thought there was nothing but body in the world. Then came Plato, seeking "to save some certain cognition of truth" by means of his theory of Ideas. But Plato seems to have erred in thinking that the form of the known must be in the knower as it is in the known. This is not necessary. In sense-perception the form of the thing is not in sense as it is in the thing. "And likewise the intelligence receives the species (Ideas) of material and mobile bodies immaterially and immutably, after its own mode; for the received is in the recipient after the mode of the recipient. Hence it is to be held that the soul through the intelligence knows bodies by immaterial, universal, and necessary cognition."
Thomas sets this matter forth in a manner very illuminating as to his general position regarding knowledge:
"It follows that material things which are known must exist in the knower, not materially, but immaterially. And the reason of this is that the act of cognition extends itself to those things which are outside of the knower. For we know things outside of us. But through matter, the form of the thing is limited to what is single (aliquid unum). Hence it is plain that the ratio (proper nature) of cognition is the opposite of the ratio of materiality. And therefore things, like plants, which receive forms only materially, are in no way cognoscitivae, as is said in the second book of De anima. The more immaterially anything possesses the form of the thing known, the more perfectly it knows. Wherefore the intelligence, which abstracts the species (Idea) not only from matter, but also from individualizing material conditions, knows more perfectly than sense, which receives the form of the thing known without matter indeed, but with material conditions. Among the senses themselves, sight is the most cognoscitivus, because least material. And among intelligences, that is the more perfect which is the more immaterial" (Qu. lxxxiv. Art. 2).
Then Thomas again differs from Plato, and holds with Aristotle, that the intelligence through which the soul knows has not its ideas written upon it by nature, but from the first is capable of receiving them all (sed est in principio in potentia ad hujusmodi species omnes). Hereupon, and with further arguments, Thomas shows "that the species intelligibiles, by which our soul knows, do not arise from separate forms" or ideas.
