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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。
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。
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。
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； thin