Objection 1. It would seem that the soul understands corporeal things through its essence. For Augustine says (De Trin. x, 5) that the soul "collects and lays hold of the images of bodies which are formed in the soul and of the soul: for in forming them it gives them something of its own substance." But the soul understands bodies by images of bodies. Therefore the soul knows bodies through its essence, which it employs for the formation of such images, and from which it forms them.
Objection 2. Further, the Philosopher says (De Anima iii, 8) that "the soul, after a fashion, is everything." Since, therefore, like is known by like, it seems that the soul knows corporeal things through itself.
Objection 3. Further, the soul is superior to corporeal creatures. Now lower things are in higher things in a more eminent way than in themselves, as Dionysius says (Coel. Hier. xii). Therefore all corporeal creatures exist in a more excellent way in the soul than in themselves. Therefore the soul can know corporeal creatures through its essence.
On the contrary, Augustine says (De Trin. ix, 3) that "the mind gathers knowledge of corporeal things through the bodily senses." But the soul itself cannot be known through the bodily senses. Therefore it does not know corporeal things through itself.
I answer that, The ancient philosophers held that the soul knows bodies through its essence. For it was universally admitted that "like is known by like." But they thought that the form of the thing known is in the knower in the same mode as in the thing known. The Platonists however were of a contrary opinion. For Plato, having observed that the intellectual soul has an immaterial nature, and an immaterial mode of knowledge, held that the forms of things known subsist immaterially. While the earlier natural philosophers, observing that things 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 the thing known, the more perfect is its knowledge. Therefore the intellect which abstracts the species not only from matter, but also from the individuating conditions of matter, has more perfect knowledge than the senses, which receive the form of the thing known, without matter indeed, but subject to material conditions. Moreover, among the senses, sight has the most perfect knowledge, because it is the least material, as we have remarked above (Question 78, Article 3): while among intellects the more perfect is the more immaterial.
It is therefore clear from the foregoing, that if there be an intellect which knows all things by its essence, then its essence must needs have all things in itself immaterially; thus the early philosophers held that the essence of the soul, that it may know all things, must be actually composed of the principles of all material things. Now this is proper to God, that His Essence comprise all things immaterially as effects pre-exist virtually in their cause. God alone, therefore, understands all things through His Essence: but neither the human soul nor the angels can do so.
Reply to Objection 1. Augustine in that passage is speaking of an imaginary vision, which takes place through the image of bodies. To the formation of such images the soul gives part of its substance, just as a subject is given in order to be informed by some form. In this way the soul makes such images from itself; not that the soul or some part of the soul be turned into this or that image; but just as we say that a body is made into something colored because of its being informed with color. That this is the sense, is clear from what follows. For he says that the soul "keeps something"--namely, not informed with such image--"which is able freely to judge of the species of these images": and that this is the "mind" or "intellect." And he says that the part which is informed with these images--namely, the imagination--is "common to us and beasts."
Reply to Objection 2. Aristotle did not hold that the soul is actually composed of all things, as did the earlier philosophers; he said that the soul is all things, "after a fashion," forasmuch as it is in potentiality to all--through the senses, to all things sensible--through the intellect, to all things intelligible.
Reply to Objection 3. Every creature has a finite and determinate essence. Wherefore although the essence of the higher creature has a certain likeness to the lower creature, forasmuch as they have something in common generically, yet it has not a complete likeness thereof, because it is determined to a certain species other than the species of the lower creature. But the Divine Essence is a perfect likeness of all, whatsoever may be found to exist in things created, being the universal principle of all.
Source: Summa Theologica (New Advent)