Summa Th. I EN Qu.78 a.4

Article: 4 Whether the interior senses are suitably distinguished?

Objection: 1. It would seem that the interior senses are not suitably distinguished. For the common is not divided against the proper. Therefore the common sense should not be numbered among the interior sensitive powers, in addition to the proper exterior senses.

2. Further, there is no need to assign an interior power of apprehension when the proper and exterior sense suffices. But the proper and exterior senses suffice for us to judge of sensible things; for each sense judges of its proper object. In like manner they seem to suffice for the perception of their own actions; for since the action of the sense is, in a way, between the power and its object, it seems that sight must be much more able to perceive its own vision, as being nearer to it, than the color; and in like manner with the other senses. Therefore for this there is no need to assign an interior power, called the common sense.

3. Further, according to the Philosopher (De Memor. et Remin. i), the imagination and the memory are passions of the "first sensitive." But passion is not divided against its subject. Therefore memory and imagination should not be assigned as powers distinct from the senses.

4. Further, the intellect depends on the senses less than any power of the sensitive part. But the intellect knows nothing but what it receives from the senses; whence we read (Poster. i, 8), that "those who lack one sense lack one kind of knowledge." Therefore much less should we assign to the sensitive part a power, which they call the "estimative" power, for the perception of intentions which the sense does not perceive.

5. Further, the action of the cogitative power, which consists in comparing, adding and dividing, and the action of the reminiscence, which consists in the use of a kind of syllogism for the sake of inquiry, is not less distant from the actions of the estimative and memorative powers, than the action of the estimative is from the action of the imagination. Therefore either we must add the cognitive and reminiscitive to the estimative and memorative powers, or the estimative and memorative powers should not be made distinct from the imagination.

6. Further, Augustine (Gn ad lit. xii, 6,7,24) describes three kinds of vision; namely, corporeal, which is the action of the sense; spiritual, which is an action of the imagination or phantasy; and intellectual, which is an action of the intellect. Therefore there is no interior power between the sense and intellect, besides the imagination.

On the contrary Avicenna (De Anima iv, 1) assigns five interior sensitive powers; namely, "common sense, phantasy, imagination, and the estimative and memorative powers."

I answer that As nature does not fail in necessary things, there must needs be as many actions of the sensitive soul as may suffice for the life of a perfect animal. If any of these actions cannot be reduced to the same one principle, they must be assigned to diverse powers; since a power of the soul is nothing else than the proximate principle of the soul's operation.

Now we must observe that for the life of a perfect animal, the animal should apprehend a thing not only at the actual time of sensation, but also when it is absent. Otherwise, since animal motion and action follow apprehension, an animal would not be moved to seek something absent: the contrary of which we may observe specially in perfect animals, which are moved by progression, for they are moved towards something apprehended and absent. Therefore an animal through the sensitive soul must not only receive the species of sensible things, when it is actually affected by them, but it must also retain and preserve them. Now to receive and retain are, in corporeal things, reduced to diverse principles; for moist things are apt to receive, but retain with difficulty, while it is the reverse with dry things. Wherefore, since the sensitive power is the act of a corporeal organ, it follows that the power which receives the species of sensible things must be distinct from the power which preserves them.

Again we must observe that if an animal were moved by pleasing and disagreeable things only as affecting the sense, there would be no need to suppose that an animal has a power besides the apprehension of those forms which the senses perceive, and in which the animal takes pleasure, or from which it shrinks with horror. But the animal needs to seek or to avoid certain things, not only because they are pleasing or otherwise to the senses, but also on account of other advantages and uses, or disadvantages: just as the sheep runs away when it sees a wolf, not on account of its color or shape, but as a natural enemy: and again a bird gathers together straws, not because they are pleasant to the sense, but because they are useful for building its nest. Animals, therefore, need to perceive such intentions, which the exterior sense does not perceive. And some distinct principle is necessary for this; since the perception of sensible forms comes by an immutation caused by the sensible, which is not the case with the perception of those intentions.

Thus, therefore, for the reception of sensible forms, the "proper sense" and the "common sense" are appointed, and of their distinction we shall speak farther on (ad 1,2). But for the retention and preservation of these forms, the "phantasy" or "imagination" is appointed; which are the same, for phantasy or imagination is as it were a storehouse of forms received through the senses. Furthermore, for the apprehension of intentions which are not received through the senses, the "estimative" power is appointed: and for the preservation thereof, the "memorative" power, which is a storehouse of such-like intentions. A sign of which we have in the fact that the principle of memory in animals is found in some such intention, for instance, that something is harmful or otherwise. And the very formality of the past, which memory observes, is to be reckoned among these intentions.

Now, we must observe that as to sensible forms there is no difference between man and other animals; for they are similarly immuted by the extrinsic sensible. But there is a difference as to the above intentions: for other animals perceive these intentions only by some natural instinct, while man perceives them by means of coalition of ideas. Therefore the power by which in other animals is called the natural estimative, in man is called the "cogitative," which by some sort of collation discovers these intentions. Wherefore it is also called the "particular reason," to which medical men assign a certain particular organ, namely, the middle part of the head: for it compares individual intentions, just as the intellectual reason compares universal intentions. As to the memorative power, man has not only memory, as other animals have in the sudden recollection of the past; but also "reminiscence" by syllogistically, as it were, seeking for a recollection of the past by the application of individual intentions. Avicenna, however, assigns between the estimative and the imaginative, a fifth power, which combines and divides imaginary forms: as when from the imaginary form of gold, and imaginary form of a mountain, we compose the one form of a golden mountain, which we have never seen. But this operation is not to be found in animals other than man, in whom the imaginative power suffices thereto. To man also does Averroes attribute this action in his book De sensu et sensibilibus (viii). So there is no need to assign more than four interior powers of the sensitive part---namely, the common sense, the imagination, and the estimative and memorative powers.

Reply to Objection: 1. The interior sense is called "common" not by predication, as if it were a genus; but as the common root and principle of the exterior senses.

2. The proper sense judges of the proper sensible by discerning it from other things which come under the same sense; for instance, by discerning white from black or green. But neither sight nor taste can discern white from sweet: because what discerns between two things must know both. Wherefore the discerning judgment must be assigned to the common sense; to which, as to a common term, all apprehensions of the senses must be referred: and by which, again, all the intentions of the senses are perceived; as when someone sees that he sees. For this cannot be done by the proper sense, which only knows the form of the sensible by which it is immuted, in which immutation the action of sight is completed, and from immutation follows another in the common sense which perceives the act of vision.

3. As one power arises from the soul by means of another, as we have seen above (Question [77], Article [7]), so also the soul is the subject of one power through another. In this way the imagination and the memory are called passions of the "first sensitive."

4. Although the operation of the intellect has its origin in the senses: yet, in the thing apprehended through the senses, the intellect knows many things which the senses cannot perceive. In like manner does the estimative power, though in a less perfect manner.

5. The cogitative and memorative powers in man owe their excellence not to that which is proper to the sensitive part; but to a certain affinity and proximity to the universal reason, which, so to speak, overflows into them. Therefore they are not distinct powers, but the same, yet more perfect than in other animals.

6. Augustine calls that vision spiritual which is effected by the images of bodies in the absence of bodies. Whence it is clear that it is common to all interior apprehensions.


The next question concerns the intellectual powers, under which head there are thirteen points of inquiry:

(1) Whether the intellect is a power of the soul, or its essence?

(2) If it be a power, whether it is a passive power?

(3) If it is a passive power, whether there is an active intellect?

(4) Whether it is something in the soul?

(5) Whether the active intellect is one in all?

(6) Whether memory is in the intellect?

(7) Whether the memory be distinct from the intellect?

(8) Whether the reason is a distinct power from the intellect?

(9) Whether the superior and inferior reason are distinct powers?

(10) Whether the intelligence is distinct from the intellect?

(11) Whether the speculative and practical intellect are distinct powers?

(12) Whether "synderesis" is a power of the intellectual part?

(13) Whether the conscience is a power of the intellectual part?

Article: 1 Whether the intellect is a power of the soul?

Objection: 1. It would seem that the intellect is not a power of the soul, but the essence of the soul. For the intellect seems to be the same as the mind. Now the mind is not a power of the soul, but the essence; for Augustine says (De Trin. ix, 2): "Mind and spirit are not relative things, but denominate the essence." Therefore the intellect is the essence of the soul.

2. Further, different genera of the soul's powers are not united in some one power, but only in the essence of the soul. Now the appetitive and the intellectual are different genera of the soul's powers as the Philosopher says (De Anima ii, 3), but they are united in the mind, for Augustine (De Trin. x, 11) places the intelligence and will in the mind. Therefore the mind and intellect of man is of the very essence of the soul and not a power thereof.

3. Further, according to Gregory, in a homily for the Ascension (xxix in ), "man understands with the angels." But angels are called "minds" and "intellects." Therefore the mind and intellect of man are not a power of the soul, but the soul itself.

4. Further, a substance is intellectual by the fact that it is immaterial. But the soul is immaterial through its essence. Therefore it seems that the soul must be intellectual through its essence.

On the contrary The Philosopher assigns the intellectual faculty as a power of the soul (De Anima ii, 3).

I answer that In accordance with what has been already shown (Question [54], Article [3]; Question [77], Article [1]) it is necessary to say that the intellect is a power of the soul, and not the very essence of the soul. For then alone the essence of that which operates is the immediate principle of operation, when operation itself is its being: for as power is to operation as its act, so is the essence to being. But in God alone His action of understanding is His very Being. Wherefore in God alone is His intellect His essence: while in other intellectual creatures, the intellect is power.

Reply to Objection: 1. Sense is sometimes taken for the power, and sometimes for the sensitive soul; for the sensitive soul takes its name from its chief power, which is sense. And in like manner the intellectual soul is sometimes called intellect, as from its chief power; and thus we read (De Anima i, 4), that the "intellect is a substance." And in this sense also Augustine says that the mind is spirit and essence (De Trin. ix, 2; xiv, 16).

2. The appetitive and intellectual powers are different genera of powers in the soul, by reason of the different formalities of their objects. But the appetitive power agrees partly with the intellectual power and partly with the sensitive in its mode of operation either through a corporeal organ or without it: for appetite follows apprehension. And in this way Augustine puts the will in the mind; and the Philosopher, in the reason (De Anima iii, 9).

3. In the angels there is no other power besides the intellect, and the will, which follows the intellect. And for this reason an angel is called a "mind" or an "intellect"; because his whole power consists in this. But the soul has many other powers, such as the sensitive and nutritive powers, and therefore the comparison fails.

4. The immateriality of the created intelligent substance is not its intellect; and through its immateriality it has the power of intelligence. Wherefore it follows not that the intellect is the substance of the soul, but that it is its virtue and power.

Article: 2 Whether the intellect is a passive power?

Objection: 1. It would seem that the intellect is not a passive power. For everything is passive by its matter, and acts by its form. But the intellectual power results from the immateriality of the intelligent substance. Therefore it seems that the intellect is not a passive power.

2. Further, the intellectual power is incorruptible, as we have said above (Question [79], Article [6]). But "if the intellect is passive, it is corruptible" (De Anima iii, 5). Therefore the intellectual power is not passive.

3. Further, the "agent is nobler than the patient," as Augustine (Gn ad lit. xii, 16) and Aristotle (De Anima iii, 5) says. But all the powers of the vegetative part are active; yet they are the lowest among the powers of the soul. Much more, therefore, all the intellectual powers, which are the highest, are active.

On the contrary The Philosopher says (De Anima iii, 4) that "to understand is in a way to be passive."

I answer that To be passive may be taken in three ways. Firstly, in its most strict sense, when from a thing is taken something which belongs to it by virtue either of its nature, or of its proper inclination: as when water loses coolness by heating, and as when a man becomes ill or sad. Secondly, less strictly, a thing is said to be passive, when something, whether suitable or unsuitable, is taken away from it. And in this way not only he who is ill is said to be passive, but also he who is healed; not only he that is sad, but also he that is joyful; or whatever way he be altered or moved. Thirdly, in a wide sense a thing is said to be passive, from the very fact that what is in potentiality to something receives that to which it was in potentiality, without being deprived of anything. And accordingly, whatever passes from potentiality to act, may be said to be passive, even when it is perfected. And thus with us to understand is to be passive. This is clear from the following reason. For the intellect, as we have seen above (Question [78], Article [1]), has an operation extending to universal being. We may therefore see whether the intellect be in act or potentiality by observing first of all the nature of the relation of the intellect to universal being. For we find an intellect whose relation to universal being is that of the act of all being: and such is the Divine intellect, which is the Essence of God, in which originally and virtually, all being pre-exists as in its first cause. And therefore the Divine intellect is not in potentiality, but is pure act. But no created intellect can be an act in relation to the whole universal being; otherwise it would needs be an infinite being. Wherefore every created intellect is not the act of all things intelligible, by reason of its very existence; but is compared to these intelligible things as a potentiality to act.

Now, potentiality has a double relation to act. There is a potentiality which is always perfected by its act: as the matter of the heavenly bodies (Question [58], Article [1]). And there is another potentiality which is not always in act, but proceeds from potentiality to act; as we observe in things that are corrupted and generated. Wherefore the angelic intellect is always in act as regards those things which it can understand, by reason of its proximity to the first intellect, which is pure act, as we have said above. But the human intellect, which is the lowest in the order of intelligence and most remote from the perfection of the Divine intellect, is in potentiality with regard to things intelligible, and is at first "like a clean tablet on which nothing is written," as the Philosopher says (De Anima iii, 4). This is made clear from the fact, that at first we are only in potentiality to understand, and afterwards we are made to understand actually. And so it is evident that with us to understand is "in a way to be passive"; taking passion in the third sense. And consequently the intellect is a passive power.

Reply to Objection: 1. This objection is verified of passion in the first and second senses, which belong to primary matter. But in the third sense passion is in anything which is reduced from potentiality to act.

2. "Passive intellect" is the name given by some to the sensitive appetite, in which are the passions of the soul; which appetite is also called "rational by participation," because it "obeys the reason" (Ethic. i, 13). Others give the name of passive intellect to the cogitative power, which is called the "particular reason." And in each case "passive" may be taken in the two first senses; forasmuch as this so-called intellect is the act of a corporeal organ. But the intellect which is in potentiality to things intelligible, and which for this reason Aristotle calls the "possible" intellect (De Anima iii, 4) is not passive except in the third sense: for it is not an act of a corporeal organ. Hence it is incorruptible.

3. The agent is nobler than the patient, if the action and the passion are referred to the same thing: but not always, if they refer to different things. Now the intellect is a passive power in regard to the whole universal being: while the vegetative power is active in regard to some particular thing, namely, the body as united to the soul. Wherefore nothing prevents such a passive force being nobler than such an active one.

Article: 3 Whether there is an active intellect?

Objection: 1. It would seem that there is no active intellect. For as the senses are to things sensible, so is our intellect to things intelligible. But because sense is in potentiality to things sensible, the sense is not said to be active, but only passive. Therefore, since our intellect is in potentiality to things intelligible, it seems that we cannot say that the intellect is active, but only that it is passive.

2. Further, if we say that also in the senses there is something active, such as light: on the contrary, light is required for sight, inasmuch as it makes the medium to be actually luminous; for color of its own nature moves the luminous medium. But in the operation of the intellect there is no appointed medium that has to be brought into act. Therefore there is no necessity for an active intellect.

3. Further, the likeness of the agent is received into the patient according to the nature of the patient. But the passive intellect is an immaterial power. Therefore its immaterial nature suffices for forms to be received into it immaterially. Now a form is intelligible in act from the very fact that it is immaterial. Therefore there is no need for an active intellect to make the species actually intelligible.

On the contrary The Philosopher says (De Anima iii, 5), "As in every nature, so in the soul is there something by which it becomes all things, and something by which it makes all things." Therefore we must admit an active intellect.

I answer that, According to the opinion of Plato, there is no need for an active intellect in order to make things actually intelligible; but perhaps in order to provide intellectual light to the intellect, as will be explained farther on (Article [4]). For Plato supposed that the forms of natural things subsisted apart from matter, and consequently that they are intelligible: since a thing is actually intelligible from the very fact that it is immaterial. And he called such forms "species or ideas"; from a participation of which, he said that even corporeal matter was formed, in order that individuals might be naturally established in their proper genera and species: and that our intellect was formed by such participation in order to have knowledge of the genera and species of things. But since Aristotle did not allow that forms of natural things exist apart from matter, and as forms existing in matter are not actually intelligible; it follows that the natures of forms of the sensible things which we understand are not actually intelligible. Now nothing is reduced from potentiality to act except by something in act; as the senses as made actual by what is actually sensible. We must therefore assign on the part of the intellect some power to make things actually intelligible, by abstraction of the species from material conditions. And such is the necessity for an active intellect.

Reply to Objection: 1. Sensible things are found in act outside the soul; and hence there is no need for an active sense. Wherefore it is clear that in the nutritive part all the powers are active, whereas in the sensitive part all are passive: but in the intellectual part, there is something active and something passive.

2. There are two opinions as to the effect of light. For some say that light is required for sight, in order to make colors actually visible. And according to this the active intellect is required for understanding, in like manner and for the same reason as light is required for seeing. But in the opinion of others, light is required for sight; not for the colors to become actually visible; but in order that the medium may become actually luminous, as the Commentator says on De Anima ii. And according to this, Aristotle's comparison of the active intellect to light is verified in this, that as it is required for understanding, so is light required for seeing; but not for the same reason.

3. If the agent pre-exist, it may well happen that its likeness is received variously into various things, on account of their dispositions. But if the agent does not pre-exist, the disposition of the recipient has nothing to do with the matter. Now the intelligible in act is not something existing in nature; if we consider the nature of things sensible, which do not subsist apart from matter. And therefore in order to understand them, the immaterial nature of the passive intellect would not suffice but for the presence of the active intellect which makes things actually intelligible by way of abstraction.

Article: 4 Whether the active intellect is something in the soul?

Objection: 1. It would seem that the active intellect is not something in the soul. For the effect of the active intellect is to give light for the purpose of understanding. But this is done by something higher than the soul: according to Jn 1,9, "He was the true light that enlighteneth every man coming into this world." Therefore the active intellect is not something in the soul.

2. Further, the Philosopher (De Anima iii, 5) says of the active intellect, "that it does not sometimes understand and sometimes not understand." But our soul does not always understand: sometimes it understands, sometimes it does not understand. Therefore the active intellect is not something in our soul.

3. Further, agent and patient suffice for action. If, therefore, the passive intellect, which is a passive power, is something belonging to the soul; and also the active intellect, which is an active power: it follows that a man would always be able to understand when he wished, which is clearly false. Therefore the active intellect is not something in our soul.

4. Further, the Philosopher (De Anima iii, 5) says that the active intellect is a "substance in actual being." But nothing can be in potentiality and in act with regard to the same thing. If, therefore, the passive intellect, which is in potentiality to all things intelligible, is something in the soul, it seems impossible for the active intellect to be also something in our soul.

5. Further, if the active intellect is something in the soul, it must be a power. For it is neither a passion nor a habit; since habits and passions are not in the nature of agents in regard to the passivity of the soul; but rather passion is the very action of the passive power; while habit is something which results from acts. But every power flows from the essence of the soul. It would therefore follow that the active intellect flows from the essence of the soul. And thus it would not be in the soul by way of participation from some higher intellect: which is unfitting. Therefore the active intellect is not something in our soul.

On the contrary The Philosopher says (De Anima iii, 5), that "it is necessary for these differences," namely, the passive and active intellect, "to be in the soul."

I answer that The active intellect, of which the Philosopher speaks, is something in the soul. In order to make this evident, we must observe that above the intellectual soul of man we must needs suppose a superior intellect, from which the soul acquires the power of understanding. For what is such by participation, and what is mobile, and what is imperfect always requires the pre-existence of something essentially such, immovable and perfect. Now the human soul is called intellectual by reason of a participation in intellectual power; a sign of which is that it is not wholly intellectual but only in part. Moreover it reaches to the understanding of truth by arguing, with a certain amount of reasoning and movement. Again it has an imperfect understanding; both because it does not understand everything, and because, in those things which it does understand, it passes from potentiality to act. Therefore there must needs be some higher intellect, by which the soul is helped to understand.

Wherefore some held that this intellect, substantially separate, is the active intellect, which by lighting up the phantasms as it were, makes them to be actually intelligible. But, even supposing the existence of such a separate active intellect, it would still be necessary to assign to the human soul some power participating in that superior intellect, by which power the human soul makes things actually intelligible. Just as in other perfect natural things, besides the universal active causes, each one is endowed with its proper powers derived from those universal causes: for the sun alone does not generate man; but in man is the power of begetting man: and in like manner with other perfect animals. Now among these lower things nothing is more perfect than the human soul. Wherefore we must say that in the soul is some power derived from a higher intellect, whereby it is able to light up the phantasms. And we know this by experience, since we perceive that we abstract universal forms from their particular conditions, which is to make them actually intelligible. Now no action belongs to anything except through some principle formally inherent therein; as we have said above of the passive intellect (Question [76], Article [1]). Therefore the power which is the principle of this action must be something in the soul. For this reason Aristotle (De Anima iii, 5) compared the active intellect to light, which is something received into the air: while Plato compared the separate intellect impressing the soul to the sun, as Themistius says in his commentary on De Anima iii. But the separate intellect, according to the teaching of our faith, is God Himself, Who is the soul's Creator, and only beatitude; as will be shown later on (Question [90], Article [3]; FS, Question [3], Article [7]). Wherefore the human soul derives its intellectual light from Him, according to Ps 4,7, "The light of Thy countenance, O Lord, is signed upon us."

Reply to Objection: 1. That true light enlightens as a universal cause, from which the human soul derives a particular power, as we have explained.

2. The Philosopher says those words not of the active intellect, but of the intellect in act: of which he had already said: "Knowledge in act is the same as the thing." Or, if we refer those words to the active intellect, then they are said because it is not owing to the active intellect that sometimes we do, and sometimes we do not understand, but to the intellect which is in potentiality.

3. If the relation of the active intellect to the passive were that of the active object to a power, as, for instance, of the visible in act to the sight; it would follow that we could understand all things instantly, since the active intellect is that which makes all things (in act). But now the active intellect is not an object, rather is it that whereby the objects are made to be in act: for which, besides the presence of the active intellect, we require the presence of phantasms, the good disposition of the sensitive powers, and practice in this sort of operation; since through one thing understood, other things come to be understood, as from terms are made propositions, and from first principles, conclusions. From this point of view it matters not whether the active intellect is something belonging to the soul, or something separate from the soul.

4. The intellectual soul is indeed actually immaterial, but it is in potentiality to determinate species. On the contrary, phantasms are actual images of certain species, but are immaterial in potentiality. Wherefore nothing prevents one and the same soul, inasmuch as it is actually immaterial, having one power by which it makes things actually immaterial, by abstraction from the conditions of individual matter: which power is called the "active intellect"; and another power, receptive of such species, which is called the "passive intellect" by reason of its being in potentiality to such species.

5. Since the essence of the soul is immaterial, created by the supreme intellect, nothing prevents that power which it derives from the supreme intellect, and whereby it abstracts from matter, flowing from the essence of the soul, in the same way as its other powers.

Article: 5 Whether the active intellect is one in all?

Objection: 1. It would seem that there is one active intellect in all. For what is separate from the body is not multiplied according to the number of bodies. But the active intellect is "separate," as the Philosopher says (De Anima iii, 5). Therefore it is not multiplied in the many human bodies, but is one for all men.

2. Further, the active intellect is the cause of the universal, which is one in many. But that which is the cause of unity is still more itself one. Therefore the active intellect is the same in all.

3. Further, all men agree in the first intellectual concepts. But to these they assent by the active intellect. Therefore all agree in one active intellect.

On the contrary The Philosopher says (De Anima iii, 5) that the active intellect is as a light. But light is not the same in the various things enlightened. Therefore the same active intellect is not in various men.

I answer that The truth about this question depends on what we have already said (Article [4]). For if the active intellect were not something belonging to the soul, but were some separate substance, there would be one active intellect for all men. And this is what they mean who hold that there is one active intellect for all. But if the active intellect is something belonging to the soul, as one of its powers, we are bound to say that there are as many active intellects as there are souls, which are multiplied according to the number of men, as we have said above (Question [76], Article [2]). For it is impossible that one same power belong to various substances.

Reply to Objection: 1. The Philosopher proves that the active intellect is separate, by the fact that the passive intellect is separate: because, as he says (De Anima iii, 5), "the agent is more noble than the patient." Now the passive intellect is said to be separate, because it is not the act of any corporeal organ. And in the same sense the active intellect is also called "separate"; but not as a separate substance.

2. The active intellect is the cause of the universal, by abstracting it from matter. But for this purpose it need not be the same intellect in all intelligent beings; but it must be one in its relationship to all those things from which it abstracts the universal, with respect to which things the universal is one. And this befits the active intellect inasmuch as it is immaterial.

3. All things which are of one species enjoy in common the action which accompanies the nature of the species, and consequently the power which is the principle of such action; but not so as that power be identical in all. Now to know the first intelligible principles is the action belonging to the human species. Wherefore all men enjoy in common the power which is the principle of this action: and this power is the active intellect. But there is no need for it to be identical in all. Yet it must be derived by all from one principle. And thus the possession by all men in common of the first principles proves the unity of the separate intellect, which Plato compares to the sun; but not the unity of the active intellect, which Aristotle compares to light.

Summa Th. I EN Qu.78 a.4