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

but the intellectual faculty becomes strong in most cases
only after this age. ‘ And the loss of insight in the intelligibles,
through illness in the body and through dotage in old age,
does not argue against this, for as long as it is proved that at
certain times the intellect is strong notwithstanding the
weakness of the body, it is clear that it exists by itself, and its
decline at the time of the declining of the body does not
imply that it exists through the body, for from a negative
consequent alternating with a positive consequent there is
no inference. For we say that, if the intellectual faculty exists
through the body, then the weakness of the body will
weaken it at all times, but the consequent is false and
therefore the antecedent is false; but, when we say the
consequent is true, sometimes it does not follow that the
antecedent is true. Further, the cause of this is that the soul
has an activity through itself, when nothing hinders it and it is
not preoccupied with something. For the soul has two kinds
of action, one in relation to the body, namely to govern and
rule it, and one in relation to its principles and essence, and
this is to perceive the intelligibles, and these two kinds of
action hinder each other and are opposed to each other, and
when it is occupied with the one action, it turns away from
the other and it cannot combine both. And its occupations
through the body are sense-perception and imagination and
the passions, anger, fear, grief, and pain, but when it sets
out to think the intelligible it neglects all these other things.
Yes, sense-perception by itself sometimes hinders the
apprehension and contemplation of the intellect without the
occurrence of any damage to the organ of the intellect or to
the intellect itself, and the reason for this is that the soul is
prevented from one action through being occupied with
another, and therefore during pain, disease, and fear-for this
also is a disease of the brain-intellectual speculation leaves
off. And why should it be impossible that through this
difference in these two kinds of action in the soul they should
hinder each other, since even two acts of the same kind may
impede each other, for fear is stunned by pain and desire by
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