Article 10 — Whether the agent intellect is one intellect belonging to all men

Tenth, it is asked whether the agent intellect is one intellect belonging to all men. (Parallel texts: In II Sent., d. 17, q. 1, a. 2; Disputed Questions on the Soul, q. 5; SCG II.76–78; Commentary on Aristotle’s On the Soul, bk. 3, lect. 4; ST I.79.5; Compendium theologiae, cc. 86–88.)

Obj. 1: And it would seem that it is. For to enlighten men is proper to God, according to that passage in John 1:9, the true light that enlightens every man was coming into the world and so on. But this pertains to the agent intellect, as is clear from On the Soul 3. (Aristotle, On the Soul 3.5, 430a15; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) Therefore, the agent intellect is God. Now God is one; therefore, the agent intellect is one only.

Obj. 2: Furthermore, nothing which is separated from the body is multiplied in consequence of the multiplication of bodies. But the agent intellect is separated from the body, as is said in On the Soul 3. (Aristotle, On the Soul 3.5, 430a17; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) Therefore, the agent intellect is not multiplied in consequence of the multiplication of bodies, and, consequently, not in consequence of the multiplication of men.

Obj. 3: Furthermore, there is nothing in our soul which always understands. But this is an attribute of the agent intellect. For it is said in On the Soul 3 that it is not the case that it sometimes understands and sometimes does not. (Aristotle, On the Soul 3.5, 430a22; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) Therefore, the agent intellect is not something belonging to the soul, and so it is not multiplied in consequence of the multiplication of souls and of men.

Obj. 4: Furthermore, nothing reduces itself from potency to act. (Cf. SCG I.16; Aristotle, Metaphysics 9.8, 1049b24–25; Aquinas, Commentary on the Metaphysics, bk. 9, lect. 7, n. 1846.) But the potential intellect is reduced to act through the agent intellect, as On the Soul 3 (Aristotle, On the Soul 3.5, 430a14–15.) makes clear. Therefore, the agent intellect is not rooted in the essence of the soul, wherein the potential intellect is rooted; thus, we reach the same conclusion as before.

Obj. 5: Furthermore, every multiplication follows upon some distinction. But the agent intellect cannot be distinguished through matter, since it is separated; nor through form, for in this case it would be different in species. Therefore, the agent intellect is not multiplied in men.

Obj. 6: Furthermore, that which is a cause of separation is in the highest degree separated. But the agent intellect is a cause of separation, for it abstracts species from matter. Therefore, it is separated, and thus is not multiplied in consequence of the multiplication of men.

Obj. 7: Furthermore, no power which can act the more, the more it acts, has a limit on its activity. But the agent intellect is of this kind, because whenever we understand some great intelligible thing, we are not less able to understand, but more, as is said in On the Soul 3. (Aristotle, On the Soul 3.4, 429b2–4; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 1.) Therefore, the agent intellect does not have any limit on its activity. Now, actual created being has a limit on its activity, since it is of finite power. Therefore, the agent intellect is not something created, and thus is one only.

Obj. 8: Furthermore, Augustine says in the Book of Eighty-Three Different Questions that everything that the corporeal sense touches . . . is changed without any temporal interruption . . . Now, something which is changed without any interruption cannot be comprehended. The clearness of truth is, therefore, not to be expected from the senses of the body. (Augustine, Book of Eighty-Three Different Questions, bk. 1, q. 9 (PL 40:13; CCSL 44A:16).) And afterwards he adds, nothing is perceptible to the sense, which does not possess a likeness to what is false, with the result that it cannot be distinguished. But nothing can be perceived which is not distinguished from the false. Judgment of the truth, then, is not established in the senses. (Augustine, Book of Eighty-Three Different Questions, bk. 1, q. 9 (PL 40:13; CCSL 44A:16–17).) In this way, therefore, he proves that we cannot judge of truth through sensible things, both because of the fact that they are changeable, and because of the fact that they have something that is similar to falsity. But this holds good of every creature; therefore, through no creature can we judge of truth. But we do judge of truth through the agent intellect; therefore, the agent intellect is not something created. Thus, we reach the same conclusion as before.

Obj. 9: Furthermore, Augustine says in On the Trinity 4 that the impious rightly censure and rightly praise many things in the customs of men. By what standards, pray, do they judge these things, unless by those in which they see how each man should live, even if they themselves do not live in the same way? Where do they see these standards? Not in their own nature, since . . . their minds are evidently changeable, but these rules are unchangeable . . . Nor do they see them in a habit of their mind, since these rules are rules of justice but their minds are evidently unjust . . . Where, therefore, have they been written except in the book of that light which is called truth? (Augustine, On the Trinity, bk. 4, ch. 15, para. 21 (PL 42:1052; CCSL 50A:450–51).) From this it would seem that we are competent to judge of what is just and what is unjust on the basis of a light which is above our minds. Now, judgment in speculative as well as in practical matters is an attribute of ours in consequence of the agent intellect. Therefore, the agent intellect is a light above our mind. Therefore, it is not multiplied along with the multiplication of souls and of men.

Obj. 10: Furthermore, Augustine says in the book On True Religion (Augustine, On True Religion, bk. 1, cc. 31–32 (PL 34:147–49; CCSL 32:224–27).) that, concerning any two things, neither of which is the best thing, we cannot judge which of them is better than the other except through something which is better than both. Now, we judge that an angel is better than a soul, although, nevertheless, neither of them is the best thing. Therefore, it must be the case that this judgment is made through something which is better than both, and this is nothing other than God. Since, therefore, we judge through the agent intellect, it would seem that the agent intellect is God; thus, we reach the same conclusion as before.

Obj. 11: Furthermore, the Philosopher says in On the Soul 3 that the agent intellect is to the potential intellect as art is to the material. (Aristotle, On the Soul 3.5, 430a12–13; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) But in no kind of artificial production do the art and the material coincide in the same object; nor in general do an agent and material coincide in an object that is the same in number, as is said in Physics 2. (Aristotle, Physics 2.7, 198a24–27; cf. Aquinas, Commentary on Aristotle’s Physics, bk. 2, lect. 11, n. 242: materia vero non est nec idem specie nec idem numero cum aliis causis; quia materia inquantum huiusmodi est ens in potentia, agens vero est ens in actu inquantum huiusmodi, forma vero vel finis est actus vel perfectio; “the matter is neither the same in species nor the same in number as the other causes. For matter as such is being in potency, whereas the agent as such is being in act, and the form or the end is act or perfection.”) Therefore, the agent intellect is not something in the essence of the soul in which the potential intellect is; and so it is not multiplied in consequence of the multiplication of souls and of men.

Obj. 12: Furthermore, Augustine says in On Free Will 2 that the true essence of number is present to all reasoning persons. (Augustine, On Free Will, bk. 2, ch. 8, para. 20 (PL 32:1251; CSEL 74:56; CCSL 29:250).) But the true essence of number is one. Therefore, there must be some one thing whereby it is present to all. Now, this is the agent intellect, by the power of which we abstract universal accounts from things. Therefore, the agent intellect is one in all men.

Obj. 13: Furthermore, in the same book it is said, if the highest good is one thing for all, it must also be the case that the truth wherein it is discerned and comprehended, that is, wisdom, is one truth common to all men. (Augustine, On Free Will, bk. 2, ch. 9, para. 27 (PL 32:1255; CSEL 74:62; CCSL 29:254).) But the highest good is discerned and comprehended by us through the intellect, and especially through the agent intellect. Therefore, the agent intellect is one in all men.

Obj. 14: Furthermore, like naturally tends to cause like. (Cf. Aristotle, On Generation and Corruption 2.10, 336a27–28.) But a universal is one thing in all men. Since, therefore, it is characteristic of the agent intellect to cause a universal, it would seem that the agent intellect is one in all men.

Obj. 15: Furthermore, if the agent intellect is a part of the soul, it must either be created clothed or filled with species, and in that case it places those species also in the potential intellect, and will not need to abstract intelligible species from the phantasms; or else it is created naked and lacking in species, and in that case it will not be effectually able to abstract species from phantasms, because it will not recognize that species which it is seeking, after it has abstracted it, unless it previously had some account of it. This is just as a man who is looking for a runaway slave does not recognize him when he has found him, unless previously he had some knowledge of him. (This example is also found in: Avicenna, Liber de anima seu Sextus de naturalibus, part 5, ch. 7. The Liber de anima (Kitāb an-nafs) is itself a section of Avicenna’s larger Book of Healing (al-Shifa’) and was available to Aquinas through a translation by Dominic Gundisalvi (Gundissalinus).) Therefore, the agent intellect is not a part of the soul; thus, it is not multiplied along with souls and men.

Obj. 16: Furthermore, once a sufficient cause has been asserted, it is superfluous to assert another cause for the same effect. (See above: a. 9, obj. 3.) But there is an extrinsic cause sufficient for the enlightenment of men, namely, God. Therefore, it is not necessary to assert that an agent intellect, whose function it is to enlighten, is something in the soul of men; thus, it is not multiplied along with souls and men.

Obj. 17: Furthermore, if the agent intellect is put down as part of the soul of man, it must be that it contributes to something in the case of man, because nothing among the things created by God is idle and vain. (See above: a. 4, obj. 1.) But the agent intellect does not contribute to man’s knowing, in the sense that it enlightens the potential intellect, because the potential intellect, once it has been actuated through an intelligible species, is fully able to act on its own account, just as anything else is which has a form. Similarly, it does not make any contribution in the matter of lighting up the phantasms, abstracting intelligible species from them, because, just as a species which is received in a sense imprints its likeness on the imagination, so it would seem that a form which is in the imagination, since it is more spiritual and for this reason more powerful, is able to imprint its likeness on a further power, namely, on the potential intellect. The agent intellect is not, therefore, a part of the soul, and thus it is not multiplied in men.

On the contrary (1), there is what the Philosopher says in On the Soul 3, (Aristotle, On the Soul 3.5, 430a13–15; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) that the agent intellect is a part of the soul. Therefore, it is multiplied in consequence of the multiplication of souls.

Furthermore (2), Augustine says in On the Trinity 4 that philosophers have not contemplated intellectually, better than others, in those supreme and eternal accounts, (Augustine, On the Trinity, bk. 4, ch. 16, para. 21 (PL 42:902; CCSL 50:188).) the things which they have discussed in a historical way. And so, it would seem that they have contemplated these things in some light that is connatural to them. Now, the light wherein we contemplate truth is the agent intellect. Therefore, the agent intellect is a part of the soul, and thus we reach the same conclusion as before.

Furthermore (3), Augustine says in On the Trinity 12 that we have to believe that the nature of the intellectual mind is so constituted . . . that it sees the above-mentioned things in a sort of incorporeal light which is unique of its kind, just as the bodily eye sees the things that lie about it in this corporeal light. (Augustine, On the Trinity, bk. 12, ch. 15, para. 24 (PL 42:1011; CCSL 50:178).) Now, that light whereby our mind understands is the agent intellect. Therefore, the agent intellect is something of the nature of the soul, and thus it is multiplied through the multiplication of souls and men.

I answer that, as has been mentioned before, (Above: a. 9, resp.) it is necessary for Aristotle to posit the agent intellect, because he did not assert that the natures of sensible things have a subsistence of their own apart from matter, so as to be actually intelligible, and consequently there had to be some power to make them actually intelligible, by abstracting from individual matter; and this power is called the agent intellect. Some have asserted that this is a sort of separated substance, not multiplied in correspondence with the number of men, but others have asserted that it is in itself a sort of power of the soul, and is multiplied in many men. (Aristotle, On the Soul 3.5, 430a13–15; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) Both of these assertions are true in a sense. (Cf. Aquinas, Disputed Questions on the Soul, q. 5, resp.; On the Unity of the Intellect against the Averroists, ch. 4.)

For it must be the case that above the human soul there is some intellect on which its understanding depends, and this can be made evident on three grounds. First of all, because everything that belongs to a thing in a partial way is previously in something in a substantial way; thus, for instance, if a piece of iron is fiery hot, there must be something among things which is “fire” in its own nature and substance. (Cf. Aquinas, Commentary on the Book of Causes, prop. 9, n. 213.) Now, the human soul is intellectual in a partial way, for it does not understand in every part of itself, but in its highest part only. There must, then, be something higher than a soul, which is intellect in its whole nature, from which the intellectuality of the soul is derived, and upon which its act of understanding depends.

Second, because it is necessary that, prior to everything that is movable, there must be something that is immovable in relation to that movement, just as above the things that are subject to alteration there is something not subject to alteration, like a heavenly body. For every movement is caused by something that is immovable. The very understanding of the human soul takes place as a movement, for the soul understands by reasoning discursively from effects to causes, and from causes to effects, and from likes to likes, and from opposites to opposites. There must, then, be above the soul some intellect whose power of understanding is fixed and at rest without discursive thinking of this sort.

Third, because it is necessary that, although in one and the same being a potency is prior to an act, nevertheless, absolutely speaking, some act is prior to any potency in another being; similarly, prior to every imperfect thing, there must be something that is perfect. Now, the human soul at the outset is in potency to intelligible things, and it is found to be imperfect in understanding because never in this life will it attain the truth of all intelligible things. There must be, then, above the soul some intellect that always exists in act and is wholly perfect in its understanding of truth.

However, it cannot be said that that higher intellect makes things actually intelligible in us immediately apart from some power from it in which our soul has a share. For it is quite generally true, even in the case of corporeal things, that in lower things there are to be found particular powers that are active in respect to definite effects, besides the universal active powers. Thus, for instance, perfect animals are not generated as a result of the universal power of the sun only, but as a result of the particular power which is in the semen; although, some imperfect animals are generated without semen as a result of the power of the sun, and yet even in their generation there is not lacking the action of a particular power that alters and disposes the matter. Now, the human soul is the most perfect of those beings which exist among inferior things. Hence, it must be the case that in addition to the universal power of the higher intellect, there should be imparted to it some power that is, as it were, particular in respect to this definite effect, namely, that things become actually intelligible. And it is clear from experience that this is true.

For one particular man, such as Socrates or Plato, makes things intelligible in act when he pleases, (Cf. Averroes, Commentary on Aristotle’s On the Soul, bk. 3, comm. 18, 36.) that is, by apprehending a universal form from particulars, when he separates that which is common to all individual men from those things which are peculiar to each. Thus, the action of the agent intellect, which is to abstract the universal, is an action of “this particular man,” as is also the act of considering or judging about a common nature, which is the action of the potential intellect. Every agent that does any action has within itself by way of a form the power which is the principle of this kind of action. Hence, just as it is necessary that the potential intellect be something that is formally inherent in man, as we showed above, (Above: a. 9, resp.) so it is necessary that the agent intellect be something that is formally inherent in man. A connection by way of phantasms does not suffice for this, as Averroes imagines, as was also shown above in regard to the potential intellect. (Above: a. 2, obj. 17; a. 9, resp.) And it seems clear that Aristotle realized this when he said that there must be these differences in the soul, (Aristotle, On the Soul 3.5, 430a13.) namely, the agent and the potential intellects; and again he says that the agent intellect is, as it were, a brightness which is a participated light. (Aristotle, On the Soul 3.5, 430a15.) But Plato, as Themistius says in his Commentary on Aristotle’s On the Soul, (Themistius (4th cent. AD), Commentary on the De anima, bk. 3, ch. 5: propter hoc enim non solem in comparatione proposuit sed lumen, Plato autem solem.) considering the intellect apart and not considering the participated power of the soul, compared the intellect to the sun.

But we must consider what that separated intellect is, upon which the human soul’s understanding depends. For some have said that this intellect is the lowest of the separated substances, which is connected with our souls by its own light. (Cf. Avicenna, Metaphysics, tract. 9, ch. 4.) But this is contrary to the truth of faith in many respects.

First of all because, since this intellectual light pertains to the nature of the soul, it comes from him alone by whom the nature of the soul is created. Now God alone is the creator of the soul, and not some separated substance which we call an angel; hence, it is said significantly in Genesis that God himself breathed into his nostrils the breath of life (Gen 2:7). Hence, the only remaining alternative is that the light of the agent intellect is not caused in the soul by any other separated substance, but is caused immediately by God.

Second, because the ultimate perfection of each individual agent is that it can attain to its own principle. (Cf. Aquinas, Commentary on Aristotle’s Physics, bk. 8, lect. 19, n. 1131; ST I.103.1, resp.) Now, the ultimate perfection or beatitude of man is based on intellectual activity, as the Philosopher also says in Ethics 10. (Aristotle, Nicomachean Ethics 10.7, 1177a12–b4. Cf. Disputed Questions on the Soul, q. 5, resp.)

If, then, the principle and cause of the intellectuality of men were some other separated substance, it would have to be the case that the ultimate beatitude of man would be situated in that created substance. Those who hold this view clearly assert this, for they assert that the ultimate felicity of man is to be connected with the agent intelligence. (Cf. Avicenna, Liber de anima, pt. 5, ch. 6; Aquinas, Disputed Questions on the Soul, q. 5, resp.) The true faith asserts that the ultimate beatitude of man is in God alone, according to this quotation from John 17:3, this is eternal life, that they know thee the only true God, and that in participating in this beatitude, men are equal to angels, as is held by Luke 20:36.

Third, because if man were to have a share in the intellectual light from an angel, it would follow that man as regards his mind would not be made to the image of God himself, but to the image of angels, contrary to what is said in Genesis 1:26, let us make man in our image, after our likeness, that is, to the common image of the Trinity, not to the image of the angels.

Hence, we say that the light of the agent intellect, of which Aristotle is speaking, is impressed upon us immediately by God, and by this light we discern truth from falsity, and good from evil. Concerning this, it is said in the Psalms, there are many who say, ‘O that we might see some good! Lift up the light of thy countenance upon us, O Lord’ (Ps 4:6–7), that is, by which good things are shown to us. Thus, then, that which makes things actually intelligible in us after the manner of a participated light is a part of the soul, and is multiplied along with the number of souls and of men. But that which makes things intelligible after the manner of the sun, which illuminates, is something that is one and separate, which is God. Hence, Augustine says in Soliloquies 1, reason promises . . . to show God to my mind as the sun is shown to the eyes; for the eyes of the mind, so to speak, are the senses of the soul. But all the most certain branches of learning are of such a nature as things illumined by the sun, so that they can be seen . . . and God himself is the one who illumines. (Augustine, The Soliloquies, bk. 1, ch. 6, para. 12 (PL 32:875; CSEL 89).) Now, this one separate principle of our knowledge cannot be understood to be the agent intellect of which the Philosopher is speaking, as Themistius says, because God is not in the nature of the soul. (Themistius, Commentary on the De anima, bk. 3, ch. 5.) But the agent intellect is the name given by Aristotle to the light that is received in our soul from God. (Aristotle, On the Soul 1.4, 408b29.) And in view of this, it remains to be said without qualification that the agent intellect is not one in all men.

Reply Obj. 1: It is proper to God to enlighten men by impressing on them the natural light of the agent intellect, and in addition to this the light of grace and glory. But the agent intellect lights up the phantasms, as a light that is impressed by God.

Reply Obj. 2: The agent intellect is called “separated” by Aristotle, not as though it were a kind of substance that has actual being outside the body, but because it is not an act of any part of the body in the sense that its activity takes place through some corporeal organ, as was said of the potential intellect. (Above: a. 9, ad 8.)

Reply Obj. 3: Aristotle does not make that statement about the agent intellect, but about the intellect in act. For first he spoke of the potential intellect, afterwards of the agent intellect, and finally he begins to speak of the intellect in act, when he says, actual knowledge of a thing is identical with its object. (Aristotle, On the Soul 3.5, 430a19–20.) And he distinguishes the intellect in act from the intellect in potency in three ways.

The first is because the intellect in potency is not the thing that is understood in potency; but the intellect in act, or knowledge in act, is the thing that is understood or known in act. (See above: a. 8, ad 14.) So too, he had said of the senses that the sense in potency and the thing that can be sensed in potency are different.

Second, he compares the intellect in act to the intellect in potency, because the intellect in potency is temporally prior in one and the same man to the intellect in act. For temporally an intellect is in potency before it is in act, but naturally act is prior to potency. Speaking in an absolute sense, we must posit some intellect in act prior even in time to an intellect in potency, which is reduced to act through some intellect in act. This is what he adds: and this in potency is temporally prior in one individual, but in general it is not prior even temporally. (Aristotle, On the Soul 3.5, 430a20–21.) He employs this comparison between potency and act also in Metaphysics 9 (Aristotle, Metaphysics 9.8, 1049b10–1050a3.) and in many other places. (Cf. Aquinas, Disputed Questions on the Soul, q. 5, resp.; Aristotle, Metaphysics 9.8, 1049b4–1051a3; 12.6, 1072a3–4.)

Third, he points out a difference in this respect, that the intellect in potency or the potential intellect is sometimes to be found understanding and sometimes not; but this cannot be said of the intellect in act. (Cf. Aristotle, Physics 2.3, 195b1–6.) So too the visual power sometimes sees and sometimes does not see, but the sight in act consists in actually seeing. This is what he says, but it is not true that it sometimes understands and sometimes does not; (Aristotle, On the Soul 3.5, 430a19.) and afterwards he adds, but that thing only is separate which truly is, (Aristotle, On the Soul 3.5, 430a22–23.) and this cannot be understood either of the agent intellect or of the potential intellect, since above he has called both separate. But it must be understood of everything that is required for the intellect in act, that is, of the whole intellectual part. Hence, also he adds, and this alone is immortal and eternal; (Aristotle, On the Soul 3.5, 430a23.) and if this be explained as referring to the agent intellect, it will follow that the potential intellect is corruptible, as Alexander understood. (Alexander of Aphrodisias (fl. 200 AD); cf. Aristotle, On the Soul 1.1, 403a8–10; Averroes, Commentary on Aristotle’s On the Soul, bk. 1, comm. 12.) But this is contrary to what Aristotle had said above about the potential intellect. (Aristotle, On the Soul 1.4, 408b18–19; 408b24–25; see also 3.4, 429a18–27; 3.5, 430a17–25.) Now, it has been necessary to explain these words of Aristotle here in order that they may not be an occasion of error to anyone.

Reply Obj. 4: Nothing prevents any two things that are related to each other from being such that each of them is both a potency and an act as regards the other, on different grounds; thus, fire, for instance, is potentially cold and actually hot, but water the opposite. For this reason, natural agents are at the same time passive and active. If, then, the intellectual part be compared to the phantasms, in one respect it will be in potency and in another it will be in act with reference to them. A phantasm actually contains a likeness of a definite nature, but this likeness of a definite species is in the phantasm in potency, able to be abstracted from material conditions. But on the intellectual side, the opposite is the case, for it does not actually possess likenesses of distinct things, but yet it actually possesses an immaterial light which has the power of abstracting those things which are able to be abstracted in potency. Thus, nothing prevents there being found in the same essence of the soul a potential intellect, which is in potency with respect to the species which are abstracted from the phantasms, and an agent intellect, which abstracts the species from the phantasms. We should have something similar if there were one and the same body which would be transparent, being in potency to all colors, and if along with this it would have a light whereby it could illuminate colors, as is somehow apparent in the eye of a cat.

Reply Obj. 5: The light of the agent intellect is multiplied immediately through the multiplication of the souls, which participate in the very light of the agent intellect. Now, souls are multiplied along with bodies, as was said above. (Above: a. 9, ad 3.)

Reply Obj. 6: This very fact that the light of the agent intellect is not an act of any corporeal organ through which it acts is sufficient for its being able to separate intelligible species from phantasms. For the separateness of intelligible species, which are received in the potential intellect, is not greater than the separateness of the agent intellect.

Reply Obj. 7: That argument would be more conclusive as regards the potential intellect than the agent intellect. For the Philosopher brings in this point concerning the potential intellect, that when it has understood the most intelligible thing, it will not less understand the least intelligible thing. But no matter what this refers to, it does not follow that the power of the intellect by means of which we understand is infinite in an absolute sense, but that it is infinite with reference to some genus. For nothing prevents a power, which is in itself finite, from not having a limit in some definite genus, but, nevertheless, it does have a limit inasmuch as it cannot extend itself to a higher genus. Thus, sight does not have a limit in the genus “color” because, if colors were multiplied to infinity, they could all be known by the sight. Yet the sight cannot know those things which belong to a higher genus, as, for instance, the universals. Similarly, our intellect does not have a limit in respect to the intelligible things which are connatural to itself, which are abstracted from things that can be sensed. Nevertheless, it has a limit, because, in regard to higher intelligible things, which are separated substances, it fails. For it is related to the most manifest of things as the eye of the owl to the light of the sun, as is said in Metaphysics 2. (Aristotle, Metaphysics 2.1, 993b9–10; cf. Aquinas, Commentary on the Metaphysics, bk. 2, lect. 1, n. 282.)

Reply Obj. 8: That argument is not to the point. For to pass judgment on a truth “by means of” something is used in two senses: in one sense, as “through the medium of”; thus we pass judgment on conclusions “by means of” principles, and on things that are regulated “by means of” a rule. And this seems to be the sense in which Augustine’s arguments are carried on. (Augustine, Book of Eighty-Three Different Questions, bk. 1, q. 9 (PL 40:13; CCSL 44A:16–17).) For that which is changeable or that which has a likeness to the false cannot be an infallible rule of truth. But in another sense, to pass judgment on some truth “by means of something” is used thus: “by means of our power of judging,” and in this sense we pass judgment on a truth by means of the agent intellect.

Yet in order to examine more searchingly the meaning of Augustine and what the truth is on this point, it must be noted that certain ancient philosophers, who did not assert any way of knowing except sensation, nor any entities besides sensible things, (Cf. Aristotle, On the Soul 3.3, 427a21–26; Physics 1.4, 187a12–188a18; Metaphysics 1.3, 983a24–985b22; 4.5, 1009a6–1011a2.) declared that no certainty concerning truth could be had by us, and this for two reasons. The first is because they asserted that sensible things are always in flux and that there is nothing stable in things. (Especially Cratylus and Heraclitus; see Aristotle, Metaphysics 1.6, 987a31–34; 4.5, 1010a7–15; 1009a6–15.) The second is because some people are to be found who make different judgments about the same thing; thus, for instance, someone who is awake judges in one way and one who is asleep in another, and one who is sick judges in one way and he who is well in another. Nor can anything be had to determine which of them has the truer estimate, since every one of them has some appearance of truth. (Cf. Aristotle, Metaphysics 4.5, 1009a38–b12.) And these are the two reasons which Augustine touches on, (Augustine, City of God, bk. 8, cc. 3–4 (PL 41:226–29; CSEL 40:356–60; CCSL 47:218–21).) because of which the ancients said that truth cannot be known by us. Hence, too, Socrates, despairing of grasping the truth of things, devoted himself entirely to moral philosophy. (Cf. Aristotle, Metaphysics 1.6, 987b1–7.) But Plato, his disciple, agreeing with the ancient philosophers that sensible things are always in flux and that the sense power has no certain judgment of things, in order to establish the certainty of scientific knowledge posited on the one hand species of things separated from sensible things and immovable, and he said that the sciences are about these; (Cf. Aristotle, Metaphysics 1.6, 987a29–b1; 987b7–14; 4.5, 1009a38–1010a15; Augustine, Book of Eighty-Three Different Questions, q. 9 (PL 40:13; CCSL 44A:16–17).) on the other hand, he posited in man a knowing power higher than sense, namely, the mind or intellect, (See Nemesius, On the Nature of Man, cc. 1, 3 (PG 40:506, 594B).) illumined by a kind of higher intelligible sun, as the sight is illumined by the visible sun. (Themistius, Commentary on the De anima, bk. 3, ch. 5.)

Augustine, however, following Plato as far as the Catholic faith allowed, did not posit species of things with a subsistence of their own, but instead of them he posited ideas of things in the divine mind and said that through these, by an intellect that is illumined by divine light, we form judgments about all things; (Augustine, Book of Eighty-Three Different Questions, q. 46, para. 2 (PL 40:30–31; CCSL 44A:71–73).) not, indeed, in such a way that we see the ideas themselves, for this would be impossible unless we were to see the essence of God, but according to what these supreme ideas imprint upon our minds. For Plato held that the sciences were concerned with the separate species in this sense, not that these latter could be seen themselves, but according as our mind participates in them it has knowledge of things. (Aristotle, Metaphysics 1.6, 987b9–14; 987b21; 1.9, 990b28; 990b30–991a3; 991a14; 991b5; 7.4, 1030a13; 7.6, 1031b18; 7.12, 1037b18; 7.15, 1040a27; 8.6, 1045b8; 1045b18; 12.12, 1075b19–20; 13.4, 1079a25; 13.5, 1079b18.) Hence, too, in a certain gloss on this passage, truths are lessened by the sons of men, (Peter Lombard, Commentary on the Davidic Psalms, on Psalm 11:1 (PL 191:155A); Augustine, Expositions on the Psalms, on Psalm 11:2 (PL 36:138; CCSL 38:82); Glossa interlinearis, at Psalm 11:1.) it is said that just as from one face many likenesses shine forth in mirrors, so from the one primary truth there result many truths in our minds. Aristotle, however, proceeded along another way. (See Aristotle, Metaphysics 4.5, 1009b12–1010a15; On the Soul 2.6, 418a8–25; 3.3, 428b16–30.)

First, he showed in many ways that there is something stable in sensible things.

Second, that the judgment of the sense is true concerning proper objects of sense, but that it is mistaken about common objects of sense, and more so about things that can be sensed by accident.

Third, that above the sense there is an intellectual power which makes judgments concerning truth, not through any intelligible things that exist outside, but through the light of the agent intellect, which makes things intelligible. (Aristotle, On the Soul 3.5, 430a10–17.)

Now, it does not matter much if we say that intelligible things themselves are participated from God, or that the light which makes them intelligible is participated from God.

Reply Obj. 9: It must be said that those rules which the impious see are the first principles of action, and that they are seen through the light of the agent intellect that is participated from God, just as are also the first principles of the speculative sciences.

Reply Obj. 10: It must be said that that whereby one judges which of two things is the better ought to be better than both, if one judges by this as by a rule or a measure. For in this sense white is the rule or measure of all other colors, and God of all beings; because each individual thing is better, the nearer it approaches the best thing. But that whereby we judge a given thing to be better than another, as by a knowing power, need not be better than both. Now, in this way we judge through the agent intellect that an angel is better than a soul.

Reply Obj. 11: The solution is clear from what has been said, for the agent intellect is related to the potential intellect as an agent and a mover is related to the material, inasmuch as it makes intelligible in act things to which the potential intellect is in potency. Now, it has been said how these two can be rooted in the one substance of the soul. (Above: ad 4.)

Reply Obj. 12: It must be said that there is one essence of numbers in all minds, just as there is also one essence of a stone; and this essence is one on the part of the thing that is understood, but not on the part of the act of understanding, which is not essential to the thing that is understood. For it is not essential to a stone that it be understood. Hence, this sort of unity of the essence of numbers or of stones or of anything whatever does not make for a unity of the possible or of the agent intellect, as was explained more fully above. (Above: resp.; a. 9, resp., ad 6.)

Reply Obj. 13: It must be said that that truth wherein the highest good is apprehended is common to all minds, either by reason of the oneness of the thing or by reason of the oneness of the primary light which flows into all minds.

Reply Obj. 14: It must be said that the universal, which the agent intellect causes, is one thing in all the beings from which it is abstracted; hence, the agent intellect is not diversified on the basis of their diversification. However, it is diversified on the basis of a diversity of intellects: because even the universal does not derive its oneness from the standpoint of its being understood by me and by you, for it is accidental to the universal that it is understood by me and by you. Hence, the diversity of intellects does not affect the oneness of the universal.

Reply Obj. 15: It must be said that it is incorrect to say that the agent intellect is naked or clothed, full of species or empty of them. For to be filled with species is characteristic of the potential intellect, but to cause them is characteristic of the agent intellect. Now, it must not be said that the agent intellect understands in isolation from the potential intellect, but that the man understands by means of both. It is he who has knowledge in particular, through the sense powers, of those things which are abstracted by means of the agent intellect.

Reply Obj. 16: It must be said that it is not because of God’s insufficiency that he attributes powers of action to created things, but because of his most perfect fullness, which is sufficient for sharing with all beings.

Reply Obj. 17: It must be said that a species which is in the imagination is of the same genus as a species which is in a sense, because both are individual and material. But a species which is in an intellect belongs to another genus, because it is universal. Consequently, an imagined species cannot imprint an intelligible species as a sensitive species imprints an imagined species, and for this reason an active intellectual power is necessary, whereas an active sense power is not.