (for they must all have in themselves that
which is at rest, and without them that against which they are supported);
but whether there is some higher and prime mover is not clear, and an origin
of that kind involves a different discussion. Animals at any rate which
move themselves are all moved supporting themselves on what is outside
them, even when they inspire and expire; for there is no essential difference
between casting a great and a small weight, and this is what men do when
they spit and cough and when they breathe in and breathe
out.
Part 5
But is it only in that which moves itself in place that there must
be a point at rest, or does this hold also of that which causes its own
qualitative changes, and its own growth? Now the question of original generation
and decay is different; for if there is, as we hold, a primary movement,
this would be the cause of generation and decay, and probably of all the
secondary movements too. And as in the universe, so in the animal world
this is the primary movement, when the creature attains maturity; and therefore
it is the cause of growth, when the creature becomes the cause of its own
growth, and the cause too of alteration. But if this is not the primary
movement then the point at rest is not necessary. However, the earliest
growth and alteration in the living creature arise through another and
by other channels, nor can anything possibly be the cause of its own generation
and decay, for the mover must exist before the moved, the begetter before
the begotten, and nothing is prior to itself.
Part 6
Now whether the soul is moved or not, and how it is moved if it
be moved, has been stated before in our treatise concerning it. And since
all inorganic things are moved by some other thing- and the manner of the
movement of the first and eternally moved, and how the first mover moves
it, has been determined before in our Metaphysics, it remains to inquire
how the soul moves the body, and what is the origin of movement in a living
creature. For, if we except the movement of the universe, things with life
are the causes of the movement of all else, that is of all that are not
moved by one another by mutual impact. And so all their motions have a
term or limit, inasmuch as the movements of things with life have such.
For all living things both move and are moved with some object, so that
this is the term of all their movement, the end, that is, in view. Now
we see that the living creature is moved by intellect, imagination, purpose,
wish, and appetite. And all these are reducible to mind and desire. For
both imagination and sensation are on common ground with mind, since all
three are faculties of judgement though differing according to distinctions
stated elsewhere. Will, however, impulse, and appetite, are all three forms
of desire, while purpose belongs both to intellect and to desire. Therefore
the object of desire or of intellect first initiates movement, not, that
is, every object of intellect, only the end in the domain of conduct. Accordingly
among goods that which moves is a practical end, not the good in its whole
extent. For it initiates movement only so far as something else is for
its sake, or so far as it is the object of that which is for the sake of
something else. And we must suppose that a seeming good may take the room
of actual good, and so may the pleasant, which is itself a seeming good.
From these considerations it is clear that in one regard that which is
eternally moved by the eternal mover is moved in the same way as every
living creature, in another regard differently, and so while it is moved
eternally, the movement of living creatures has a term. Now the eternal
beautiful, and the truly and primarily good (which is not at one time good,
at another time not good), is too divine and precious to be relative to
anything else. The prime mover then moves, itself being unmoved, whereas
desire and its faculty are moved and so move. But it is not necessary for
the last in the chain of things moved to move something else; wherefore
it is plainly reasonable that motion in place should be the last of what
happens in the region of things happening, since the living creature is
moved and goes forward by reason of desire or purpose, when some alteration
has been set going on the occasion of sensation or imagination.
Part 7
But how is it that thought (viz. sense, imagination, and thought
proper) is sometimes followed by action, sometimes not; sometimes by movement,
sometimes not? What happens seems parallel to the case of thinking and
inferring about the immovable objects of science. There the end is the
truth seen (for, when one conceives the two premisses, one at once conceives
and comprehends the conclusion), but here the two premisses result in a
conclusion which is an action- for example, one conceives that every man
ought to walk, one is a man oneself: straightway one walks; or that, in
this case, no man should walk, one is a man: straightway one remains at
rest. And one so acts in the two cases provided that there is nothing in
the one case to compel or in the other to prevent. Again, I ought to create
a good, a house is good: straightway I make a house. I need a covering,
a coat is a covering: I need a coat. What I need I ought to make, I need
a coat: I make a coat. And the conclusion I must make a coat is an action.
And the action goes back to the beginning or first step. If there is to
be a coat, one must first have B, and if B then A, so one gets A to begin
with. Now that the action is the conclusion is clear. But the premisses
of action are of two kinds, of the good and of the possible.
And as in some cases of speculative inquiry we suppress one premise
so here the mind does not stop to consider at all an obvious minor premise;
for example if walking is good for man, one does not dwell upon the minor
'I am a man'. And so what we do without reflection, we do quickly. For
when a man actualizes himself in relation to his object either by perceiving,
or imagining or conceiving it, what he desires he does at once. For the
actualizing of desire is a substitute for inquiry or reflection. I want
to drink, says appetite; this is drink, says sense or imagination or mind:
straightway I drink. In this way living creatures are impelled to move
and to act, and desire is the last or immediate cause of movement, and
desire arises after perception or after imagination and conception. And
things that desire to act now create and now act under the influence of
appetite or impulse or of desire or wish.
The movements of animals may be compared with those of automatic
puppets, which are set going on the occasion of a tiny movement; the levers
are released, and strike the twisted strings against one another; or with
the toy wagon. For the child mounts on it and moves it straight forward,
and then again it is moved in a circle owing to its wheels being of unequal
diameter (the smaller acts like a centre on the same principle as the cylinders).
Animals have parts of a similar kind, their organs, the sinewy tendons
to wit and the bones; the bones are like the wooden levers in the automaton,
and the iron; the tendons are like the strings, for when these are tightened
or leased movement begins. However, in the automata and the toy wagon there
is no change of quality, though if the inner wheels became smaller and
greater by turns there would be the same circular movement set up. In an
animal the same part has the power of becoming now larger and now smaller,
and changing its form, as the parts increase by warmth and again contract
by cold and change their quality. This change of quality is caused by imaginations
and sensations and by ideas. Sensations are obviously a form of change
of quality, and imagination and conception have the same effect as the
objects so imagined and conceived For in a measure the form conceived be
it of hot or cold or pleasant or fearful is like what the actual objects
would be, and so we shudder and are frightened at a mere idea. Now all
these affections involve changes of quality, and with those changes some
parts of the body enlarge, others grow smaller. And it is not hard to see
that a small change occurring at the centre makes great and numerous changes
at the circumference, just as by shifting the rudder a hair's breadth you
get a wide deviation at the prow. And further, when by reason of heat or
cold or some kindred affection a change is set up in the region of the
heart, even in an imperceptibly small part of the heart, it produces a
vast difference in the periphery of the body,- blushing, let us say, or
turning white, goose-skin and shivers and their opposites.
Part 8
But to return, the object we pursue or avoid in the field of action
is, as has been explained, the original of movement, and upon the conception
and imagination of this there necessarily follows a change in the temperature
of the body. For what is painful we avoid, what is pleasing we pursue.
We are, however, unconscious of what happens in the minute parts; still
anything painful or pleasing is generally speaking accompanied by a definite
change of temperature in the body. One may see this by considering the
affections. Blind courage and panic fears, erotic motions, and the rest
of the corporeal affections, pleasant and painful, are all accompanied
by a change of temperature, some in a particular member, others in the
body generally. So, memories and anticipations, using as it were the reflected
images of these pleasures and pains, are now more and now less causes of
the same changes of temperature. And so we see the reason of nature's handiwork
in the inward parts, and in the centres of movement of the organic members;
they change from solid to moist, and from moist to solid, from soft to
hard and vice versa. And so when these are affected in this way, and when
besides the passive and active have the constitution we have many times
described, as often as it comes to pass that one is active and the other
passive, and neither of them falls short of the elements of its essence,
straightway one acts and the other responds. And on this account thinking
that one ought to go and going are virtually simultaneous, unless there
be something else to hinder action. The organic parts are suitably prepared
by the affections, these again by desire, and desire by imagination. Imagination
in its turn depends either upon conception or sense-perception. And the
simultaneity and speed are due to the natural correspondence of the active
and passive.
However, that which first moves the animal organism must be situate
in a definite original. Now we have said that a joint is the beginning
of one part of a limb, the end of another. And so nature employs it sometimes
as one, sometimes as two. When movement arises from a joint, one of the
extreme points must remain at rest, and the other be moved (for as we explained
above the mover must support itself against a point at rest); accordingly,
in the case of the elbow-joint, the last point of the forearm is moved
but does not move anything, while, in the flexion, one point of the elbow,
which lies in the whole forearm that is being moved, is moved, but there
must also be a point which is unmoved, and this is our meaning when we
speak of a point which is in potency one, but which becomes two in actual
exercise. Now if the arm were the living animal, somewhere in its elbow-joint
would be situate the original seat of the moving soul. Since, however,
it is possible for a lifeless thing to be so related to the hand as the
forearm is to the upper (for example, when a man moves a stick in his hand),
it is evident that the soul, the original of movement, could not lie in
either of the two extreme points, neither, that is, in the last point of
the stick which is moved, nor in the original point which causes movement.
For the stick too has an end point and an originative point by reference
to the hand. Accordingly, this example shows that the moving original which
derives from the soul is not in the stick and if not, then not in the hand;
for a precisely similar relation obtains between the hand and the wrist,
as between the wrist and the elbow. In this matter it makes no difference
whether the part is a continuous part of the body or not; the stick may
be looked at as a detached part of the whole. It follows then of necessity
that the original cannot lie in any individual origin which is the end
of another member, even though there may lie another part outside the one
in question. For example, relatively to the end point of the stick the
hand is the original, but the original of the hand's movement is in the
wrist. And so if the true original is not in the hand, be-there is still
something higher up, neither is the true original in the wrist, for once
more if the elbow is at rest the whole part below it can be moved as a
continuous whole.
Part 9
Now since the left and the right sides are symmetrical, and these
opposites are moved simultaneously, it cannot be that the left is moved
by the right remaining stationary, nor vice versa; the original must always
be in what lies above both. Therefore, the original seat of the moving
soul must be in that which lies in the middle, for of both extremes the
middle is the limiting point; and this is similarly related to the movements
from above [and below,] those that is from the head, and to the bones which
spring from the spinal column, in creatures that have a spinal
column.
And this is a reasonable arrangement. For the sensorium is in our
opinion in the centre too; and so, if the region of the original of movement
is altered in structure through sense-perception and thus changes, it carries
with it the parts that depend upon it and they too are extended or contracted,
and in this way the movement of the creature necessarily follows. And the
middle of the body must needs be in potency one but in action more than
one; for the limbs are moved simultaneously from the original seat of movement,
and when one is at rest the other is moved. For example, in the line BAC,
B is moved, and A is the mover. There must, however, be a point at rest
if one is to move, the other to be moved. A (AE) then being one in potency
must be two in action, and so be a definite spatial magnitude not a mathematical
point. Again, C may be moved simultaneously with B. Both the originals
then in A must move and be, and so there must be something other than them
which moves but is not moved. For otherwise, when the movement begins,
the extremes, i.e. the originals, in A would rest upon one another, like
two men putting themselves back to back and so moving their legs. There
must then be some one thing which moves both. This something is the soul,
distinct from the spatial magnitude just described and yet located
therein.
Part 10
Although from the point of view of the definition of movement-
a definition which gives the cause- desire is the middle term or cause,
and desire moves being moved, still in the material animated body there
must be some material which itself moves being moved. Now that which is
moved, but whose nature is not to initiate movement, is capable of being
passive to an external force, while that which initiates movement must
needs possess a kind of force and power. Now experience shows us that animals
do both possess connatural spirit and derive power from this. (How this
connatural spirit is maintained in the body is explained in other passages
of our works.) And this spirit appears to stand to the soul-centre or original
in a relation analogous to that between the point in a joint which moves
being moved and the unmoved. Now since this centre is for some animals
in the heart, in the rest in a part analogous with the heart, we further
see the reason for the connatural spirit being situate where it actually
is found. The question whether the spirit remains always the same or constantly
changes and is renewed, like the cognate question about the rest of the
parts of the body, is better postponed. At all events we see that it is
well disposed to excite movement and to exert power; and the functions
of movement are thrusting and pulling. Accordingly, the organ of movement
must be capable of expanding and contracting; and this is precisely the
characteristic of spirit. It contracts and expands naturally, and so is
able to pull and to thrust from one and the same cause, exhibiting gravity
compared with the fiery element, and levity by comparison with the opposites
of fire. Now that which is to initiate movement without change of structure
must be of the kind described, for the elementary bodies prevail over one
another in a compound body by dint of disproportion; the light is overcome
and kept down by the heavier, and the heavy kept up by the
lighter.
We have now explained what the part is which is moved when the
soul originates movement in the body, and what is the reason for this.
And the animal organism must be conceived after the similitude of a well-governed
commonwealth. When order is once established in it there is no more need
of a separate monarch to preside over each several task. The individuals
each play their assigned part as it is ordered, and one thing follows another
in its accustomed order. So in animals there is the same orderliness- nature
taking the place of custom- and each part naturally doing his own work
as nature has composed them. There is no need then of a soul in each part,
but she resides in a kind of central governing place of the body, and the
remaining parts live by continuity of natural structure, and play the parts
Nature would have them play.
Part 11
So much then for the voluntary movements of animal bodies, and
the reasons for them. These bodies, however, display in certain members
involuntary movements too, but most often non-voluntary movements. By involuntary
I mean motions of the heart and of the privy member; for often upon an
image arising and without express mandate of the reason these parts are
moved. By non-voluntary I mean sleep and waking and respiration, and other
similar organic movements. For neither imagination nor desire is properly
mistress of any of these; but since the animal body must undergo natural
changes of quality, and when the parts are so altered some must increase
and other decrease, the body must straightway be moved and change with
the changes that nature makes dependent upon one another. Now the causes
of the movements are natural changes of temperature, both those coming
from outside the body, and those taking place within it. So the involuntary
movements which occur in spite of reason in the aforesaid parts occur when
a change of quality supervenes. For conception and imagination, as we said
above, produce the conditions necessary to affections, since they bring
to bear the images or forms which tend to create these states. And the
two parts aforesaid display this motion more conspicuously than the rest,
because each is in a sense a separate vital organism, the reason being
that each contains vital moisture. In the case of the heart the cause is
plain, for the heart is the seat of the senses, while an indication that
the generative organ too is vital is that there flows from it the seminal
potency, itself a kind of organism. Again, it is a reasonable arrangement
that the movements arise in the centre upon movements in the parts, and
in the parts upon movements in the centre, and so reach one another. Conceive
A to be the centre or starting point. The movements then arrive at the
centre from each letter in the diagram we have drawn, and flow back again
from the centre which is moved and changes, (for the centre is potentially
multiple) the movement of B goes to B, that of C to C, the movement of
both to both; but from B to C the movements flow by dint of going from
B to A as to a centre, and then from A to C as from a
centre.
Moreover a movement contrary to reason sometimes does and sometimes
does not arise in the organs on the occasion of the same thoughts; the
reason is that sometimes the matter which is passive to the impressions
is there in sufficient quantity and of the right quality and sometimes
not.
And so we have finished our account of the reasons for the parts
of each kind of animal, of the soul, and furthere of sense-perception,
of sleep, of memory, and of movement in general; it remains to speak of
animal generation.
THE END
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