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

are the conclusions of other proofs, and this is something which may be
ascertained in their books. And likewise it is not self-evident that every
thing moved by an exterior mover must finally terminate in a thing moved
by itself: what is posed here as a set of self-evident premisses is, as a
matter of fact, a mixture of the two kinds of assertions; that is to say they
are partly conclusions and partly self-evident. Indeed, that what is moved
by itself and not by an external body is moved either by its substance and
nature or by an interior principle, and that it cannot be moved by
something which cannot be seen or touched and which is connected with
it from the outside (or in other words by an incorporeal entity) is selfevident.
You can claim to have a proof for this, namely by saying that if
this were not so, upward movement would not be proper to fire rather than
to earth; but it is, indeed, evident in itselfAnd as to that which does not
move by its own substance and nature, this is evident in the things which
are sometimes in motion and sometimes at rest, since that which is by
nature cannot perform both of two opposites;’ for those things, however,
which are perceived to move continually, a proof is necessary.
Again, as to his assertion that what is moved by itself is moved through a
principle in itself, either a principle called ‘nature’ or a principle called ‘soul’
and ‘choice’, this is true, when previously it has been proved that nothing
exists which is moved by itself. As concerns his affirmation that the
principle called nature does not move by itself in space, except when it is
not its proper place (for then it moves to its proper place and stays there),
this is true. And his further remark that what moves in a circle has neither
an improper nor a proper place, so that it could move from the one to the
other either totally or partially, this is nearly self-evident and easy to
uphold, and he has in this section mentioned something of its explanation
and proof; and therefore, when we understand ‘nature’ in the sense he
has established here, circular movement cannot move by nature.
And as to his further remark that, when it does not move by nature, it
moves through soul or through a potency which resembles the soul, it
appears that the term ‘soul’ is predicated only equivocally of the soul in the
celestial bodies, ‘ and the learned for the most part apply the term ‘nature’
to every potency which performs a rational act, namely an act which
conforms to the order and arrangement which exist in rational things; but
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