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

I say:
does not reside in the hand, for it does not cease when the
hand is cut off; nor is the will in the hand, for often a man
wants to write, when his hand has withered and he is not
able to do so, not because his will has gone, but because his
power has. “
This discussion is not an independent one, but only a complement to the
first, for in the first discussion it was merely assumed that knowledge is
not divided by the division of its substratum, and here an attempt is made
to prove this by making use of a division into three categories. And he
repeats the same objection, which presented itself to him because he did
not carry out the division of matter in the two senses in which it can be
taken. For when the philosophers denied that the intellect could be divided
through the division of its substratum in the way in which accidents are
divided through the division of their substratum, and there exists another
way of division in body which must be applied to the bodily functions of
perception, they had a doubt about these faculties. The proof is only
completed by denying that the intellect can be divided in either of these
ways, and by showing that everything which exists in a body is necessarily
divisible in one of them.
For of those things in the body which are divided in this second way, i. e.
which are not by definition divisible through the division of their
substratum’ it was sometimes doubted whether they are separable from
their substratum or not. For we see it happen that most parts of the
substratum decay and still this kind of existence, i. e. the individual
perception, does not decay; and it was thought that it might happen that,
just as the form does not disappear through the disappearance of one or
more parts of its substratum, in the same way the form might not
disappear when the whole was destroyed, and that the decay of the act of
the form through its substratum was similar to the decay of the act of the
artisan through the deterioration of his tools. And therefore Aristotle says
that if an old man had the eye of a young man, he would see as well as
the young one, meaning that it is thought that the decrepitude which
occurs to the sight of the old man does not happen because of the decay
of the faculty but because of the decay of the organs. And he tries to
prove this by the inactivity of the organ or the greater part of it in sleep,
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