f man we must needs
suppose a superior intellect, from which the soul acquires the power
of understanding. For what is such by participation, and what is
mobile, and what is imperfect always requires the pre-existence of
something essentially such, immovable and perfect. Now the human soul
is called intellectual by reason of a participation in intellectual
power; a sign of which is that it is not wholly intellectual but only
in part. Moreover it reaches to the understanding of truth by
arguing, with a certain amount of reasoning and movement. Again it
has an imperfect understanding; both because it does not understand
everything, and because, in those things which it does understand, it
passes from potentiality to act. Therefore there must needs be some
higher intellect, by which the soul is helped to understand.

Wherefore some held that this intellect, substantially separate, is
the active intellect, which by lighting up the phantasms as it were,
makes them to be actually intelligible. But, even supposing the
existence of such a separate active intellect, it would still be
necessary to assign to the human soul some power participating in
that superior intellect, by which power the human soul makes things
actually intelligible. Just as in other perfect natural things,
besides the universal active causes, each one is endowed with its
proper powers derived from those universal causes: for the sun alone
does not generate man; but in man is the power of begetting man: and
in like manner with other perfect animals. Now among these lower
things nothing is more perfect than the human soul. Wherefore we must
say that in the soul is some power derived from a higher intellect,
whereby it is able to light up the phantasms. And we know this by
experience, since we perceive that we abstract universal forms from
their particular conditions, which is to make them actually
intelligible. Now no action belongs to anything except through some
principle formally inherent therein; as we have said above of the
passive intellect (Q. 76, A. 1). Therefore the power which is the
principle of this action must be something in the soul. For this
reason Aristotle (De Anima iii, 5) compared the active intellect to
light, which is something received into the air: while Plato compared
the separate intellect impressing the soul to the sun, as Themistius
says in his commentary on _De Anima_ iii. But the separate intellect,
according to the teaching of