Chapter 78 — That Aristotle’s opinion concerning the active intellect was not that it is a separate substance, but rather that it is part of the soul

Since, however, some agree with the above opinion (Ch. 76.) in the belief that it reflects the mind of Aristotle, we must show from his words that in his opinion the active intellect is not a separate substance.

For he says, (Aristotle, On the Soul 3.5, 430a10–16.) in the first place, that just as in every nature there is something like the matter in every genus, which is in potency to all that comes under that genus; while there is also a cause like the efficient cause, as art in relation to matter, so must these differences be in the soul. The latter, that which is as matter in the soul, is the (possible) intellect wherein all things intelligible are made: whereas the former, which is as the efficient cause in the soul, is the intellect by which we make all things (namely, actually intelligible), and this is the active intellect, which is like a habit, and not like a power. In what sense he calls the active intellect a habit, he explains by adding that it is as a light, since in a manner light makes potential colors to be colors actually, that it, insofar as it makes them to be actually visible, because this is what is ascribed to the active intellect in regard to intelligibles.

From this we gather that the active intellect is not a separate substance but rather a part of the soul: for he says explicitly that the possible and active intellect are differences of the soul and that they are in the soul. Therefore, neither of them is a separate substance.

Again. His argument proves this also. Because in every nature in which we find potency and act, there is something by way of matter that is in potency to the things of that genus, and something by way of agent that reduces the potency to act: even as in the products of art, there is art and matter. Now the intellective soul is a nature in which we find potency and act, since sometimes it is actually understanding and sometimes potentially. Therefore, in the nature of the intellective soul, there is something by way of matter that is in potency to all intelligibles, and this is called the possible intellect, and there is something by way of efficient cause which makes all things actual, and is called the active intellect. Consequently, both intellects are in the nature of the soul, according to the argument of Aristotle, and are not something separate as to being from the body of which the soul is the act.

Moreover. Aristotle says that the active intellect is like a habit that is a light. Now a “habit” does not designate something existing by itself, but something belonging to one who has it. Therefore, the active intellect is not a substance existing separately by itself, but is part of the human soul.

The text of Aristotle, however, does not mean that the effect of the active intellect may be described as a habit, as though the sense were that the active intellect makes man to understand all things, which is like a habit. For the meaning of habit, as the commentator Averroes says on this passage, is that he who has the habit understands by that which is proper to him, by himself, and whenever he will, without any need in him of something extrinsic, since he explicitly likens to a habit, not the effect itself, but the intellect by which we make all things.

And yet we are not to understand that the active intellect is a habit in the same way as a habit is in the second species of quality, in which sense some have said that the active intellect is the habit of principles. Because this habit of principles is derived from sensibles, as Aristotle proves in Posterior Analytics 2. (Aristotle, Posterior Analytics 2.19, 99b36–100b4.) Consequently, it must be the effect of the active intellect, to which it belongs to make actually intelligible the phantasms that are understood potentially. But habit is to be taken as contrasted with privation and potency, in which sense every form and act may be called a habit. This is evident since he asserts that the active intellect is a habit in the same way as light is a habit.

After this he adds that this intellect, namely the active, is separate, unmixed, impassible, and an actually existing substance. Now of these four conditions which he ascribes to the active intellect, he had already (Aristotle, On the Soul 3.4, 429a8; 429b4; 3.5, 430a17.) explicitly ascribed two to the possible intellect, namely, that it is unmixed and separate. He had applied the third (Aristotle, On the Soul 3.4, 429a29–b4; 429b22–430a9.) (namely, that it is impassible) with a distinction; for he proves in the first place that it is not passible as the senses are, and afterwards he shows that, taking passion broadly, it is passive insofar as it is in potency to intelligibles. But as to the fourth, he absolutely denies it of the possible intellect, and says that it was in potency to intelligibles, and none of these things was actual before the act of intelligence. (Aristotle, On the Soul 3.4, 429b29–430a9.) Accordingly, in the first two the possible intellect agrees with the active; in the third it agrees partly, and partly differs; while in the fourth the active differs altogether from the possible intellect. He proves these four conditions of the active intellect by one argument, when he goes on to say: for the agent is always more noble than the patient, and the (active) principle than matter. (Aristotle, On the Soul 3.5, 430a17–19.) For he had said above that the active intellect is like an efficient cause, and the possible intellect like matter. Now by this middle proposition the two first conditions are proved thus. The agent is more noble than the patient and matter. But the possible intellect, which is as patient and matter, is separate and unmixed, as proved above. Much more, therefore, is the agent. The others are proved by this middle proposition thus: the agent is more noble than the patient and matter, in that it is compared to it as agent and actual being to patient and potential being. Now, the possible intellect is, in a sense, patient and potential being. Therefore, the active intellect is a non-passive agent and an actual being. And it is evident that neither from these words of Aristotle can we gather that the active intellect is a separate substance, but that it is separate in the same sense as he had already said of the possible intellect—namely, as not having an organ. When he says that it is an actually existing substance, this is not inconsistent with the substance of the soul being in potency, as we have shown above. (Ch. 77.)

Then he goes on to say: now knowledge, when actual, is identical with the thing. (Aristotle, On the Soul 3.5, 430a19; cf. ST I.79.4, ad 2.) On this, the Commentator says that the active intellect differs from the possible because that which understands and that which is understood are the same in the active intellect, but not in the possible. But this is clearly contrary to the meaning of Aristotle. For he had employed the same words before in speaking of the possible intellect, where he says of the possible intellect that it is intelligible as intelligibles are, since in things void of matter, understanding and that which is understood are the same, because speculative knowledge is identified with that which it speculates. (Aristotle, On the Soul 3.4, 430a2–4.) For he clearly wishes to show that the possible intellect is understood like other intelligibles from the fact that the possible intellect, as understanding actually, is the same as that which is understood. Moreover, he had said a little earlier that, in a manner, the possible intellect is potentially the intelligibles, but is nothing actually before it understands, (Aristotle, On the Soul 3.4, 429b30–430a1.) where he clearly gives one to understand that by understanding actually it becomes the intelligibles. Nor is it surprising that he should say this of the possible intellect, since he had already (Aristotle, On the Soul 3.2, 425b22–426a1; cf. 3.8, 431b20–432a2.) said this of sense and the sensible in act. For the sense becomes actual by the species actually sensed; and in like manner the possible intellect becomes actual by the intelligible species in act; and for this reason the intellect in act is said to be the intelligible itself in act. Accordingly, we must say that Aristotle, after defining the possible and active intellects, begins here to describe the intellect in act, when he says that actual knowledge is the same as the thing actually known.

Afterwards, he says: but that which is in potency, in point of time, precedes in one subject, but not altogether in point of time. (Aristotle, On the Soul 3.5, 430a19–21.) Which distinction between potency and act is employed by him in several places, namely, that act is naturally prior to potency, but that in point of time, potency precedes act in one and the same subject that is changed from potency to act; yet, absolutely speaking, potency does not precede act even in point of time, since potency is not reduced to act except by an act. He says, therefore, that the intellect which is in potency (namely, the possible intellect considered as being in potency) is prior in time to the intellect in act, and this I say is in one and the same subject. But not altogether, that is, universally, because the possible intellect is reduced to act by the active intellect, which again is in act, as he said, by some possible intellect made actual. Therefore, he said (Aristotle, Physics 3.3.) that before learning a man needs a teacher to reduce him from potency to act. Accordingly, in these words he shows the relation of the possible intellect, as in potency, to the intellect in act.

Then he says: but it does not sometimes understand and sometimes not understand. (Aristotle, On the Soul 3.5, 430a21.) In this he indicates the difference between the intellect in act and the possible intellect. For he said above (Aristotle, On the Soul 3.4, 430a2–9.) of the possible intellect that it does not understand always, but sometimes does not understand, when it is in potency to intelligibles, and sometimes understands, namely, when it is them actually. Now the intellect becomes actual by becoming the intelligibles, as he had already stated. Consequently, it is not competent to it to understand sometimes, and sometimes not to understand.

Afterwards he adds: but that alone is separate which is truly (intellect). This cannot apply to the active intellect, since it alone is not separate, for he had already said the same of the possible intellect. Nor can it apply to the possible intellect, since he had already said this of the active intellect. It follows, then, that it is said of that which includes both, namely, the intellect in act, of which he was speaking, because this alone in our soul is separate and uses no organ, which belongs to the intellect in act (namely, that part of the soul by which we understand actually and which includes both the possible and active intellect). Therefore, he adds that only this part of the soul is immortal and everlasting, as being independent of the body through being separate from it.

The human soul