intellect are derived from some separate
substances.

Obj. 3: Further, whatever is in potentiality is reduced to act by
something actual. If, therefore, our intellect, previously in
potentiality, afterwards actually understands, this must needs be
caused by some intellect which is always in act. But this is a
separate intellect. Therefore the intelligible species, by which we
actually understand, are caused by some separate substances.

_On the contrary,_ If this were true we should not need the senses in
order to understand. And this is proved to be false especially from
the fact that if a man be wanting in a sense, he cannot have any
knowledge of the sensibles corresponding to that sense.

_I answer that,_ Some have held that the intelligible species of our
intellect are derived from certain separate forms or substances. And
this in two ways. For Plato, as we have said (A. 1), held that the
forms of sensible things subsist by themselves without matter; for
instance, the form of a man which he called _per se_ man, and the
form or idea of a horse which is called _per se_ horse, and so forth.
He said therefore that these forms are participated both by our soul
and by corporeal matter; by our soul, to the effect of knowledge
thereof, and by corporeal matter to the effect of existence: so that,
just as corporeal matter by participating the idea of a stone,
becomes an individuating stone, so our intellect, by participating
the idea of a stone, is made to understand a stone. Now participation
of an idea takes place by some image of the idea in the participator,
just as a model is participated by a copy. So just as he held that
the sensible forms, which are in corporeal matter, are derived from
the ideas as certain images thereof: so he held that the intelligible
species of our intellect are images of the ideas, derived therefrom.
And for this reason, as we have said above (A. 1), he referred
sciences and definitions to those ideas.

But since it is contrary to the nature of sensible things that their
forms should subsist without matter, as Aristotle proves in many ways
(Metaph. vi), Avicenna (De Anima v) setting this opinion aside, held
that the intelligible species of all sensible things, instead of
subsisting in themselves without matter, pre-exist immaterially in the
separate intellects: from the first of which, said he, such species
are derived by a second, and so on to the last separate intellect
which he called th