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

simply the hot body that accepts coldness by divesting itself of hotness
and accepting coldness, and vice versa. Now when the philosophers
found that this was the case with activity and passivity, they understood
that all existents of this description were composed of two substances, a
substance which is the act and a substance which is the potency, and they
realized that the substance in act is the perfection of the substance in
potency and that the substance in act stands in relation to the substance
in potency as if it were the end of its actualization, for there is no actual
difference between them. ‘ Then, when they looked through all the
different forms of existents, they found that all these substances must
necessarily lead up to a substance in act which is absolutely devoid of
matter, and this substance must necessarily be active and cannot have
any passivity and cannot be subject to exhaustion, weariness, and decay;
for such things occur to the substance in act only because it is the
perfection of the substance in potency, not because it is pure act. For
since the substance in potency only goes forth into act through a
substance in act, the series of substances which are at the same time
both active and passive must terminate in a substance which is pure act,
and the series must terminate in that substance. And the proof of the
existence of this substance, in so far as it is a mover and agent, through
essential particular premisses, can be found in the eighth book of
Aristotle’s Physics.
Having established the existence of this substance by special and
general arguments according to what is known in their books, the
philosophers now investigated the nature of the forms in matter which
produce motion, and they found some of them nearer to actuality and
farther from potency because they are less than others involved in
passivity, which is the special sign of the matter which exists in them. And
they realized that that which among these forms is most destitute of matter
is the soul, and especially the intellect, so that they started to doubt
whether the intellect belongs to the forms which are in matter or not. z But
when they investigated the perceiving forms amongst the forms of the soul
and found that they were free from matter, they understood that the cause
of perception consists in freedom from matter, ; and since they discovered
that the intellect is without passivity they understood that the reason why
one form is inorganic and another perceptive consists in the fact that when
it is the perfection of a potency it is inorganic or not percipient, ‘ and when
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