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

I say:
anger and the observation of one intelligible by that of
another? And a sign that the illness which enters the body
does not occur in the substratum of the sciences is that,
when the sick man recovers, he does not need to learn the
sciences anew, but the disposition of his soul becomes the
same as it was before, and those sciences come back to him
exactly as they were without any new learning.
The objection is that we say that there may be
innumerable causes for the increase and the decrease of the
faculties, for some of the faculties increase in power at the
beginning of life, some in middle life, some at the end, and
the same is the case with the intellect and only a topical
proof can be claimed. And it is not impossible that smell and
sight should differ in this, that smell becomes stronger after
forty years and sight weaker, although they both inhere in
the body, just as those faculties differ in animals; for in some
animals smell is stronger, in others hearing and sight
because of the difference in their temperaments, and it is not
possible to ascertain these facts absolutely. Nor is it
impossible that the temperament of the organs also should
differ with individual persons and conditions. One of the
reasons why the decay of sight is earlier than the decay of
the intellect is that sight is earlier, for a man sees when he is
first created, Whereas his intellect is not mature before
fifteen years or more, ‘ according to the different opinions we
find people to have about this problem; and it is even said
that greyness comes earlier to the hair on the head than to
that on the beard, because the hair on the head grows
earlier. If one goes deeper into these causes and does not
simply refer them to the usual course of nature, one cannot
base any sure knowledge thereon, because the possibilities
for certain faculties to become stronger and others weaker
are unlimited, and nothing evident results from this.
When it is assumed that the substratum of the perceptive faculties is the
natural heat, and that natural heat suffers diminution after forty years, then
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