Objection 1. It would seem that our intellect knows singulars. For whoever knows composition, knows the terms of composition. But our intellect knows this composition; "Socrates is a man": for it belongs to the intellect to form a proposition. Therefore our intellect knows this singular, Socrates.
Objection 2. Further, the practical intellect directs to action. But action has relation to singular things. Therefore the intellect knows the singular.
Objection 3. Further, our intellect understands itself. But in itself it is a singular, otherwise it would have no action of its own; for actions belong to singulars. Therefore our intellect knows singulars.
Objection 4. Further, a superior power can do whatever is done by an inferior power. But sense knows the singular. Much more, therefore, can the intellect know it.
On the contrary, The Philosopher says (Phys. i, 5), that "the universal is known by reason; and the singular is known by sense."
I answer that, Our intellect cannot know the singular in material things directly and primarily. The reason of this is that the principle of singularity in material things is individual matter, whereas our intellect, as have said above (Question 85, Article 1), understands by abstracting the intelligible species from such matter. Now what is abstracted from individual matter is the universal. Hence our intellect knows directly the universal only. But indirectly, and as it were by a kind of reflection, it can know the singular, because, as we have said above (Question 85, Article 7), even after abstracting the intelligible species, the intellect, in order to understand, needs to turn to the phantasms in which it understands the species, as is said De Anima iii, 7. Therefore it understands the universal directly through the intelligible species, and indirectly the singular represented by the phantasm. And thus it forms the proposition "Socrates is a man." Wherefore the reply to the first objection is clear.
Reply to Objection 2. The choice of a particular thing to be done is as the conclusion of a syllogism formed by the practical intellect, as is said Ethic. vii, 3. But a singular proposition cannot be directly concluded from a universal proposition, except through the medium of a singular proposition. Therefore the universal principle of the practical intellect does not move save through the medium of the particular apprehension of the sensitive part, as is said De Anima iii, 11.
Reply to Objection 3. Intelligibility is incompatible with the singular not as such, but as material, for nothing can be understood otherwise than immaterially. Therefore if there be an immaterial singular such as the intellect, there is no reason why it should not be intelligible.
Reply to Objection 4. The higher power can do what the lower power can, but in a more eminent way. Wherefore what the sense knows materially and concretely, which is to know the singular directly, the intellect knows immaterially and in the abstract, which is to know the universal.
Source: Summa Theologica (New Advent)