Objection 1. It seems that falsity is not in the intellect. For Augustine says (Qq. lxxxiii, 32), "Everyone who is deceived, understands not that in which he is deceived." But falsity is said to exist in any knowledge in so far as we are deceived therein. Therefore falsity does not exist in the intellect.
Objection 2. Further, the Philosopher says (De Anima iii, 51) that the intellect is always right. Therefore there is no falsity in the intellect.
On the contrary, It is said in De Anima iii, 21, 22 that "where there is composition of objects understood, there is truth and falsehood." But such composition is in the intellect. Therefore truth and falsehood exist in the intellect.
I answer that, Just as a thing has being by its proper form, so the knowing faculty has knowledge by the likeness of the thing known. Hence, as natural things cannot fall short of the being that belongs to them by their form, but may fall short of accidental or consequent qualities, even as a man may fail to possess two feet, but not fail to be a man; so the faculty of knowing cannot fail in knowledge of the thing with the likeness of which it is informed; but may fail with regard to something consequent upon that form, or accidental thereto. For it has been said (2) that sight is not deceived in its proper sensible, but about common sensibles that are consequent to that object; or about accidental objects of sense. Now as the sense is directly informed by the likeness of its proper object, so is the intellect by the likeness of the essence of a thing. Hence the intellect is not deceived about the essence of a thing, as neither the sense about its proper object. But in affirming and denying, the intellect may be deceived, by attributing to the thing of which it understands the essence, something which is not consequent upon it, or is opposed to it. For the intellect is in the same position as regards judging of such things, as sense is as to judging of common, or accidental, sensible objects. There is, however, this difference, as before mentioned regarding truth (16, 2), that falsity can exist in the intellect not only because the intellect is conscious of that knowledge, as it is conscious of truth; whereas in sense falsity does not exist as known, as stated above (Article 2).
But because falsity of the intellect is concerned essentially only with the composition of the intellect, falsity occurs also accidentally in that operation of the intellect whereby it knows the essence of a thing, in so far as composition of the intellect is mixed up in it. This can take place in two ways. In one way, by the intellect applying to one thing the definition proper to another; as that of a circle to a man. Wherefore the definition of one thing is false of another. In another way, by composing a definition of parts which are mutually exclusive. For thus the definition is not only false of the thing, but false in itself. A definition such as " a reasonable four-footed animal" would be of this kind, and the intellect false in making it; for such a statement as "some reasonable animals are four-footed" is false in itself. For this reason the intellect cannot be false in its knowledge of simple essences; but it is either true, or it understands nothing at all.
Reply to Objection 1. Because the essence of a thing is the proper object of the intellect, we are properly said to understand a thing when we reduce it to its essence, and judge of it thereby; as takes place in demonstrations, in which there is no falsity. In this sense Augustine's words must be understood, "that he who is deceived, understands not that wherein he is deceived;" and not in the sense that no one is ever deceived in any operation of the intellect.
Reply to Objection 2. The intellect is always right as regards first principles; since it is not deceived about them for the same reason that it is not deceived about what a thing is. For self-known principles are such as are known as soon as the terms are understood, from the fact that the predicate is contained in the definition of the subject.
Source: Summa Theologica (New Advent)