Knowing agents differ from those that do not know in the fact that the nonknowers possess their own form only, while the knower is adapted from its origin to possess also the form of another thing, in the sense that the species of the known thing may be present in the knower.

It is clear, then, that the nature of a noncognitive thing is more restricted and limited, while the nature of knowing things has greater fullness and extension. This is why the Philosopher says in the third book, On the Soul (8, 431b21); that “the soul is in a way all things.”

Now, the limitation of the form is due to matter. Hence we said above that the more immaterial forms are, the more do they approach some sort of infinity. It is plain, then, that the immateriality of a thing is the reason why it is able to know; and the level of cognition depends on the level of immaterality.