Article 12 — Whether falsity is in the intellect

Twelfth, it is asked whether falsity is in the intellect? (Parallel texts: ST I.12.3; I.17.3; I.58.5; I.85.6; In I Sent., d. 19, q. 5, a. 1, ad 7; Commentary on Aristotle's On Interpretation, bk. 1, lect. 3; Commentary on Aristotle's On the Soul, bk. 3, lect. 11; Commentary on the Metaphysics, bk. 6, lect. 4; SCG I.59; III.108.)

Obj. 1: It seems not, for the intellect has two operations. By one it forms quiddities, (Cf. Averroes, On the Soul, bk. 3, comm. 21; Aristotle, On the Soul 3.6, 430a26.) and, as the Philosopher says, the false is not in this. (Aristotle, On the Soul 3.6, 430a27.) By the other it joins and separates, and the false is not in this either, as is clear from Augustine’s saying: no one has intellectual knowledge of false things. (Augustine, On True Religion, cc. 34, 36 (PL 34:150).) Consequently, falsity is not in the intellect.

Obj. 2: According to Augustine: whoever is deceived does not understand that in which he is deceived. (Augustine, Book of Eighty-Three Different Questions, qq. 31–32 (PL 40:22; CCSL 44A).) The intellect is always true, therefore, and there can be no falsity in it.

Obj. 3: Algazel says: either we understand something as it is or we do not understand. (Algazel, Metaphysics, part 1, tract. 3, sent. 11.) But whoever understands a thing as it is truly understands it. Therefore, the intellect is always true, and there is no falsity in it.

On the contrary, the Philosopher says: where there is a joining of concepts, there the true and the false begin to be. (Aristotle, On the Soul 3.6, 430a27.) Hence, falsity is found in the intellect.

I answer that the name intellect arises from the intellect’s ability to know the most profound elements of a thing; for to understand (intelligere) means to read what is inside a thing (intus legere). (Cf. Peter Helias, Summa super Priscianum, "De syllaba"; William of Auvergne, De universo, bk. 2, ch. 15.) Sense and imagination know only external accidents, but the intellect alone penetrates to the interior and to the essence of a thing. But even beyond this, the intellect, having perceived essences, operates in different ways by reasoning and inquiring. Hence, intellect can be taken in two senses. First, it can be taken merely according to its relation to that from which it first received its name. We are said to understand, properly speaking, when we apprehend the quiddity of things or when we understand those truths that are immediately known by the intellect, once it knows the quiddities of things. For example, first principles are immediately known when we know their terms, (Aristotle, Posterior Analytics 1.3, 72b23.) and for this reason intellect or understanding is called "a habit of principles.” (Aristotle, Ethics 6.6, 1141a7.) The proper object of the intellect, however, is the quiddity of a thing. Hence, just as the sensing of proper sensibles is always true, so the intellect is always true in knowing what a thing is, as is said in On the Soul. (Aristotle, On the Soul 3.6, 430b27.) By accident, however, falsity can occur in this knowing of quiddities, if the intellect falsely joins and separates. This happens in two ways: when it attributes the definition of one thing to another, as would happen were it to conceive that “mortal rational animal” were the definition of an ass; or when it joins together parts of definitions that cannot be joined, as would happen were it to conceive that “irrational, immortal animal” were the definition of an ass. For it is false to say that some irrational animal is immortal. So it is clear that a definition cannot be false except to the extent that it implies a false affirmation. This twofold mode of falsity is touched upon in the Metaphysics. (Aristotle, Metaphysics 5.29, 1024b26.) Similarly, the intellect is not deceived in any way with respect to first principles. It is plain, then, that if intellect is taken in the first sense—according to that action from which it receives the name intellect—falsity is not in the intellect. Intellect can also be taken in a second sense—in general, that is, as extending to all its operations, including opinion and reasoning. In that case, there is falsity in the intellect. But it never occurs if a reduction to first principles is made correctly.

From this the answers to the difficulties are clear.