s known are corporeal and material, held that
things known must exist materially even in the soul that knows them.
And therefore, in order to ascribe to the soul a knowledge of all
things, they held that it has the same nature in common with all. And
because the nature of a result is determined by its principles, they
ascribed to the soul the nature of a principle; so that those who
thought fire to be the principle of all, held that the soul had the
nature of fire; and in like manner as to air and water. Lastly,
Empedocles, who held the existence of our four material elements and
two principles of movement, said that the soul was composed of these.
Consequently, since they held that things exist in the soul
materially, they maintained that all the soul's knowledge is
material, thus failing to discern intellect from sense.

But this opinion will not hold. First, because in the material
principle of which they spoke, the various results do not exist save
in potentiality. But a thing is not known according as it is in
potentiality, but only according as it is in act, as is shown
_Metaph._ ix (Did. viii, 9): wherefore neither is a power known
except through its act. It is therefore insufficient to ascribe to
the soul the nature of the principles in order to explain the fact
that it knows all, unless we further admit in the soul natures and
forms of each individual result, for instance, of bone, flesh, and
the like; thus does Aristotle argue against Empedocles (De Anima i,
5). Secondly, because if it were necessary for the thing known to
exist materially in the knower, there would be no reason why things
which have a material existence outside the soul should be devoid of
knowledge; why, for instance, if by fire the soul knows fire, that
fire also which is outside the soul should not have knowledge of fire.

We must conclude, therefore, that material things known must needs
exist in the knower, not materially, but immaterially. The reason of
this is, because the act of knowledge extends to things outside the
knower: for we know things even that are external to us. Now by
matter the form of a thing is determined to some one thing. Wherefore
it is clear that knowledge is in inverse ratio of materiality. And
consequently things that are not receptive of forms save materially,
have no power of knowledge whatever--such as plants, as the
Philosopher says (De Anima ii, 12). But the more immaterially a thing
receives the form of t