Article 1 — Whether the human soul can be a form and a particular thing

And first it is asked whether the human soul can be a form and a particular thing. (Parallel texts: Summa theologiae (ST) I, q. 75, a. 2; q. 76, a. 1; Commentary on the Sentences, bk. 2 (In II Sent.), d. 1, q. 2, a. 4 ; d. 17, q. 2, a. 1; Summa contra Gentiles (SCG), bk. 2, cc. 56–57, 59, 68–70; Disputed Questions on the Power of God, q. 3, aa. 9, 11; Disputed Question on Spiritual Creatures, a. 2; Commentary on Aristotle’s On the Soul, bk. 2, lect. 4; bk. 3, lect. 7; On the Unity of the Intellect against the Averroists (De unitate intellectus), ch. 3; Compendium theologiae, bk. 1, cc. 80, 87.)

Obj. 1: It seems that it cannot. For if the human soul is a particular thing, it is a subsisting thing having a complete act of existing in virtue of its own nature. (Bonaventure, Commentary on the Sentences, bk. 2, d. 17, a. 1, q. 2, obj. 6.) Now whatever accrues to a thing over and above its complete existence (That is, its substantial existence.) is an accident of that thing, just as whiteness and clothing are accidents of man. (Cf. Plato, Phaedo 87b; Nemesius of Emesa (fl. ca. 390 AD), On the Nature of Man, ch. 3.) Therefore, when the body is united to the soul, it is united to it accidentally. Consequently, if the soul is a particular thing, it is not the substantial form of the body. (Cf. Aquinas, On Spiritual Creatures, a. 2, obj. 3.)

Obj. 2: Further, if the soul is a particular thing, it must be an individuated thing, for a universal is not a particular thing. (Cf. Aristotle, Metaphysics 9.7, 1049a27–28; 7.13, 1038b34–1039b2; Aquinas, Commentary on Aristotle’s Metaphysics, bk. 9, lect. 6, n. 1841; bk. 7, lect. 13, n. 1535.) Now the soul is individuated either by something other than itself, or by itself. If the soul is individuated by something other than itself, and is the form of the body, it must be individuated by the body, for forms are individuated by their proper matter. (Cf. Aristotle, Metaphysics 5.6, 1016b32; 7.8, 1034a5–8; 7.10, 1035b30; 10.3, 1054a33; 10.9, 1058b5; 12.8, 1074a33; Avicenna (Ibn Sīnā, ca. 980‒1037 AD), Liber de anima seu Sextus de naturalibus, part 5, ch. 3; Avicenna’s Liber de anima (Kitāb an-nafs) is itself a section of his Book of Healing (al-Shifa’).) And thus, it follows that when the body is separated from the soul, the latter loses its individuation. In that case, the soul could not subsist of itself nor be a particular thing. On the other hand, if the soul is individuated by itself, it is either a form in its entirety or is something composed of matter and form. If it is a form in its entirety, it follows that one individuated soul could differ from another only according to form. But difference in form causes difference in species. (Cf. Aquinas, Commentary on Aristotle’s Metaphysics, bk. 2, lect. 4, n. 320; bk. 5, lect. 2, n. 764.) Hence it would follow that the souls of different men are diverse in species; and if the soul is the form of the body, men differ among themselves in species because each and every thing derives its species from its proper form. On the other hand, if the soul is composed of matter and form, it would be impossible for the soul as a whole to be the form of the body, for the matter of a thing never has the nature of a form. It follows, then, that the soul cannot be at once both a particular thing and a form.

Obj. 3: Further, if the soul is a particular thing, it follows that it is an individual. Now every individual belongs to a species and a genus. Consequently, the soul will have a proper species and a proper genus. But a thing possessing its own species cannot have anything else superadded to it in order to give it its species, because, as the Philosopher points out in the Metaphysics, the forms or species of things are like numbers whose species change if a unit is added or subtracted. (Aristotle, Metaphysics, 8.3, 1043b33–1044a2.) Matter and form, however, are united in order to constitute a species. Therefore, if the soul is a particular thing, it is not united to the body as a form to matter.

Obj. 4: Further, since God made things because of his goodness, which is manifested in the different grades of things, he instituted as many grades of beings as nature could admit. (Cf. SCG II.45; ST I.76.5, ad 1; Augustine, On the Literal Interpretation of Genesis, bk. 2, ch. 1 (Corpus Scriptorum Ecclesiasticorum Latinorum [CSEL] 28.1:32–33; Patrologia Latina [PL] 34:263).) Hence, if the human soul can subsist in itself, which must be maintained if it is a particular thing, it would then constitute a distinct grade of being. But forms without matter do not themselves constitute a distinct grade of being. Thus, if the soul is a particular thing, it will not be the form of any matter.

Obj. 5: Further, if the soul is a particular thing subsisting in itself, it must be incorruptible, for neither has it a contrary, nor is it composed of contraries. (Cf. Aristotle, Categories 5, 3b24–25; Metaphysics 11.12, 1068a10–12.) But if the soul is incorruptible, it cannot be proportioned to a corruptible body such as the human body is. Now every form is proportioned to its matter. So if the soul is a particular thing, it will not be the form of the human body.

Obj. 6: Further, the only subsisting being that is pure act is God. (Cf. Boethius, On the Trinity, ch. 2 (PL 64:1250); Aquinas, In II Sent., d. 3, q. 1, a. 1, resp.) Therefore, if the soul is a particular self-subsisting thing, it will be composed of act and potentiality, and thus will not be a form, because no potentiality is an act. Consequently, if the soul is a particular thing, it will not be a form.

Obj. 7: Further, if the soul is a particular thing capable of subsisting in itself, it would need to be united to a body only for a good accruing to the soul, either for an essential good or an accidental one. It is not united for an essential good, however, because it can subsist without the body. Nor is it even for an accidental good; for the knowledge of truth which the human soul can acquire through the senses, themselves incapable of existing without bodily organs, is evidently a preeminent good of this sort. But some hold that the souls of stillborn infants have a perfect knowledge of things, and these certainly never acquired that knowledge through their senses. (Cf. Aquinas, Disputed Questions on Truth, q. 18, a. 7, resp.; ST I.84.3, obj. 3.) Consequently, if the soul is a particular thing, there is no reason why it should be united as a form to the body.

Obj. 8: Further, a form and a particular thing are distinguished from each other as opposites; for the Philosopher says in On the Soul 2 that substance has a threefold division: the first is form, the second, matter, and the third, this particular thing. (Aristotle, On the Soul 2.2, 414a15; 2.1, 412a6–9.) But opposites are not predicated of one and the same thing. Therefore, the human soul cannot be a form and a particular thing.

Obj. 9: Further, it belongs to the very essence of a particular thing to subsist of itself. But it is proper to a form to exist in something else. These seem to be contradictory. Consequently, if the soul is a particular thing, it is apparently not a form.

Obj. 10: But it might be said that when the body corrupts, the soul still remains a particular self-subsisting thing, but the account of a form then perishes in it.

On the contrary, whatever can exist apart from a thing and retain the nature of a substance exists in that thing accidentally. Therefore, if the soul continues to exist after the body corrupts, the soul ceases to have the account of a form; and thus, the account of a form belongs to it only accidentally. But it is only as a form that the soul is united to the body in order to constitute a man. Hence, the soul is united to the body accidentally, and thus man will be a being accidentally. This is incongruous.

Obj. 11: Further, if the human soul is a particular self-subsisting thing, it must have an operation of its own because a thing that exists of itself has its own proper operation. But the human soul does not have its own proper operation, because the act of understanding itself, which seems to be proper above all to the soul, is not an activity of the soul, but that of a man through his soul, as is stated in On the Soul 1. (Aristotle, On the Soul 1.4, 408a14; 408b13–15; 1.1, 403a8; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 2, n. 17; ST I.75.2, obj. 2–3.) Therefore, the human soul is not a particular thing.

Obj. 12: Further, if the human soul is the form of the body, it must depend in some way on the body, for form and matter depend on each other. (Cf. Aristotle, Physics 2.2, 194b8–9.) But whatever depends on something else in this way is not a particular thing. Therefore, if the soul is the form of the body, it will not be a particular thing.

Obj. 13: Further, if the soul is the form of the body, there must be one act of existing common to the soul and the body; because from the union of matter and form there results a thing having one act of existing. (Cf. Aristotle, Metaphysics 8.6, 1045b18–20; Aquinas, Commentary on Aristotle’s Metaphysics, bk. 8, lect. 5, n. 1767.) But there cannot be one act of existing common to the soul and the body, since they are diverse in genus; for the soul belongs to the genus of incorporeal substance, and the body to that of corporeal substance. Hence the soul cannot be the form of the body.

Obj. 13b: (Some manuscripts do not include this objection, since no reply to it is found below; however, as the Leonine edition notes, it is not unusual for Aquinas to neglect to provide a response to an objection in the disputed questions.) The soul has its own proper act of existing from its own principles. Therefore, if it has some act of existing in common with the body, it follows that it would have a double act of existing, which is impossible.

Obj. 14: Further, the body’s act of existing is a corruptible one resulting from quantitative parts. The soul’s act of existing, on the other hand, is incorruptible and simple. Therefore, there is not one act of existing possessed in common by the body and the soul.

Obj. 15: But it was said that the human body itself has the act of existing of a body through the soul. (Cf. SCG IV.81; Commentary on Aristotle’s On the Soul, bk. 2, lect. 1, n. 225.)

On the contrary, the Philosopher says that the soul is the act of a physical organic body. (Aristotle, On the Soul 2.1, 412b5–6; 412a27–28.) Therefore, that entity which is related to the soul as matter to act is now a physical organic body; and this body can exist only through a form whereby it is placed in the genus of body. Consequently, the human body possesses its own act of existing distinct from that of the soul.

Obj. 16: Further, the essential principles of matter and form are ordered to the act of existing. But whatever can be brought about in nature by one principle does not require two. (Cf. ST I-II.51.1, obj. 2; I-II.63.3, obj. 3; Aristotle, On the Heavens 2.11, 291b13–14.) Therefore, if the soul has in itself its own act of existing because it is a particular thing, then the body by nature is united to the soul only as a matter to a form.

Obj. 17: Further, the act of existing is related to the substance of the soul as its act. Hence, the act of existing must be supreme in the soul. (Cf. Aquinas, In I Sent., d. 33, q. 1, a. 1, ad 1; In II Sent., d. 1, q. 1, a. 4; ST I.4.1, ad 3; ST I.8.1, resp.; I.50.2, ad 3; I.54.1, resp.) But an inferior being is not related to a superior one with respect to that which is supreme in the superior, but rather with respect to that which is lowest in it. For Dionysius says in On the Divine Names 7 (Pseudo-Dionysius, On the Divine Names, ch. 7, para. 3 (Patrologia Graeca [PG] 3:870–72); cf. Aquinas, In II Sent., d. 39, q. 3, a. 1, resp.; SCG II.68; III.97.) that divine wisdom joins that which is highest in primary things (That is, those having less perfection.) to that which is lowest in secondary ones. (That is, those having greater perfection.) Therefore, the body, which is inferior to the soul, does not attain to that act of existing which is supreme in the soul.

Obj. 18: Further, things having one and the same act of existing have one and the same operation. (Cf. SCG II.56, obj. 5.) Therefore, if the act of existing of the human soul, when joined to the body, belongs also to the body, the act of understanding, which is the operation of the soul, will belong both to the soul and the body. This is impossible, as is proved in On the Soul 3. (Aristotle, On the Soul 3.4, 429a18–27; 430a17–25; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 10; bk. 3, lect. 1.) Consequently, there is not one act of existing for both the human soul and the body. Hence, it follows that the soul is not the form of the body and a particular thing.

On the contrary (1), a thing receives its species through its proper form. (Cf. SCG II.59, 60, 73; ST I.76.1, resp.; On the Unity of the Intellect, ch. 3; On Spiritual Creatures, a. 9, resp.) But man is man because he is rational. Hence, the rational soul is the proper form of man. Moreover, the soul is a particular self-subsisting thing because it operates of itself; for its act of understanding is not performed through a bodily organ, as is proved in On the Soul 3. (Aristotle, On the Soul 3.4, 429a24–27.) Consequently, the human soul is a particular thing and a form.

Furthermore (2), the highest perfection of the human soul consists in the knowledge of truth, which is acquired through the intellect. (Cf. Aristotle, Nicomachean Ethics 10.7, 1177a17; Aquinas, Commentary on the Nicomachean Ethics, bk. 10, lect. 10; ST I.76.1, resp.) Moreover, the soul must be united to the body in order to be perfected in knowledge of truth because it understands through phantasms which are non-existent without the body. Consequently, the soul must be united as a form to the body and must be a particular thing as well.

I answer that, “a particular thing,” properly speaking, designates an individual in the genus of substance. For the Philosopher says that first substances undoubtedly signify particular things; second substances, indeed, although they seem to signify particular things, rather signify the specific essence. (Aristotle, Categories 5, 3b10–23; 2a10.) Furthermore, an individual in the genus of substance is capable not only of subsisting of itself, but is also a complete entity belonging to a definite species and genus of substance. (Cf. Aristotle, Metaphysics 7.5, 1031a1–2; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 1.) Wherefore the Philosopher also calls a hand and a foot and things of this sort parts of substances rather than first or second substances. (Aristotle, Categories 5, 3a28–31.) For although they do not exist in another as a subject, which is characteristic of a substance, they still do not possess completely the nature of a species. Hence, they belong to a species or to a genus only by reduction. (Cf. ST I.75.2, ad 1–2.)

Now some men have denied that the human soul possesses these two real characteristics belonging to a particular thing by its very nature because they said that the soul is a harmony, as Empedocles did, (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 9; SCG II.64.) or a combination of the elements, as Galen did, or something of this kind. (Cf. Nemesius, On the Nature of Man, ch. 2; SCG II.63.) For then the soul will neither be able to subsist of itself, nor will it be a complete thing belonging to a species or genus of substance, but will be a form similar only to other material forms.

But this position is untenable as regards the vegetative soul, whose operations necessarily require some principle surpassing the active and passive qualities of the elements, which play only an instrumental role in nutrition and growth, as is proved in On the Soul 2. (Aristotle, On the Soul 2.4, 415b28; 416b17–30.) Moreover, a combination and a harmony do not transcend the elemental qualities. This position is likewise untenable as regards the sensitive soul, whose operations consist in receiving species separated from matter, as is shown in On the Soul 2. (Aristotle, On the Soul 2.12, 424a16–24.) For inasmuch as active and passive qualities are dispositions of matter, they do not transcend matter. Again, this position is even less tenable as regards the rational soul, whose operation consists in understanding, and in abstracting species not only from matter, but from all individuating conditions, this being required for the understanding of universals. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 12.)

However, in the case of the rational soul, something of special importance must still be considered because not only does it receive intelligible species without matter and material conditions, but it is also quite impossible for it, in performing its proper operation, to have anything in common with a bodily organ, as though something bodily might be an organ of understanding, just as the eye is the organ of sight, as is proved in On the Soul 3. (Aristotle, On the Soul 3.4, 429a24–27.) Thus, the intellective soul, inasmuch as it performs its proper operation without communicating in any way with the body, must act of itself. And because a thing acts so far as it is actual, (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 3, lect. 1; Commentary on Aristotle’s On Sense and What Is Sensed, tract. 1, ch. 9.) the intellective soul must have a complete act of existing in itself, depending in no way on the body. (Cf. Aquinas, In I Sent., d. 8, q. 5, a. 2, ad 1; On the Power of God, q. 3, a. 9, resp.) For forms whose act of existing depends on matter or on a subject do not operate of themselves. Heat, for instance, does not act, but something hot.

For this reason, the later Greek philosophers came to the conclusion that the intellective part of the soul is a self-subsisting thing. For the Philosopher says that the intellect is a substance, and is not corrupted. (Aristotle, On the Soul 3.5, 430a23–24; 1.4, 408b18–19.) The teaching of Plato, who maintains that the soul is incorruptible and subsists of itself, in view of the fact that it moves itself, amounts to the same thing. (Plato, Phaedrus 245c–e; cf. Macrobius, Commentary on the Dream of Scipio, bk. 2, ch. 13, nn. 9–11; Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 10; bk. 3, lect. 6.) For he took “motion” in a broad sense to signify every operation; hence, he understands that the soul moves itself because it moves itself by itself. But elsewhere, Plato maintained that the human soul not only subsists of itself but also has the complete nature of a species. (Plato, Alcibiades 129b–131e.) For he held that the complete nature of the human species is found in the soul, saying that a man is not a composite of soul and body, but a soul joined to a body in such a way that it is related to the body as a pilot is to a ship, or as one clothed to his clothing. (Cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 2; Nemesius, On the Nature of Man, ch. 1.)

However, this position is untenable because it is obvious that the soul is the reality which gives life to the body. Moreover, vital activity is the act of existing of living things. Consequently, the soul is that which gives the human body its act of existing. Now a form is of this nature. (Cf. SCG II.59; ST I.76.1, resp.; Commentary on Aristotle’s On the Soul, bk. 2, lect. 4, n. 271; On the Unity of the Intellect, cc. 1, 3.) Therefore, the human soul is the form of the body. But if the soul were in the body as a pilot is in a ship, it would give neither the body nor its parts their specific nature. The contrary of this is seen to be true because when the soul leaves the body, the body’s individual parts retain their original names only in an equivocal sense. For the eye of a dead man, like the eye of a portrait or that of a statue, is called an eye equivocally; (Cf. Aristotle, On the Soul 2.1, 412b13–15; 412b20–21; Metaphysics 7.10, 1035b24–25; 7.11, 1036b30–36.) and similarly for the other parts of the body. Furthermore, if the soul were in the body as a pilot in a ship, it would follow that the union of soul and body would be an accidental one. Then death, which brings about their separation, would not be a substantial corruption, which is clearly false. So it follows that the soul is a particular thing, and that it can subsist of itself, not as a thing having a complete species of its own, but as completing the human species by being the form of the body. (Cf. Aquinas, In III Sent., d. 5, q. 3, a. 2, ad 1, 3; ST I.75.2, ad 1; On Spiritual Creatures, a. 2, ad 5, ad 16; Commentary on Aristotle’s On the Soul, bk. 2, lect. 1.) Hence, it likewise follows that it is both a form and a particular thing.

Indeed, this can be shown from the order of natural forms. (Cf. Nemesius, On the Nature of Man, ch. 1.) For we find among the forms of lower bodies that the higher a form is, the more it resembles and approaches higher principles. This can be seen from the proper operation of forms. For the forms of the elements, being lowest and nearest to matter, possess no operation surpassing their active and passive qualities, such as rarefaction and condensation, and the like, which appear to be material dispositions. Over and above these forms are those of the mixed bodies, and these forms have, in addition to the above mentioned operations, a certain activity, consequent upon their species, which they receive from the heavenly bodies. The magnet, for instance, attracts iron not because of its heat or its cold or anything of this sort, but because it shares in the powers of the heavens. (Cf. Aristotle, On the Soul 1.2, 405a19–21; Averroes (Ibn Rushd, 1126‒1198 AD), Long Commentary on Aristotle’s On the Soul, bk. 1, comm. 32; Aquinas, Commentary on Aristotle’s On the Soul, bk. 1, lect. 5.) Again, surpassing these forms are the souls of plants, which resemble not only the forms of earthly bodies but also the movers of the heavenly bodies inasmuch as they are principles of a certain motion, themselves being moved. Still higher are brute beasts’ forms, which now resemble a substance moving a heavenly body not only because of the operation whereby they move bodies but also because they are capable of knowledge, although their knowledge is concerned merely with material things and belongs to the material order, for which reason they require bodily organs. Again, over and above these forms, and in the highest place, are human souls, which certainly resemble superior substances with respect to the kind of knowledge they possess because they are capable of knowing immaterial things by their act of understanding. However, human souls differ from superior substances inasmuch as the human soul’s intellective power, by its very nature, must acquire its immaterial knowledge from the knowledge of material things attained through the senses.

Consequently, the human soul’s mode of existing can be known from its operation. For inasmuch as the human soul has an operation transcending the material order, its act of existing transcends the body and does not depend on the body. Indeed, inasmuch as the soul is naturally capable of acquiring immaterial knowledge from material things, evidently its species can be complete only when it is united to a body. For a thing’s species is complete only if it has the things necessary for the proper operation of its species. Consequently, if the human soul, inasmuch as it is united as a form to the body, has an act of existing which transcends the body and does not depend on it, obviously the soul itself is established on the boundary line dividing corporeal from separate substances. (Cf. Nemesius, On the Nature of Man, ch. 1; The Book of Causes, propositions 2, 8; Aquinas, In IV Sent., d. 50, q. 1, a. 1; SCG II.68, 81, 91; III.61; IV.55; Quodlibet III, q. 9, a. 1; On Spiritual Creatures, a. 2; On the Unity of the Intellect, ch. 1.)

Reply Obj. 1: Although the soul has a complete act of existing of its own, it does not follow that the body is united to it accidentally: first, because the same act of existing that belongs to the soul is conferred on the body by the soul so that there is one act of existing for the whole composite; (Aquinas, In I Sent., d. 8, q. 5, a. 2, ad 2; cf. ST I.76.1, ad 5; On the Unity of the Intellect, ch. 1.) second, because, while the soul can subsist of itself, it does not have a complete species, for the soul needs the body in order to complete its species.

Reply Obj. 2: The act of existing and individuation of a thing are always found together. For universals do not exist in reality inasmuch as they are universals, but only inasmuch as they are individuated. Therefore, although the soul receives its act of existing from God as from an active principle, and exists in the body as in matter, nevertheless the soul’s act of existing does not cease when the body corrupts, nor does the soul’s individuation cease when the body corrupts, even though it has a relationship to the body. (See Aristotle, Metaphysics 5.6, 1016b32; 7.8, 1034a5–8; 7.10, 1035b30; 10.3, 1054a33; 10.9, 1058b5; 12.8, 1074a33; Avicenna, De anima, part 5, ch. 3; Aquinas, In I Sent., d. 8, q. 5, a. 2, ad 6; On the Power of God, q. 3, a. 10, ad 16; SCG II.81.)

Reply Obj. 3: The human soul is not a particular thing as though it were a substance having a complete species in itself, but inasmuch as it is part of a thing having a complete species, as is clear from what has been said. Therefore, the conclusion in the objection is false.

Reply Obj. 4: Although the human soul subsists of itself, it does not have a complete species in virtue of its very nature. Consequently, souls existing apart from bodies cannot constitute a distinct grade of being. (Cf. Aquinas, In III Sent., d. 5, q. 3, a. 2; On the Power of God, q. 3, a. 10, resp.; ST I.75.4, ad 2.)

Reply Obj. 5: The human body is the matter proportioned to the human soul, for the body is related to the soul as potentiality is to act. However, as regards its capacity for existing, the soul need not be on a par with the body, because the human soul is not a form totally embraced by matter. This is evident from the fact that one of the soul’s operations transcends matter. However, another explanation can be given in accordance with the position of faith, namely, that in the beginning the human body was in some way created incorruptible and incurred the necessity of dying through sin, from which necessity it will be freed once again at the resurrection. (Cf. Romans 5:12.) Hence, it is accidental that the body does not share in the incorruptibility of the soul.

Reply Obj. 6: Since the human soul is a subsisting being, it is composed of potentiality and act. For the substance itself of the soul is not its own act of existing, but is related to its act of existing as potentiality is to act. (See below: q. 6, resp.; Aquinas, On Spiritual Creatures, a. 1.; ST I.75.5, resp., ad 4.) However, it does not follow that the soul cannot be the form of the body, because, even in the case of other forms, whatever is like form and act in relation to one thing is like potentiality in relation to something else; just as transparency is formally present to the atmosphere, which is in potency in relation to light. (Aristotle, On the Soul 2.7, 418b9–10; Averroes, Commentary on Aristotle’s On the Soul, bk. 2, comm. 69; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14.)

Reply Obj. 7: The soul is united to the body both for a good which is a substantial perfection, namely, the completion of the human species, (Cf. Aquinas, On Spiritual Creatures, a. 2, ad 5.) and for a good which is an accidental perfection, namely, the perfecting of the soul in intellectual knowledge which it acquires from the senses; (Cf. SCG II.83; ST I.50.1, resp.) for this mode of understanding is natural to man. Nor is this position rendered untenable if the separated souls of infants and those of other men employ a different mode of understanding, for these souls are capable of such understanding rather by reason of being separated from the body than by reason of their human species.

Reply Obj. 8: It is not of the very account of a particular thing to be composed of matter and form, but only to be capable of subsisting in itself. Consequently, although a composite is a particular thing, this does not prevent other beings (That is, those not composed of matter and form.) from being particular things.

Reply Obj. 9: For a thing to exist in another as an accident in a subject prevents that thing from having the account of a particular thing. However, for a thing to exist in another as part of it—and the soul exists in man in this way—does not altogether prevent a thing having such an existence from being called a particular thing.

Reply Obj. 10: When the body is corrupted, the soul does not lose the nature which belongs to it as a form, despite the fact that it does not in act perfect matter as a form.

Reply Obj. 11: Understanding is the operation proper to the soul, if the soul is considered to be the principle from which the operation flows, for this operation is not exercised by the soul through some bodily organ as sight is exercised through the eye. Nevertheless, the body shares in this operation on the side of the object, for phantasms, which are the objects of the intellect, cannot exist without bodily organs.

Reply Obj. 12: Although the soul has some dependence on the body, inasmuch as the soul’s species is not complete without the body, the soul does not depend on the body in such a way that it cannot exist without the body.

Reply Obj. 13: If the soul is the form of the body, the soul and the body must have one common act of existing, which is the act of existing of the composite. Nor is this prevented by the fact that the soul and the body belong to two different genera, for the soul and the body belong to the same species or genus only by reduction, just as the parts of a whole are reduced to the species or genus of the whole. (Cf. Aquinas, On Spiritual Creatures, a. 2, ad 16; ST I.3.5 resp.)

[No Reply to Objection 13b is provided.] (Cf. SCG II.68.)

Reply Obj. 14: The thing that is properly corrupted is neither the form nor the matter, nor the act of existing itself, but the composite. Moreover, the body’s act of existing is said to be corruptible inasmuch as the body by corrupting is deprived of the act of existing which it possessed in common with the soul, and this act of existing remains in the subsisting soul. The same thing is to be said for the parts composing the body, because the body is constituted of its parts in such a way that it can receive its act of existing from the soul.

Reply Obj. 15: Sometimes in the definitions of forms a subject is considered independently of its form, as when it is said that motion is the act of a being in potentiality. (Cf. Aristotle, Physics 3.2, 202a7; Metaphysics 11.9, 1065b20–23.) Sometimes, however, the subject is regarded as informed, as when it is said that motion is the act of a mobile thing, just as light is the act of that which is transparent. (Cf. ST I.76.4, ad 1; Commentary on Aristotle’s On the Soul, bk. 2, lect. 14, nn. 421–22; Aristotle, On the Soul 2.1, 412b25–413a3.) Now it is in this way that the soul is said to be the act of a physical organic body, because the soul causes it to be a physical organic body, just as light makes something lucid. (Cf. Aristotle, Metaphysics 7.4–5, 1030b14–1031a14; Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 1.)

Reply Obj. 16: The essential principles of a species are not related merely to an act of existing, but to the act of existing of this particular species. Consequently, although the soul can exist of itself, it cannot be complete in its species without the body.

Reply Obj. 17: While the act of existing is the most formal of all principles, it is also the most communicable, although it is not shared in the same measure both by inferior beings and by superior ones. Hence, the body shares in the soul’s act of existing, but not as perfectly as the soul does.

Reply Obj. 18: Although the soul’s act of existing belongs in a certain measure to the body, the body does not succeed in sharing in the soul’s act of existing to the full extent of its perfection and actuality. Therefore, the soul has an operation in which the body does not share.