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

intellect must behave in the same way in this respect; that is, if its
substratum is natural heat, then it is necessary that the intellect should
become old as the natural heat becomes old. If, however, it is thought that
the substrata for the intellect and the senses are different, then it is not
necessary that both should be similar in their lifetimes.
Ghazali says:
I say
The ninth proof is that the philosophers say: How can
man be attributed to body with its accidents, for those bodies
are continually in dissolution, and nutrition replaces what is
dissolved, so that when we see a child after its separation
from its mother’s womb fall ill a few times and become thin
and then fat again and grow up, we may safely say that after
forty years no particle remains of what was there when his
mother was delivered of it. Indeed, the child began its
existence out of the parts of the sperm alone, but nothing of
the particles of the sperm remains in it; no, all this is
dissolved and has changed into something else, and then
this body has become another. Still we say that the identical
man remains and his notions remain with him from the
beginning of his youth, although all the bodily parts have
changed. And this shows that the soul has an existence
outside the body and that the body is its organ!
The objection is that this is contradicted by what happens
to animals and plants, for when the condition of their being
small is compared to the condition of their being big, their
identity is asserted equally with the identity of man; still, it
does not prove that they have an incorporeal existence. ‘
And what is said about knowledge is refuted by the retention
of imaginative forms, for they remain in the boy from youth -
to old age, although the particles of his brain change.
None of the ancient philosophers used this proof for the survival of the
soul; they only used it to show that in individuals there is an essence
which remains from birth to death and that things are not in an eternal flux,
as was believed by many ancients who denied necessary knowledge, so
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