Article 4 — Whether it is necessary to admit that an agent intellect exists

Fourth, it is asked whether it is necessary to admit that an agent intellect exists. (Parallel texts: ST I.79.3; I.54.4; SCG II.76–78; On Spiritual Creatures, a. 9; Compendium theologiae, bk. 1, ch. 83; Commentary on Aristotle’s On the Soul, bk. 3, lect. 4, 10; Quodlibet VIII, q. 2, a. 1.)

Obj. 1: It seems that it is not. For whatever can be accomplished in nature by one thing is not done by many. (Cf. Aristotle, On the Soul 3.3, 432b21–22; On the Heavens 1.4, 271a33.) Now a man can understand quite well by means of one intellect, namely, the potential intellect. Therefore, it is unnecessary to admit that an agent intellect exists. The minor premise is proved thus: powers that are rooted in one and the same essence of the soul influence one another. It is for this reason that an impression is made on the imagination as a result of a change in the sense powers, for the imagination is moved when the external senses are actuated, as is said in On the Soul 3. (Aristotle, On the Soul 3.3, 429a1–2.) Therefore, if the potential intellect belongs to our soul and is not a separate substance, as we have explained above, (Above: qq. 2–3.) it must belong to the same essence of the soul as the imagination does. Hence, a change in the imagination flows into the potential intellect, and thus it is unnecessary to admit that an agent intellect exists that makes phantasms intelligible by abstracting species from the phantasms themselves.

Obj. 2: Further, touch and sight are different powers. Now in the case of one who is blind, it happens that the imagination is moved to imagine something that belongs to the sense of sight from a change produced in the imagination by the sense of touch, and this occurs because sight and touch are rooted in one and the same essence of the soul. Therefore, if the potential intellect is a certain power of the soul, then, for a similar reason, an impression will be produced in the potential intellect from a change in the imagination. Thus, it is unnecessary to admit that an agent intellect exists.

Obj. 3: Further, an agent intellect is held to exist in order that the potentially intelligible may be made actually intelligible. Moreover, some things are made actually intelligible by being abstracted from matter and from material conditions. Thus, an agent intellect is held to exist in order that intelligible species may be abstracted from matter. However, this can be accomplished without an agent intellect, for, since the potential intellect is immaterial, it must receive things in an immaterial way, because whatever is received is in the recipient according to the mode of the recipient. (Pseudo-Dionysius, On the Divine Names, ch. 4, para. 1 (PG 3:694–95); The Celestial Hierarchy, ch. 12, para. 2 (PG 3:291–94); The Book of Causes, proposition 9 [10]; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 12.) Therefore, it is unnecessary to admit that an agent intellect exists.

Obj. 4: Further, in On the Soul 3, Aristotle compares the agent intellect to light. (Aristotle, On the Soul 3.5, 430a15–16.) But light is necessary for sight only inasmuch as it makes the medium to be actually luminous; (Cf. Averroes, Commentary on Aristotle’s On the Soul, bk. 2, comm. 68, 73; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14, n. 404.) for color is visible in virtue of its own nature, and moves the medium which is actually luminous, as explained in On the Soul 2. (Aristotle, On the Soul 2.7, 418a26; 418a31–b1; 418b27.) However, the agent intellect is not required in order to prepare the potential intellect for the reception of species, because the potential intellect is in potency to intelligible species by its very nature. Therefore, it is unnecessary to maintain that an agent intellect exists.

Obj. 5: Further, just as our intellect is related to intelligible things, so are our senses related to sensible things. Now sensible things require no agent sense in order that they may move the senses, yet sensible things are present with an immaterial mode of existence, both in the senses, which are receptive of sensible things without matter, as is said in On the Soul 3, (Aristotle, On the Soul 3.8, 431b25; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 10; bk. 2, lect. 26.) as well as in the medium, which receives the species of sensible things in an immaterial way. (Aristotle, On the Soul 2.12, 424a18–19; 3.2, 425b23–24.) This is evident from the very fact that the species of contrary qualities, such as white and black, are received in the same part of the medium. (Cf. Aristotle, Metaphysics 7.7, 1032b1–5; SCG II.76; Commentary on Aristotle’s Metaphysics, bk. 7, lect. 6, n. 1405.) Therefore, neither do intelligible things require an agent intellect.

Obj. 6: Further, in order for something in potency to be made actual in the case of natural things, something actual belonging to the same genus is sufficient. (Cf. Aristotle, Metaphysics 9.8, 1049b24–25; On the Generation of Animals 2.1, 734b21–22.) For example, in the case of matter, whatever is ablaze potentially is set ablaze actually by fire, which is in act. Therefore, for our intellect, which is in potency, to be actuated, nothing more is required than the intellect in act, either of the knower himself, as when we proceed from a knowledge of principles to a knowledge of conclusions, or of someone else, as when someone learns from a teacher. (Cf. Aristotle, Posterior Analytics 1.3, 72a30–33; 72b17–24; 2.19, 100b10–17; Aquinas, On Truth, q. 11, a. 1, resp.) Consequently, it seems unnecessary to maintain that an agent intellect exists.

Obj. 7: Further, an agent intellect is held to exist in order that it may illuminate our phantasms, just as the light of the sun illuminates colors. But the divine light, which enlightens every man was coming into this world (John 1:9), suffices for our illumination. Consequently, it is unnecessary to maintain that an agent intellect exists.

Obj. 8: Further, understanding is the activity of an intellect. Therefore, if there are two intellects, that is, an agent and a potential intellect, the understanding of one and the same man will be twofold, which is incongruous.

Obj. 9: Further, it is seen that an intelligible species perfects our intellect. (Cf. SCG II.76; II.78.) Therefore, if there are two intellects, namely, a possible and an agent intellect, then there are two distinct acts of understanding, which seems unnecessary.

On the contrary, in On the Soul 3, Aristotle concludes that, since in every nature there is something active and something potential, these two things must be found within the soul itself, and one of these is the agent intellect, the other the potential intellect. (Aristotle, On the Soul 3.5, 430a10–14.)

I answer that we must admit that an agent intellect exists. To make this evident, we must observe that, since the potential intellect is in potency to intelligibles, the intelligibles themselves must move the potential intellect. But that which is non-existent cannot move anything. Moreover, the intelligible as such, that which the potential intellect understands, does not exist in reality, for our potential intellect understands something as though it were a one-in-many and common to many. (Aristotle, Metaphysics 1.9, 990b7; 991a2; Posterior Analytics 1.11, 77a6; 1.4, 73b26–27.) However, such an entity is not found subsisting in reality, as Aristotle proves in Metaphysics 7. (Aristotle, Metaphysics 7.13, 1038b35; 1039a15; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 12.) Therefore, if the potential intellect has to be moved by an intelligible, this intelligible must be produced by an intellective power. And since it is impossible for anything in potency, in a certain respect, to actuate itself, we must admit that an agent intellect exists, in addition to the potential intellect, and that this agent intellect causes the actual intelligibles which actuate the potential intellect. (Cf. Aquinas, In III Sent., d. 14, q. 1, a. 1, qa. 2, ad 2; On Truth, q. 10, a. 6, resp.)

Moreover, it produces these intelligibles by abstracting them from matter and from material conditions which are the principles of individuation. (Cf. SCG II.49.) And since the nature as such of the species does not possess these principles by which the nature is given a multiple existence among different things, because individuating principles of this sort are distinct from the account itself, (Cf. Aristotle, Metaphysics 10.3, 1058a34–b15; Aquinas, Commentary on Aristotle’s Metaphysics, bk. 10, lect. 11, n. 2131.) the intellect will be able to receive this account apart from all material conditions and consequently will receive it as a unity. For the same reason, the intellect receives the nature of a genus by abstracting from specific differences, so that it is a one-in-many and common to many species.

However, if universals subsisted in reality in virtue of themselves, as the Platonists maintained, (Aristotle, Metaphysics 1.6, 987b1–8; 7.14, 1039a24–33; Averroes, Commentary on Aristotle’s On the Soul, bk. 3, comm. 18.) it would not be necessary to admit than an agent intellect exists, because things which are intelligible in virtue of their own nature move the potential intellect. Therefore, it appears that Aristotle was led by this necessity to posit an agent intellect, because he did not agree with the opinion of Plato on the question of ideas.

Nevertheless, there are some subsistent things in the real order which are actual intelligibles in virtue of themselves; the immaterial substances, for instance, are of this nature. (See below: q. 7.) However, the potential intellect cannot attain a knowledge of these immediately but acquires its knowledge of them through what it abstracts from material and sensible things. (See below: q. 16.)

Reply Obj. 1: Our act of understanding cannot be accomplished by the potential intellect alone, for the potential intellect can understand only when it is moved by an intelligible, and this intelligible, since it does not already exist in the real order, must be produced by the agent intellect. (Cf. Averroes, Commentary on Aristotle’s On the Soul, bk. 3, comm. 18; Aquinas, In II Sent., d. 3, q. 3, a. 4, ad 4.) Moreover, it is true that two powers, which are rooted in one and the same substance of the soul, do influence each other. But this influence can be understood to occur in two ways: first, inasmuch as one power is hindered or totally prevented from performing its operation when another power operates intensely—however, this has no bearing on the problem; (Cf. Avicenna, De anima, part 4, ch. 2; part 5, cc. 3, 7; SCG II.58; ST I.76.3, resp.) second, inasmuch as one power is moved by another, as the imagination is moved by the senses. Now this is possible because the forms in the imagination and those in the external senses are the same in genus, for all are individual forms. Therefore, the forms which are in the external senses can impress those forms which exist in the imagination by moving the imagination because they are similar to these forms. (Aristotle, On the Soul 3.3, 428b10–14; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 30.) However, the forms in the imagination, since they represent things as individuals, cannot cause intelligible forms, because these are universal.

Reply Obj. 2: The species received in the imagination from the sense of touch are not enough to cause the imagination to produce forms (Cf. Aristotle, On the Soul 3.3, 428b11–12.) belonging to the sense of sight unless forms previously received by the sense of sight are stored up in the repertory of memory or imagination. (Cf. Aquinas, On Truth, q. 10, a. 6, ad 5; SCG II.73–74; ST I.78.4, resp.; Averroes, Commentary on Aristotle’s On the Soul, bk. 2, comm. 153.) For one who is born blind cannot imagine color by any other kind of sensible species whatsoever. (Cf. Aristotle, Posterior Analytics 1.18, 81a38–39; Aquinas, Commentary on Aristotle’s Posterior Analytics, bk. 1, lect. 30, n. 249; On Truth, q. 10, a. 6, s.c.)

Reply Obj. 3: The condition of the recipient cannot cause a species which has been received to be transferred from one genus to another; however, it can alter a received species of the same genus according to some mode of being. Hence, since a universal species and a particular species differ in genus, (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 12; bk. 3, lect. 1, n. 692.) it follows that the cognitive activity of the potential intellect alone is not enough to give the particular species in the imagination the universality which they possess in the intellect, but that an agent intellect is required to do this.

Reply Obj. 4: There are two opinions concerning light, as the Commentator points out in Long Commentary on the De anima of Aristotle 2. (Averroes, Commentary on Aristotle’s On the Soul, bk. 2, comm. 67; cf. ST I.79.3, ad 2.) For some said that light is necessary for sight inasmuch as it gives to colors the power of moving the sense of sight, as if color were not visible of itself, but only through light. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14.) But this seems to contradict Aristotle, since he points out in On the Soul 2 that color is visible in virtue of itself, and this would not be the case if it were made visible by light alone. (Aristotle, On the Soul 2.7, 418a29–b1; 419a10–11.) For this reason, others offer a different and more acceptable explanation, namely, that light is necessary for sight inasmuch as it perfects the medium, making it to be actually luminous. (Cf. SCG II.76; Compendium theologiae, bk. 1, ch. 83; Commentary on Aristotle’s On the Soul, bk. 2, lect. 14.) Wherefore, the Philosopher says in On the Soul 2 that color has the power of moving what is actually luminous. (Aristotle, On the Soul 2.7, 418a33–b1.) Nor is this position rendered untenable by the fact that someone in the dark sees things which are in the light, but not vice versa. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14.) For this occurs because the medium which surrounds a visible thing must be illuminated in order that the medium may receive the visible species; and this remains visible as long as the act of light continues to illuminate the medium, although it illuminates it more perfectly the nearer it is, and more weakly the farther it is away. Consequently, the comparison between light and the agent intellect does not hold in all respects, because the agent intellect is necessary for this reason, that it may make the potentially intelligible to be actually intelligible. Aristotle pointed this out in On the Soul 3, when he said that the agent intellect is like light in some respects. (Aristotle, On the Soul 3.5, 430a15–16; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14, n. 421.)

Reply Obj. 5: Since a sensible is something particular, (Aristotle, On the Soul 2.5, 417b27.) it does not impress a species of a higher genus either on the sense or on the medium, because the species existing in the medium and in the sense is a particular and nothing more. The potential intellect, however, receives species of a higher genus than those present in the imagination because the potential intellect receives universal species, whereas the imagination contains only particular species. (Cf. Aquinas, On Truth, q. 10, a. 6, ad 1; SCG II.77.) Therefore, we require an agent intellect in the case of intelligible things but need no additional agent power in the case of sensible things. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 27.) Indeed, all the sensitive powers are passive powers.

Reply Obj. 6: The potential intellect in act is not sufficient to cause knowledge in us unless an agent intellect is presupposed. For if we speak of the intellect in act of one who is learning, it so happens that his potential intellect is in potency with respect to some things, and in act with respect to others; (Cf. Aquinas, On Truth, q. 11, a. 1, resp.) and his intellect can be put into act, so far as the things to which it is in potency are concerned, by the things that are already actually known, just as one is made to be actually knowing conclusions, which were previously known only potentially, by actually knowing principles. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 11, n. 372; ST I.117.1, resp.) However, the potential intellect can be actually knowing principles only through the activity of the agent intellect; for our knowledge of principles is received from sensible things, as is stated at the end of Posterior Analytics 2. (Aristotle, Posterior Analytics 2.19, 100a9–11; cf. Aquinas, Commentary on Aristotle’s Posterior Analytics, bk. 2, lect. 20.) Moreover, intelligibles can be derived from sensible things only by the abstractive activity of the agent intellect. Thus, it is evident that the intellect in act with respect to principles does not suffice to move the potential intellect from potentiality to act without the agent intellect. Indeed, in this actuating of the potential intellect, the agent intellect acts like an artisan and the principles of demonstration like tools. (Cf. Aquinas, On Truth, q. 11, a. 3, resp.)

However, if we speak of the intellect in act of the teacher, it is evident that when the teacher causes knowledge in one who is learning, he does not act as an interior agent, but as an external administrator, just as, in the production of health, a physician acts as an external administrator, whereas the nature of the patient acts as an interior agent. (Cf. Aquinas, On Truth, q. 11, a. 1, resp.; ST I.117.1, resp., ad 1; Commentary on Aristotle’s On the Soul, bk. 2, lect. 11.)

Reply Obj. 7: Just as real things of any kind require proper active principles, even though God is the first and universal agent, (See Aristotle, Physics 2.2, 194b13; cf. SCG II.76; On Truth, q. 11, a. 1, resp.) so too does man require a proper intellective light even though God is the first light illuminating all men in common.

Reply Obj. 8: There is a proper activity for each one of the two intellects, that is, the potential intellect and the agent intellect. For the activity of the potential intellect consists in receiving intelligibles, whereas that of the agent intellect consists in abstracting them. (Cf. SCG II.76.) Nor does it follow that there are two distinct acts of understanding in a man, because it is necessary that the activities of both intellects concur to produce one act of understanding.

Reply Obj. 9: The same intelligible species is related to the agent intellect and to the potential intellect. However, it is related to the potential intellect as a recipient, and to the agent intellect as the one producing species of this sort by abstraction. (Cf. SCG II.77; ST I.87.1, resp.; Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.)