Now it seems that Plato strayed from the truth because, having observed that all knowledge takes places through some kind of similitude, he thought that the form of the thing known must of necessity be in the knower in the same manner as in the thing known itself. But it was his opinion that the form of the thing understood is in the intellect under conditions of universality, immateriality, and immobility; which is apparent from the very operation of the intellect, whose act of understanding is universal, and characterized by a certain necessity; for the mode of action corresponds to the mode of the agent's form. Therefore he concluded that the things which we understand must subsist in themselves under the same conditions of immateriality and immobility.
But there is no necessity for this. ... the sensible form is in one way in the thing whcih is external to the soul, and in another way in the senses, which receive the forms of sensible things without receiving matter.... So, too, the intellect, according to its own mode, receives under conditions of immateriality and immobility the species of material and movable bodies; for the received is in the receiver according to the mode of the receiver. We must conclude, therefore, that the soul knows bodies through the intellect by a knowledge which is immaterial, universal and necessary. (STh I.84.1 c. Basic Writings of Saint Thomas Aquinas. Vol. I. Ed. Anton C. Pegis. New York: Random House, 1945.)See also STh I.84.2 c for more on why the principle that like is known by like is to be rejected. Here Aquinas concludes that the soul does not understand corporeal things by its essence; only God does.