an immaterial power not making use of a
corporeal organ for its action. And since the incorporeal cannot be
affected by the corporeal, he held that intellectual knowledge is not
brought about by sensible things affecting the intellect, but by
separate intelligible forms being participated by the intellect, as
we have said above (AA. 4 ,5). Moreover he held that sense is a power
operating of itself. Consequently neither is sense, since it is a
spiritual power, affected by the sensible: but the sensible organs
are affected by the sensible, the result being that the soul is in a
way roused to form within itself the species of the sensible.
Augustine seems to touch on this opinion (Gen. ad lit. xii, 24) where
he says that the "body feels not, but the soul through the body,
which it makes use of as a kind of messenger, for reproducing within
itself what is announced from without." Thus according to Plato,
neither does intellectual knowledge proceed from sensible knowledge,
nor sensible knowledge exclusively from sensible things; but these
rouse the sensible soul to the sentient act, while the senses rouse
the intellect to the act of understanding.

Aristotle chose a middle course. For with Plato he agreed that
intellect and sense are different. But he held that the sense has not
its proper operation without the cooperation of the body; so that to
feel is not an act of the soul alone, but of the "composite." And he
held the same in regard to all the operations of the sensitive part.
Since, therefore, it is not unreasonable that the sensible objects
which are outside the soul should produce some effect in the
"composite," Aristotle agreed with Democritus in this, that the
operations of the sensitive part are caused by the impression of the
sensible on the sense: not by a discharge, as Democritus said, but by
some kind of operation. For Democritus maintained that every
operation is by way of a discharge of atoms, as we gather from _De
Gener._ i, 8. But Aristotle held that the intellect has an operation
which is independent of the body's cooperation. Now nothing corporeal
can make an impression on the incorporeal. And therefore in order to
cause the intellectual operation according to Aristotle, the
impression caused by the sensible does not suffice, but something
more noble is required, for "the agent is more noble than the
patient," as he says (De Gener. i, 5). Not, indeed, in the sense that
the intellectual operation is eff