articipation in
order to have knowledge of the genera and species of things. But
since Aristotle did not allow that forms of natural things exist
apart from matter, and as forms existing in matter are not actually
intelligible; it follows that the natures or forms of the sensible
things which we understand are not actually intelligible. Now nothing
is reduced from potentiality to act except by something in act; as
the senses as made actual by what is actually sensible. We must
therefore assign on the part of the intellect some power to make
things actually intelligible, by abstraction of the species from
material conditions. And such is the necessity for an active
intellect.

Reply Obj. 1: Sensible things are found in act outside the soul; and
hence there is no need for an active sense. Wherefore it is clear
that in the nutritive part all the powers are active, whereas in the
sensitive part all are passive: but in the intellectual part, there
is something active and something passive.

Reply Obj. 2: There are two opinions as to the effect of light. For
some say that light is required for sight, in order to make colors
actually visible. And according to this the active intellect is
required for understanding, in like manner and for the same reason as
light is required for seeing. But in the opinion of others, light is
required for sight; not for the colors to become actually visible;
but in order that the medium may become actually luminous, as the
Commentator says on _De Anima_ ii. And according to this, Aristotle's
comparison of the active intellect to light is verified in this, that
as it is required for understanding, so is light required for seeing;
but not for the same reason.

Reply Obj. 3: If the agent pre-exist, it may well happen that its
likeness is received variously into various things, on account of
their dispositions. But if the agent does not pre-exist, the
disposition of the recipient has nothing to do with the matter. Now
the intelligible in act is not something existing in nature; if we
consider the nature of things sensible, which do not subsist apart
from matter. And therefore in order to understand them, the
immaterial nature of the passive intellect would not suffice but for
the presence of the active intellect which makes things actually
intelligible by way of abstraction.
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FOURTH ARTICLE [I, Q. 79, Art. 4]

Whether the Active Intellect Is Something in the Soul?