Article 11 — Whether the rational, sensitive, and vegetative souls in man are substantially one and the same

Eleventh, it is asked whether the rational, sensitive, and vegetative souls in man are substantially one and the same. (Parallel texts: ST I.76.3; SCG II.58; On the Power of God, q. 3, a. 9, ad 9; Quodlibet II, q. 3, a. 1; Quodlibet XI, q. 5, a. 1; On Spiritual Creatures, a. 3; Compendium theologiae, bk. 1, cc. 90–92; see also Averroes, Commentary on Aristotle’s On the Soul, bk. 1, comm. 92.)

Obj. 1: It seems that they are not substantially one and the same. For wherever an act of a soul exists, there also does a soul exist. Now in the embryo the act of a vegetative soul precedes that of a sensitive soul, and the act of a sensitive soul precedes that of a rational soul. Therefore, in pregnancy, the vegetative soul is prior to the sensitive, and the sensitive is prior to the rational. (Cf. Aristotle, On the Generation of Animals 2.3, 736a35–b5; Aquinas, On the Power of God, q. 3, a. 9, obj. 14; Compendium theologiae, bk. 1, ch. 91.) Consequently, they are not substantially one and the same.

Obj. 2: But it has been said that the act of a vegetative soul and that of a sensitive soul which are present in the embryo do not belong to a soul existing in the embryo, but to a power of the parent’s soul existing therein.

On the contrary, a finite agent acts by its power only with respect to a limited distance, as is evident in the motion of throwing. For a thrower casts an object to a definite place according to the power which he possesses. (Cf. Aristotle, Physics 8.10, 266a24–30; 266b25–267a12.) But the movements and operations of a soul appear in the embryo no matter how distant the male parent may be, although his power is finite. Therefore, the operations of a soul do not exist in the embryo as a result of a power of the parent’s soul.

Obj. 3: Further, the Philosopher, in the book On the Generation of Animals, states that the embryo is an animal before it is a man. (Aristotle, On the Generation of Animals 2.3, 736a35–b5; cf. Aquinas, On the Power of God, q. 3, a. 9, obj. 12; ST I.76.3, obj. 3.) But an animal is such because of its sensitive soul, and a man is a man because of his rational soul. Therefore, a sensitive soul, and not merely a power of such a soul, exists in the embryo prior to a rational soul.

Obj. 4: Further, living and sensing are operations which can issue only from an intrinsic principle, because they are acts of a soul. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 5.) Hence, since the embryo lives and senses before it has a rational soul, the acts of living and sensing in the embryo will not come from the soul of the external parent, but from a soul existing within the embryo itself.

Obj. 5: Further, the Philosopher says in On the Soul 2 that the soul is not only the formal cause of the body, but its efficient and final cause as well. (Aristotle, On the Soul 2.4, 415b7–9.) But it would not be the efficient cause of the body unless it were present to the body when it is formed. However, the body is formed before the rational soul is infused. Therefore, there is a soul and not merely a power of a soul in the embryo preceding the infusion of the rational soul.

Obj. 6: But it has been said that the formation of the body is brought about not by a soul existing in the embryo, but by the soul of the parent. (Below: ad 1.)

On the contrary, living bodies move themselves by their own movements. (Cf. Aristotle, Physics 5.1, 225a35–36.) But the generation of a living body is a certain movement of its own because the generative power is one of its principles. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 6.) Therefore, with respect to this activity, a living thing moves itself. But a thing that moves itself is composed of a part that causes movement and one that is moved, as is proved in Physics 8. (Aristotle, Physics 8.4, 254b30; 8.5, 257b12.) Hence, the principle of generation, which forms the living body, is a soul existing in the embryo.

Obj. 7: Further, it is obvious that the embryo increases in size. Now increase is a species of local motion, as is stated in Physics 4. (Aristotle, Physics 4.6, 211a14–17; 213b19.) Therefore, since the animal moves itself locally, it will also move itself augmentatively. Hence, this movement must come from a principle existing in the embryo and not from an extrinsic soul.

Obj. 8: Further, the Philosopher says in On the Generation of Animals that it cannot be said that no soul exists in the embryo, but that first a nutritive soul exists there and then a sensitive soul. (Aristotle, On the Generation of Animals 2.3, 736a32–35.)

Obj. 9: But it has been argued that the Philosopher does not mean that a soul exists in the embryo actually, but only potentially.

On the contrary, a thing acts only so far as it is in act. But there are activities of a soul in the embryo. Therefore, a soul actually exists there, and thus it follows that the souls in the embryo are not substantially one and the same. (Cf. Aquinas, Commentary on Aristotle’s On Sense and What Is Sensed, tract. 1, ch. 9.)

Obj. 10: Further, it is impossible for one and the same thing to be caused both by an intrinsic principle and by an extrinsic one. But the rational soul in man is caused by an extrinsic principle. The vegetative and sensitive souls, however, are caused by an intrinsic principle contained in the semen, as is clear from what the Philosopher says in the book On the Generation of Animals. (Aristotle, On the Generation of Animals 2.3, 736b21–29.) Therefore, in man the vegetative, sensitive, and rational souls are not substantially one and the same.

Obj. 11: Further, it is impossible for a substantial principle of one thing to be an accident of another. (Cf. ST I.67.3.) Consequently, the Philosopher says in Metaphysics 8 that heat is not the substantial form of fire, because heat is an accident of other things. (According to Aristotle, heat is the proper quality of fire but not the form of its substance; Aristotle, Metaphysics 8.2, 1042b9–1043a5; 8.4, 1044a15–b10; Averroes, Commentary on Aristotle’s Metaphysics, bk. 8, comm. 5; cf. Aquinas, Commentary on Aristotle’s Metaphysics, bk. 11, lect. 12, nn. 2368–84.) But the sensitive soul is a substantial principle in brute animals. Therefore, it is not merely a power in man, because powers are certain properties and accidents of a soul.

Obj. 12: Further, man is a nobler animal than the brute. Now an animal is said to be such because it possesses a sensitive soul. Therefore, the sensitive soul in man is nobler than that in brute animals. (Cf. Aquinas, In II Sent., d. 18, q. 2, a. 3, ad 4; SCG II.89.) But it is a certain substantial entity in brute animals and not merely a power of their soul. Consequently, in man the sensitive soul is a substance in virtue of its very nature to an even greater degree.

Obj. 13: Further, it is impossible for the substance of one and the same being to be both corruptible and incorruptible. (Cf. ST I.76.3, obj. 1; Compendium theologiae, bk. 1, ch. 91.) Now the rational soul is incorruptible. sensitive and vegetative souls, however, are corruptible. Therefore, it is impossible for the rational, sensitive, and vegetative souls to be substantially one and the same.

Obj. 14: (Cf. ST I.76.3, obj. 2.) But it has been said that the sensitive soul in man is incorruptible.

On the contrary, the corruptible and the incorruptible differ in genus, as the Philosopher states in Metaphysics 10. (Aristotle, Metaphysics 10.10, 1058b26–29; 1059a10; cf. SCG II.89; ST I.76.3, obj. 2.) Now the sensitive soul in brutes is corruptible. Therefore, if the sensitive soul in man is incorruptible, the sensitive soul in man and that in a horse will not be the same in genus. Hence, since an animal is said to be such because of its sensitive soul, a man and a horse would not exist in one and the same genus, which is obviously false.

Obj. 15: Further, it is impossible for the substance of the same being to be both rational and irrational, because a contradiction is not true of one and the same being. (Cf. Aristotle, On Interpretation 1.6, 17a33–34.) But sensitive and vegetative souls are irrational. Therefore, they cannot be substantially the same as the rational soul.

Obj. 16: Further, the body is proportioned to the soul. But in a body there are diverse principles of operation of the soul which are called principal members. (Cf. Averroes, Commentary on Aristotle’s On the Soul, bk. 1, comm. 89.) Therefore, in man there is not one soul alone but many souls.

Obj. 17: Further, the powers of a soul flow naturally from its essence. However, only one thing proceeds naturally from one thing. (Avicenna, Metaphysics, bk. 9, ch. 4; cf. Averroes, Commentary on Aristotle’s Metaphysics, bk. 12, comm. 44; Aquinas, On the Power of God, q. 3, a. 16, ad 1, ad 11.) Therefore, if there were only one soul in man, there would not come from this soul certain powers that operate through organs, and others that do not.

Obj. 18: Further, genus is taken from matter, difference from form. (Cf. Avicenna, Metaphysics, bk. 5, ch. 5; Aquinas, In II Sent., d. 3, q. 1, a. 5, obj. 4; On Being and Essence, ch. 2; ST I.76.3, obj. 4; Commentary on Aristotle’s Metaphysics, bk. 5, lect. 22, nn. 1120–23; Compendium theologiae, bk. 1, ch. 91; Aristotle, Metaphysics 8.2, 1043a2–19; 8.6, 1045a7.) But the genus of man is “animal,” his difference, “rational.” Therefore, since an animal is said to be such because of its sensitive soul, it appears that not only the body but also the sensitive soul is related as matter to the rational soul. Therefore, the rational and sensitive souls are not substantially one and the same.

Obj. 19: Further, a man and a horse share animality in common. Moreover, an animal is said to be such because of its sensitive soul. (Cf. Aristotle, On the Soul 2.2, 413b2.) Therefore, their sensitive souls are the same. But the sensitive soul in a horse is not rational. Therefore, it is not rational in man.

Obj. 20: Further, if in man the rational, sensitive, and vegetative souls are substantially one and the same, then in every part of the body in which one of them exists, another must exist. But this is false. For the vegetative soul is in the bones because they are nourished and increased in size. However, the sensitive soul does not exist there, (Cf. Aristotle, On the Soul 1.5, 410a30–b1; 3.13, 435a24–25.) because the bones lack sense. Therefore, these souls are not substantially one and the same.

On the contrary, it is pointed out, in the work Book of Church Doctrine, that we do not say that there are two souls in man as Jacobus and other Syrians write: an animal soul by which the body is animated . . . and a rational soul which exercises reasoning, but in man we speak of one and the same soul which gives life to the body by being united to it, and which disposes itself by its reason. (Gennadius of Marseilles (d. ca. 496 AD), Book of Ecclesiastical Dogmas, ch. 15 (PL 58:984; 42:1216); cf. Aquinas, On the Power of God, q. 3, a. 9, ad 9; SCG II.58; ST I.76.3, s.c.)

I answer that there are different opinions about this question, not only among the moderns, but also among the ancients. For Plato maintained that there are different souls in the body, and, indeed, this followed from his principles because he maintained that the soul is united as a mover and not as a form to the body, saying that the soul exists in the body as a sailor in a ship. (Plato, Timaeus 41d–42a; 69e–70a; Phaedrus 246c; Republic, bk. 9, 580d–581c; cf. Aristotle, On the Soul 1.5, 411b5; Averroes, Commentary on Aristotle’s On the Soul, bk. 1, comm. 90; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 2; SCG II.58.) Moreover, he maintained that it is necessary to posit different movers wherever there are operations diverse in genus. For instance, in a ship there is one who steers, and another who rows. Hence, Plato argued in this way: the fact that these movers hold different positions does not destroy the unity existing in the ship, because just as the actions of these movers are subordinated to one another, so also are the movers themselves. In like manner it does not seem to be inconsistent with the unity of a man or that of an animal if in one body there exist many souls subordinated to one another as movers in accordance with the subordination of operations.

However, according to this position, neither a man nor an animal would have its unity absolutely and in virtue of its very nature, (Cf. Aristotle, Physics 8.4, 254b30–33; 256a2–3.) because a being that has unity absolutely and in its own right does not result from the conjunction of a mover and something movable. Nor, on Plato’s hypothesis, would generation and corruption strictly speaking exist when a body receives a soul or loses it. Consequently, it must be said that the soul is united to the body not only as a mover but as a form. This is also evident from the preceding arguments. (Above: q. 1, resp.; q. 8; q. 9, ad 2, 3, and 6.)

But even if this position is adopted, it still follows from Plato’s principles that there are several souls in man and in the animal. For the Platonists maintained that universals are separate forms which are predicated of sensible things inasmuch as sensible things participate in them. (Cf. Aristotle, Metaphysics 1.6, 987b1–14; 7.16, 1040b30–34; Aquinas, In II Sent., d. 18, q. 2, a. 3; ST I.76.3.) For example, Socrates is called an animal inasmuch as he participates in the idea “animal,” and a man inasmuch as he participates in the idea “man.” The consequence of this is that there is one form essentially in virtue of which Socrates is said to be an animal, and another in virtue of which he is said to be a man. Hence, it follows in turn that the sensitive and rational souls differ substantially.

But this cannot be maintained, for a being having unity in its own right cannot be constituted of diverse things having actual existence, because if terms are predicated of some subject by reason of diverse forms having existence of themselves, one is predicated of another accidentally. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 1; SCG II.58; ST I.76.3, resp.; Commentary on Aristotle’s Metaphysics, bk. 5, lect. 9, n. 885.) For instance, it is said of Socrates that he is white according to whiteness, and musical according to music. Therefore, musical is predicated of white accidentally. (Cf. Aristotle, Posterior Analytics 1.22, 83a1–11; Metaphysics 5.7, 1017a7–20; Aquinas, Commentary on Aristotle’s Posterior Analytics, bk. 1, lect. 33.) Hence, if Socrates is said to be a man and an animal according to different forms, it would follow that this predication, “man is an animal,” is an accidental one, and that man is not really what an animal is.

However, it happens that an essential predication is made through different forms when they are intrinsically related to one another, as when it is said that a thing having surface is colored. (Cf. Aristotle, Posterior Analytics 1.4, 73a34–b36; Metaphysics 5.18, 1022a24–36; 7.10, 1034b20–32; Aquinas, Commentary on Aristotle’s Posterior Analytics, bk. 1, lect. 10; Commentary on Aristotle’s On the Soul, bk. 2, lect. 14.) For color exists in a substance through the medium of its surface. But this mode of essential predication occurs, not because the thing predicated is placed in the definition of the subject, but rather the reverse. For surface is placed in the definition of color just as number is placed in the definition of the equal. Therefore, if animal were predicated of man through this mode of essential predication, when the sensitive soul is related as matter to the rational soul, assuming they are diverse, it would follow that animal is not predicated essentially of man but rather the reverse. (Cf. Aquinas, Commentary on Aristotle’s Posterior Analytics, bk. 1, lect. 10; SCG II.58.)

An additional difficulty follows. For a being having unity absolutely cannot be constituted of diverse things having actual existence unless there is something uniting them and binding them to one another in some manner. (Cf. Averroes, Commentary on Aristotle’s On the Soul, bk. 2, comm. 7.) Therefore, if Socrates were an animal and rational according to different forms, these two forms would need a unitary principle to make them substantially one. Therefore, since, on Plato’s hypothesis, no such unitary principle is to be found, it will follow that the unity of a man will be a unity of aggregation alone, like that of a heap of things which is relatively one and absolutely many. So neither will man be a being absolutely, because, so far as a thing is a being, to that extent it is one. (Cf. SCG II.58; ST I.76.3, resp.)

Still, another incongruity follows. For the form in accordance with which an individual substance receives a generic predication must be a substantial one because a genus is a substantial predicate. And thus, the sensitive soul, in virtue of which Socrates is said to be an animal, must be his substantial form, and so must give to the body its act of existing in the absolute sense, and make it to be this particular thing. Therefore, if the rational soul differs substantially from the sensitive, it does not make the body to be this particular thing, nor does it give to the body an act of existing in the absolute sense, but only relatively. For in that case a rational form will accrue to a thing already actually subsisting. Consequently, it will not be a substantial form but an accidental one, and thus it will not give Socrates his species, for a species is also a substantial predicate. (Cf. SCG II.58; Quodlibet XI, q. 5, a. 1.)

It follows, therefore, that a man’s soul, which is rational, sensitive, and vegetative, is substantially one only. This is a consequence of the argument given in a preceding article (Above: q. 9, resp.; cf. Boethius, Consolation of Philosophy, bk. 5, prose 4 (CCSL 94:98; PL 63:849).) concerning the order of substantial forms, namely, that no substantial form is united to matter through the medium of another, but that a more perfect form gives to matter whatever an inferior form does, and something over and above. Hence, the rational soul gives to the human body everything that the sensitive soul gives to the brute and the vegetative soul gives to the plant, and something over and above. (Cf. Aristotle, On the Soul 2.3, 414b28–32; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 1 and 5; ST I.76.3; On Spiritual Creatures, a. 3.) For this reason, the soul in man is both vegetative and sensitive and rational. The following example also attests to this, namely, that when the operation of one power is intense, that of another is impeded; (Cf. SCG II.58; ST I.76.3, resp.) and contrariwise, there is an overflowing of one power into another, which would occur only if all the powers were rooted in one and the same essence of the soul. (See above: q. 10, obj. 13.)

Reply Obj. 1: Having premised that the soul existing in the human body is substantially one only, this argument is answered in different ways by diverse men. For some claim that no soul exists in the embryo prior to the rational soul, but a certain power deriving from the soul of the parent. (Cf. Aristotle, On the Generation of Animals 2.1, 733b31–734a16; 2.3, 736a32–b27; 2.4, 740b24–741a2; On the Power of God, q. 3, a. 11, obj. 5.) They maintain that the operations perceived in the embryo are caused by this power, which is called a formative one. (Cf. Aquinas, In II Sent., d. 18, q. 1, a. 2, resp.; d. 18, q. 2, a. 3, resp.; SCG II.89; On the Power of God, q. 3, a. 9, ad 9; ST I.118.1.) But this is not altogether true, because there appears in the embryo not only the forming of the body, which can be attributed to the aforesaid power, but other operations as well, and these cannot be attributed to anything but a soul, such as growth, sensation, and the like. Nevertheless, this position could be held if the aforesaid active principle in the embryo were called a power of a soul temporarily, and not a soul, because it is not yet a perfect one, just as the embryo is not a perfect animal. But then the same difficulty will remain. (Cf. Aquinas, On the Power of God, q. 3, a. 9, ad 9.) For some say that the vegetative soul exists in the embryo before the sensitive soul, and the sensitive before the rational, but not concomitantly. (Albert the Great, Treatise on Animals, bk. 16, tract. 1, ch. 11; Summa de homine, tract. 1, q. 17, a. 3.) Indeed, they maintain that the semen gives rise to an actual vegetative soul as a result of an active principle existing in the semen. This soul in the course of time is brought to a higher degree of perfection by the process of generation and itself becomes a sensitive soul, which in turn is brought to a still higher degree of perfection by an extrinsic principle and becomes the rational soul. Now, according to this position, it follows that the substance itself of the rational soul comes from an active principle existing in the semen, but that an additional perfection accrues to it finally from an extrinsic principle. And thus, it would follow that the very substance of the rational soul is corruptible. For that cannot be incorruptible which is caused by a power existing in the semen. (Cf. SCG II.89; On the Power of God, q. 3, a. 9, ad 9; ST I.118.2, ad 2; Aristotle, On the Generation of Animals 1.13, 720a9–33.)

Therefore, our explanation must be different, namely, that the generation of an animal is not one simple generation alone, but that many generations and corruptions follow one another. (Cf. Aquinas, In II Sent., d. 18, q. 2, a. 3, ad 4; SCG II.89; On the Power of God, q. 3, a. 9, ad 9; ST I.118.2, ad 2; Compendium theologiae, bk. 1, ch. 92; On Spiritual Creatures, a. 3, ad 13.) For it is said that first the animal has the form of semen, then the form of blood, and so on successively until generation is completed. And, therefore, since corruption and generation do not take place without the loss of one form and the acquisition of another, the imperfect form which first exists within the embryo must be discarded and a more perfect one assumed. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 11.) This continues until the thing conceived has acquired its perfect form. Consequently, it is said that the vegetative soul first exists in the semen, but that it is lost in the process of generation, and that another soul succeeds it which is not only vegetative, but also sensitive. Then another soul is added to this which is at once vegetative, sensitive, and rational.

Reply Obj. 2: The power existing in the semen which is derived from the father is a permanent intrinsic power, not one coming from an extrinsic principle, just as the power of the mover which exists in the thing thrown is intrinsic. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 9.) Hence, the power which is in the semen operates no matter how far away the father may be. For the active power which is in the semen cannot be caused by the mother, although some indeed maintain this, (Cf. Aquinas, In II Sent., d. 18, q. 2, a. 3; On the Power of God, q. 3, a. 9, ad 9; SCG II.89; ST I.118.2, ad 2; On Spiritual Creatures, a. 3, ad 13.) because the woman is not an active principle but a passive one. Nevertheless, there is some similarity here because, just as the thrower’s power, which is finite, moves an object to a definite place some distance away by local motion, so does the power of one generating move a thing to a determinate form by the movement of generation.

Reply Obj. 3: That power has the nature of a soul as has been explained. Therefore, the embryo can be called an animal because of it.

Reply Obj. 4–8: The fourth, fifth, sixth, seventh and eighth objections are answered in the same way.

Reply Obj. 9: Inasmuch as the soul exists actually, but imperfectly, in the embryo, to that extent does it perform imperfect operations.

Reply Obj. 10: Although the sensitive soul in the brute comes from an intrinsic principle, nevertheless in man the substance of the soul, which is at once vegetative, sensitive, and rational, comes from an extrinsic principle, as has just been shown.

Reply Obj. 11: The sensitive soul does not exist in man accidentally, but substantially, because it is substantially the same as the rational soul. However, a sensitive power is an accident in man just as it is in other animals.

Reply Obj. 12: The sensitive soul in man is nobler than that in other animals because in man it is not only sensitive but also rational. (Cf. Aquinas, On the Power of God, q. 3, a. 11, ad 1; Quodlibet XI, q. 5, a. 1, ad 4; ST I.76.5; Commentary on Aristotle’s On Sense and What Is Sensed, tract. 1, ch. 8.)

Reply Obj. 13: As regards its very substance, the sensitive soul in man is incorruptible because its substance is the substance of the rational soul, although perhaps the sensitive powers, which are acts of the body, do not remain in existence after the body has corrupted, as some maintain. (Cf. ST I.76.3, ad 1; Compendium theologiae, bk. 1, ch. 92.)

Reply Obj. 14: If the sensitive soul existing in brutes and that existing in man could of themselves be placed in a genus and a species, they would belong to one genus only logically speaking, according to some common intention. (Cf. ST I.76.3, ad 4.) But the thing that exists expressly in a genus and a species is the composite of body and soul, and this composite is corrupted both in the case of man and in that of the animal.

Reply Obj. 15: The sensitive soul in man is not a non-rational soul but is at once a sensitive and rational soul. (Cf. Aristotle, On the Soul 2.3, 414b28–32; SCG II.89; On the Power of God, q. 3, a. 11, ad 2; ST I.76.3, resp.) However, it is true that the soul’s sensitive powers as such are, indeed, non-rational, but participate in reason inasmuch as they obey reason. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 4.) Moreover, the powers of the vegetative soul are wholly non-rational because they do not obey reason, as is clear from what the Philosopher says in Ethics 1. (Aristotle, Nicomachean Ethics 1.13, 1102b28–1103a3.)

Reply Obj. 16: Although in the body there are several principal members in which the principles of certain operations of a soul are manifested, nevertheless all depend upon the heart as the first bodily principle.

Reply Obj. 17: Powers operating through organs spring from the human soul inasmuch as it is united to the body. But powers not operating through organs spring from the soul inasmuch as it exceeds by its power the capacity of the body. (Cf. Aquinas, On Spiritual Creatures, a. 11, ad 12; Compendium theologiae, bk. 1, ch. 92.)

Reply Obj. 18: As is clear from the preceding questions, matter receives different grades of perfection from one and the same form. (See above: q. 9, resp.; Aquinas, Compendium theologiae, bk. 1, ch. 92.) And so far as matter is perfected by an inferior grade of perfection, the material disposition for a higher grade of perfection still remains. Thus, so far as the body is perfected in sensible being by the human soul, the body retains to that extent the nature of matter with respect to a higher perfection. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 1.) And according to this, animal, which is a genus, is derived from matter, and rational, which is a difference, is derived from form.

Reply Obj. 19: As an animal as such is neither rational nor non-rational, (Cf. Aristotle, Metaphysics 7.12, 1037b18–20.) but rational animal is man, and non-rational animal is brute animal, so is the sensitive soul as such neither rational nor non-rational, but is rational in man and non-rational in the brute.

Reply Obj. 20: Although the sensitive and vegetative soul is one soul, still it is not necessary that the operation of one appear wherever the operation of the other appears, because of the diverse dispositions of parts. For this reason, it also happens that all of the sensitive soul’s operations are not exercised through one part, but sight is exercised through the eye, hearing through the ear, and so on for the rest.