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ON THE MOTION OF ANIMALS
by Aristotle
translated by A。 S。 L。 Farquharson
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。
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。
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 he