ase of lethargy, we
see that a man is hindered from actually understanding things of which
he had a previous knowledge. Secondly, anyone can experience this of
himself, that when he tries to understand something, he forms certain
phantasms to serve him by way of examples, in which as it were he
examines what he is desirous of understanding. For this reason it is
that when we wish to help someone to understand something, we lay
examples before him, from which he forms phantasms for the purpose of
understanding.

Now the reason of this is that the power of knowledge is proportioned
to the thing known. Wherefore the proper object of the angelic
intellect, which is entirely separate from a body, is an intelligible
substance separate from a body. Whereas the proper object of the human
intellect, which is united to a body, is a quiddity or nature existing
in corporeal matter; and through such natures of visible things it
rises to a certain knowledge of things invisible. Now it belongs to
such a nature to exist in an individual, and this cannot be apart from
corporeal matter: for instance, it belongs to the nature of a stone to
be in an individual stone, and to the nature of a horse to be in an
individual horse, and so forth. Wherefore the nature of a stone or any
material thing cannot be known completely and truly, except in as much
as it is known as existing in the individual. Now we apprehend the
individual through the senses and the imagination. And, therefore, for
the intellect to understand actually its proper object, it must of
necessity turn to the phantasms in order to perceive the universal
nature existing in the individual. But if the proper object of our
intellect were a separate form; or if, as the Platonists say, the
natures of sensible things subsisted apart from the individual; there
would be no need for the intellect to turn to the phantasms whenever
it understands.

Reply Obj. 1: The species preserved in the passive intellect exist
there habitually when it does not understand them actually, as we
have said above (Q. 79, A. 6). Wherefore for us to understand
actually, the fact that the species are preserved does not suffice;
we need further to make use of them in a manner befitting the things
of which they are the species, which things are natures existing in
individuals.

Reply Obj. 2: Even the phantasm is the likeness of an individual
thing; wherefore the imagination does not need any further likeness
o