Tahafut_al-Tahafut-transl-Engl-van-den-Bergh

I say:
will, because His will has the same relation to all bodies, and
why should this body be specially disposed so that God
should move it rather than another? One cannot suppose
this; for it is impossible, as has been shown in the question
about the temporal beginning of the world. When it is
therefore established that this body needs as a principle of
movement a special qualification, the first division, that of the
movement through constraint, is ruled out.
So there remains the possibility that this movement
occurs by nature. But this is not possible, for nature by itself
is not the cause of motion, because the meaning of ‘motion’
is the withdrawal from one place to another place; and a
body does not move from the place in which it is when that
place is its proper place. For this reason a bladder full of air
on the surface of the water does not move, but when it is
immersed it moves towards the surface of the water, and
then it has found its proper place and has come to rest and
its nature is stabilized; when, however, it is transferred to a
place which is not its proper one, it withdraws to its proper
place, just as it withdraws from mid-water to the border of
the air. Now it cannot be imagined that the circular
movement is natural, since it returns to every position and
place which it would be supposed to abandon, and it is not
by nature that a body seeks the place which it abandons,
and therefore the bladder of air does not seek the interior of
the water, nor the stone when it has come to rest on the
earth the air. Thus only the third division remains, that of
movement by will. ‘
What he lays down in this section, that every thing moved either is
moved by itself or through a body from outside and that it is this which is
called constraint, is self-evident. But that for every thing which is moved by
itself there is no mover but the movedz is not a self-evident proposition; it
is only a common notion, and the philosophers indeed try to prove that
every thing moved by itself has an interior mover different from it, through
the use of other premisses which are self-evident, and of premisses which
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