Objection 1: It seems that the intellectual principle is not united to the body as its form. For the Philosopher says (De Anima iii, 4) that the intellect is "separate," and that it is not the act of any body. Therefore it is not united to the body as its form.

Objection 2: Further, every form is determined according to the nature of the matter of which it is the form; otherwise no proportion would be required between matter and form. Therefore if the intellect were united to the body as its form, since every body has a determinate nature, it would follow that the intellect has a determinate nature; and thus, it would not be capable of knowing all things, as is clear from what has been said (Question [75], Article [2]); which is contrary to the nature of the intellect. Therefore the intellect is not united to the body as its form.

Objection 3: Further, whatever receptive power is an act of a body, receives a form materially and individually; for what is received must be received according to the condition of the receiver. But the form of the thing understood is not received into the intellect materially and individually, but rather immaterially and universally: otherwise the intellect would not be capable of the knowledge of immaterial and universal objects, but only of individuals, like the senses. Therefore the intellect is not united to the body as its form.

Objection 4: Further, power and action have the same subject; for the same subject is what can, and does, act. But the intellectual action is not the action of a body, as appears from above (Question [75], Article [2]). Therefore neither is the intellectual faculty a power of the body. But virtue or power cannot be more abstract or more simple than the essence from which the faculty or power is derived. Therefore neither is the substance of the intellect the form of a body.

Objection 5: Further, whatever has "per se" existence is not united to the body as its form; because a form is that by which a thing exists: so that the very existence of a form does not belong to the form by itself. But the intellectual principle has "per se" existence and is subsistent, as was said above (Question [75], Article [2]). Therefore it is not united to the body as its form.

Objection 6: Further, whatever exists in a thing by reason of its nature exists in it always. But to be united to matter belongs to the form by reason of its nature; because form is the act of matter, not by an accidental quality, but by its own essence; otherwise matter and form would not make a thing substantially one, but only accidentally one. Therefore a form cannot be without its own proper matter. But the intellectual principle, since it is incorruptible, as was shown above (Question [75], Article [6]), remains separate from the body, after the dissolution of the body. Therefore the intellectual principle is not united to the body as its form.

On the contrary, According to the Philosopher, Metaph. viii (Did. vii 2), difference is derived from the form. But the difference which constitutes man is "rational," which is applied to man on account of his intellectual principle. Therefore the intellectual principle is the form of man.

I answer that, We must assert that the intellect which is the principle of intellectual operation is the form of the human body. For that whereby primarily anything acts is a form of the thing to which the act is to be attributed: for instance, that whereby a body is primarily healed is health, and that whereby the soul knows primarily is knowledge; hence health is a form of the body, and knowledge is a form of the soul. The reason is because nothing acts except so far as it is in act; wherefore a thing acts by that whereby it is in act. Now it is clear that the first thing by which the body lives is the soul. And as life appears through various operations in different degrees of living things, that whereby we primarily perform each of all these vital actions is the soul. For the soul is the primary principle of our nourishment, sensation, and local movement; and likewise of our understanding. Therefore this principle by which we primarily understand, whether it be called the intellect or the intellectual soul, is the form of the body. This is the demonstration used by Aristotle (De Anima ii, 2).

But if anyone says that the intellectual soul is not the form of the body he must first explain how it is that this action of understanding is the action of this particular man; for each one is conscious that it is himself who understands. Now an action may be attributed to anyone in three ways, as is clear from the Philosopher (Phys. v, 1); for a thing is said to move or act, either by virtue of its whole self, for instance, as a physician heals; or by virtue of a part, as a man sees by his eye; or through an accidental quality, as when we say that something that is white builds, because it is accidental to the builder to be white. So when we say that Socrates or Plato understands, it is clear that this is not attributed to him accidentally; since it is ascribed to him as man, which is predicated of him essentially. We must therefore say either that Socrates understands by virtue of his whole self, as Plato maintained, holding that man is an intellectual soul; or that intelligence is a part of Socrates. The first cannot stand, as was shown above (Question [75], Article [4]), for this reason, that it is one and the same man who is conscious both that he understands, and that he senses. But one cannot sense without a body: therefore the body must be some part of man. It follows therefore that the intellect by which Socrates understands is a part of Socrates, so that in some way it is united to the body of Socrates.

The Commentator held that this union is through the intelligible species, as having a double subject, in the possible intellect, and in the phantasms which are in the corporeal organs. Thus through the intelligible species the possible intellect is linked to the body of this or that particular man. But this link or union does not sufficiently explain the fact, that the act of the intellect is the act of Socrates. This can be clearly seen from comparison with the sensitive faculty, from which Aristotle proceeds to consider things relating to the intellect. For the relation of phantasms to the intellect is like the relation of colors to the sense of sight, as he says De Anima iii, 5,7. Therefore, as the species of colors are in the sight, so are the species of phantasms in the possible intellect. Now it is clear that because the colors, the images of which are in the sight, are on a wall, the action of seeing is not attributed to the wall: for we do not say that the wall sees, but rather that it is seen. Therefore, from the fact that the species of phantasms are in the possible intellect, it does not follow that Socrates, in whom are the phantasms, understands, but that he or his phantasms are understood.

Some, however, tried to maintain that the intellect is united to the body as its motor; and hence that the intellect and body form one thing so that the act of the intellect could be attributed to the whole. This is, however, absurd for many reasons. First, because the intellect does not move the body except through the appetite, the movement of which presupposes the operation of the intellect. The reason therefore why Socrates understands is not because he is moved by his intellect, but rather, contrariwise, he is moved by his intellect because he understands. Secondly, because since Socrates is an individual in a nature of one essence composed of matter and form, if the intellect be not the form, it follows that it must be outside the essence, and then the intellect is the whole Socrates as a motor to the thing moved. Whereas the act of intellect remains in the agent, and does not pass into something else, as does the action of heating. Therefore the action of understanding cannot be attributed to Socrates for the reason that he is moved by his intellect. Thirdly, because the action of a motor is never attributed to the thing moved, except as to an instrument; as the action of a carpenter to a saw. Therefore if understanding is attributed to Socrates, as the action of what moves him, it follows that it is attributed to him as to an instrument. This is contrary to the teaching of the Philosopher, who holds that understanding is not possible through a corporeal instrument (De Anima iii, 4). Fourthly, because, although the action of a part be attributed to the whole, as the action of the eye is attributed to a man; yet it is never attributed to another part, except perhaps indirectly; for we do not say that the hand sees because the eye sees. Therefore if the intellect and Socrates are united in the above manner, the action of the intellect cannot be attributed to Socrates. If, however, Socrates be a whole composed of a union of the intellect with whatever else belongs to Socrates, and still the intellect be united to those other things only as a motor, it follows that Socrates is not one absolutely, and consequently neither a being absolutely, for a thing is a being according as it is one.

There remains, therefore, no other explanation than that given by Aristotle---namely, that this particular man understands, because the intellectual principle is his form. Thus from the very operation of the intellect it is made clear that the intellectual principle is united to the body as its form.

The same can be clearly shown from the nature of the human species. For the nature of each thing is shown by its operation. Now the proper operation of man as man is to understand; because he thereby surpasses all other animals. Whence Aristotle concludes (Ethic. x, 7) that the ultimate happiness of man must consist in this operation as properly belonging to him. Man must therefore derive his species from that which is the principle of this operation. But the species of anything is derived from its form. It follows therefore that the intellectual principle is the proper form of man.

But we must observe that the nobler a form is, the more it rises above corporeal matter, the less it is merged in matter, and the more it excels matter by its power and its operation; hence we find that the form of a mixed body has another operation not caused by its elemental qualities. And the higher we advance in the nobility of forms, the more we find that the power of the form excels the elementary matter; as the vegetative soul excels the form of the metal, and the sensitive soul excels the vegetative soul. Now the human soul is the highest and noblest of forms. Wherefore it excels corporeal matter in its power by the fact that it has an operation and a power in which corporeal matter has no share whatever. This power is called the intellect.

It is well to remark that if anyone holds that the soul is composed of matter and form, it would follow that in no way could the soul be the form of the body. For since the form is an act, and matter is only in potentiality, that which is composed of matter and form cannot be the form of another by virtue of itself as a whole. But if it is a form by virtue of some part of itself, then that part which is the form we call the soul, and that of which it is the form we call the "primary animate," as was said above (Question [75], Article [5]).

Reply to Objection 1: As the Philosopher says (Phys. ii, 2), the ultimate natural form to which the consideration of the natural philosopher is directed is indeed separate; yet it exists in matter. He proves this from the fact that "man and the sun generate man from matter." It is separate indeed according to its intellectual power, because the intellectual power does not belong to a corporeal organ, as the power of seeing is the act of the eye; for understanding is an act which cannot be performed by a corporeal organ, like the act of seeing. But it exists in matter so far as the soul itself, to which this power belongs, is the form of the body, and the term of human generation. And so the Philosopher says (De Anima iii) that the intellect is separate, because it is not the faculty of a corporeal organ.

From this it is clear how to answer the Second and Third objections: since, in order that man may be able to understand all things by means of his intellect, and that his intellect may understand immaterial things and universals, it is sufficient that the intellectual power be not the act of the body.

Reply to Objection 4: The human soul, by reason of its perfection, is not a form merged in matter, or entirely embraced by matter. Therefore there is nothing to prevent some power thereof not being the act of the body, although the soul is essentially the form of the body.

Reply to Objection 5: The soul communicates that existence in which it subsists to the corporeal matter, out of which and the intellectual soul there results unity of existence; so that the existence of the whole composite is also the existence of the soul. This is not the case with other non-subsistent forms. For this reason the human soul retains its own existence after the dissolution of the body; whereas it is not so with other forms.

Reply to Objection 6: To be united to the body belongs to the soul by reason of itself, as it belongs to a light body by reason of itself to be raised up. And as a light body remains light, when removed from its proper place, retaining meanwhile an aptitude and an inclination for its proper place; so the human soul retains its proper existence when separated from the body, having an aptitude and a natural inclination to be united to the body.

Objection 1: It would seem that the intellectual principle is not multiplied according to the number of bodies, but that there is one intellect in all men. For an immaterial substance is not multiplied in number within one species. But the human soul is an immaterial substance; since it is not composed of matter and form as was shown above (Question [75], Article [5]). Therefore there are not many human souls in one species. But all men are of one species. Therefore there is but one intellect in all men.

Objection 2: Further, when the cause is removed, the effect is also removed. Therefore, if human souls were multiplied according to the number of bodies, it follows that the bodies being removed, the number of souls would not remain; but from all the souls there would be but a single remainder. This is heretical; for it would do away with the distinction of rewards and punishments.

Objection 3: Further, if my intellect is distinct from your intellect, my intellect is an individual, and so is yours; for individuals are things which differ in number but agree in one species. Now whatever is received into anything must be received according to the condition of the receiver. Therefore the species of things would be received individually into my intellect, and also into yours: which is contrary to the nature of the intellect which knows universals.

Objection 4: Further, the thing understood is in the intellect which understands. If, therefore, my intellect is distinct from yours, what is understood by me must be distinct from what is understood by you; and consequently it will be reckoned as something individual, and be only potentially something understood; so that the common intention will have to be abstracted from both; since from things diverse something intelligible common to them may be abstracted. But this is contrary to the nature of the intellect; for then the intellect would seem not to be distinct from the imagination. It seems, therefore, to follow that there is one intellect in all men.

Objection 5: Further, when the disciple receives knowledge from the master, it cannot be said that the master's knowledge begets knowledge in the disciple, because then also knowledge would be an active form, such as heat is, which is clearly false. It seems, therefore, that the same individual knowledge which is in the master is communicated to the disciple; which cannot be, unless there is one intellect in both. Seemingly, therefore, the intellect of the disciple and master is but one; and, consequently, the same applies to all men.

Objection 6: Further, Augustine (De Quant. Animae xxxii) says: "If I were to say that there are many human souls, I should laugh at myself." But the soul seems to be one chiefly on account of the intellect. Therefore there is one intellect of all men.

On the contrary, The Philosopher says (Phys. ii, 3) that the relation of universal causes to universals is like the relation of particular causes to individuals. But it is impossible that a soul, one in species, should belong to animals of different species. Therefore it is impossible that one individual intellectual soul should belong to several individuals.

I answer that, It is absolutely impossible for one intellect to belong to all men. This is clear if, as Plato maintained, man is the intellect itself. For it would follow that Socrates and Plato are one man; and that they are not distinct from each other, except by something outside the essence of each. The distinction between Socrates and Plato would be no other than that of one man with a tunic and another with a cloak; which is quite absurd.

It is likewise clear that this is impossible if, according to the opinion of Aristotle (De Anima ii, 2), it is supposed that the intellect is a part or a power of the soul which is the form of man. For it is impossible for many distinct individuals to have one form, as it is impossible for them to have one existence, for the form is the principle of existence.

Again, this is clearly impossible, whatever one may hold as to the manner of the union of the intellect to this or that man. For it is manifest that, supposing there is one principal agent, and two instruments, we can say that there is one agent absolutely, but several actions; as when one man touches several things with his two hands, there will be one who touches, but two contacts. If, on the contrary, we suppose one instrument and several principal agents, we might say that there are several agents, but one act; for example, if there be many drawing a ship by means of a rope; there will be many drawing, but one pull. If, however, there is one principal agent, and one instrument, we say that there is one agent and one action, as when the smith strikes with one hammer, there is one striker and one stroke. Now it is clear that no matter how the intellect is united or coupled to this or that man, the intellect has the precedence of all the other things which appertain to man; for the sensitive powers obey the intellect, and are at its service. Therefore, if we suppose two men to have several intellects and one sense---for instance, if two men had one eye---there would be several seers, but one sight. But if there is one intellect, no matter how diverse may be all those things of which the intellect makes use as instruments, in no way is it possible to say that Socrates and Plato are otherwise than one understanding man. And if to this we add that to understand, which is the act of the intellect, is not affected by any organ other than the intellect itself; it will further follow that there is but one agent and one action: that is to say that all men are but one "understander," and have but one act of understanding, in regard, that is, of one intelligible object.

However, it would be possible to distinguish my intellectual action form yours by the distinction of the phantasms---that is to say, were there one phantasm of a stone in me, and another in you---if the phantasm itself, as it is one thing in me and another in you, were a form of the possible intellect; since the same agent according to divers forms produces divers actions; as, according to divers forms of things with regard to the same eye, there are divers visions. But the phantasm itself is not a form of the possible intellect; it is the intelligible species abstracted from the phantasm that is a form. Now in one intellect, from different phantasms of the same species, only one intelligible species is abstracted; as appears in one man, in whom there may be different phantasms of a stone; yet from all of them only one intelligible species of a stone is abstracted; by which the intellect of that one man, by one operation, understands the nature of a stone, notwithstanding the diversity of phantasms. Therefore, if there were one intellect for all men, the diversity of phantasms which are in this one and that one would not cause a diversity of intellectual operation in this man and that man. It follows, therefore, that it is altogether impossible and unreasonable to maintain that there exists one intellect for all men.

Reply to Objection 1: Although the intellectual soul, like an angel, has no matter from which it is produced, yet it is the form of a certain matter; in which it is unlike an angel. Therefore, according to the division of matter, there are many souls of one species; while it is quite impossible for many angels to be of one species.

Reply to Objection 2: Everything has unity in the same way that it has being; consequently we must judge of the multiplicity of a thing as we judge of its being. Now it is clear that the intellectual soul, by virtue of its very being, is united to the body as its form; yet, after the dissolution of the body, the intellectual soul retains its own being. In like manner the multiplicity of souls is in proportion to the multiplicity of the bodies; yet, after the dissolution of the bodies, the souls retain their multiplied being.

Reply to Objection 3: Individuality of the intelligent being, or of the species whereby it understands, does not exclude the understanding of universals; otherwise, since separate intellects are subsistent substances, and consequently individual, they could not understand universals. But the materiality of the knower, and of the species whereby it knows, impedes the knowledge of the universal. For as every action is according to the mode of the form by which the agent acts, as heating is according to the mode of the heat; so knowledge is according to the mode of the species by which the knower knows. Now it is clear that common nature becomes distinct and multiplied by reason of the individuating principles which come from the matter. Therefore if the form, which is the means of knowledge, is material---that is, not abstracted from material conditions---its likeness to the nature of a species or genus will be according to the distinction and multiplication of that nature by means of individuating principles; so that knowledge of the nature of a thing in general will be impossible. But if the species be abstracted from the conditions of individual matter, there will be a likeness of the nature without those things which make it distinct and multiplied; thus there will be knowledge of the universal. Nor does it matter, as to this particular point, whether there be one intellect or many; because, even if there were but one, it would necessarily be an individual intellect, and the species whereby it understands, an individual species.

Reply to Objection 4: Whether the intellect be one or many, what is understood is one; for what is understood is in the intellect, not according to its own nature, but according to its likeness; for "the stone is not in the soul, but its likeness is," as is said, De Anima iii, 8. Yet it is the stone which is understood, not the likeness of the stone; except by a reflection of the intellect on itself: otherwise, the objects of sciences would not be things, but only intelligible species. Now it happens that different things, according to different forms, are likened to the same thing. And since knowledge is begotten according to the assimilation of the knower to the thing known, it follows that the same thing may happen to be known by several knowers; as is apparent in regard to the senses; for several see the same color, according to different likenesses. In the same way several intellects understand one object understood. But there is this difference, according to the opinion of Aristotle, between the sense and the intelligence---that a thing is perceived by the sense according to the disposition which it has outside the soul ---that is, in its individuality; whereas the nature of the thing understood is indeed outside the soul, but the mode according to which it exists outside the soul is not the mode according to which it is understood. For the common nature is understood as apart from the individuating principles; whereas such is not its mode of existence outside the soul. But, according to the opinion of Plato, the thing understood exists outside the soul in the same condition as those under which it is understood; for he supposed that the natures of things exist separate from matter.

Reply to Objection 5: One knowledge exists in the disciple and another in the master. How it is caused will be shown later on (Question [117], Article [1]).

Reply to Objection 6: Augustine denies a plurality of souls, that would involve a plurality of species.

Objection 1: It would seem that besides the intellectual soul there are in man other souls essentially different from one another, such as the sensitive soul and the nutritive soul. For corruptible and incorruptible are not of the same substance. But the intellectual soul is incorruptible; whereas the other souls, as the sensitive and the nutritive, are corruptible, as was shown above (Question [75], Article [6]). Therefore in man the essence of the intellectual soul, the sensitive soul, and the nutritive soul, cannot be the same.

Objection 2: Further, if it be said that the sensitive soul in man is incorruptible; on the contrary, "corruptible and incorruptible differ generically," says the Philosopher, Metaph. x (Did. ix, 10). But the sensitive soul in the horse, the lion, and other brute animals, is corruptible. If, therefore, in man it be incorruptible, the sensitive soul in man and brute animals will not be of the same "genus." Now an animal is so called from its having a sensitive soul; and, therefore, "animal" will not be one genus common to man and other animals, which is absurd.

Objection 3: Further, the Philosopher says, Metaph. viii (Did. vii, 2), that the genus is taken from the matter, and difference from the form. But "rational," which is the difference constituting man, is taken from the intellectual soul; while he is called "animal" by reason of his having a body animated by a sensitive soul. Therefore the intellectual soul may be compared to the body animated by a sensitive soul, as form to matter. Therefore in man the intellectual soul is not essentially the same as the sensitive soul, but presupposes it as a material subject.

On the contrary, It is said in the book De Ecclesiasticis Dogmatibus xv: "Nor do we say that there are two souls in one man, as James and other Syrians write; one, animal, by which the body is animated, and which is mingled with the blood; the other, spiritual, which obeys the reason; but we say that it is one and the same soul in man, that both gives life to the body by being united to it, and orders itself by its own reasoning."

I answer that, Plato held that there were several souls in one body, distinct even as to organs, to which souls he referred the different vital actions, saying that the nutritive power is in the liver, the concupiscible in the heart, and the power of knowledge in the brain. Which opinion is rejected by Aristotle (De Anima ii, 2), with regard to those parts of the soul which use corporeal organs; for this reason, that in those animals which continue to live when they have been divided in each part are observed the operations of the soul, as sense and appetite. Now this would not be the case if the various principles of the soul's operations were essentially different, and distributed in the various parts of the body. But with regard to the intellectual part, he seems to leave it in doubt whether it be "only logically" distinct from the other parts of the soul, "or also locally."

The opinion of Plato might be maintained if, as he held, the soul was supposed to be united to the body, not as its form, but as its motor. For it involves nothing unreasonable that the same movable thing be moved by several motors; and still less if it be moved according to its various parts. If we suppose, however, that the soul is united to the body as its form, it is quite impossible for several essentially different souls to be in one body. This can be made clear by three different reasons.

In the first place, an animal would not be absolutely one, in which there were several souls. For nothing is absolutely one except by one form, by which a thing has existence: because a thing has from the same source both existence and unity; and therefore things which are denominated by various forms are not absolutely one; as, for instance, "a white man." If, therefore, man were 'living' by one form, the vegetative soul, and 'animal' by another form, the sensitive soul, and "man" by another form, the intellectual soul, it would follow that man is not absolutely one. Thus Aristotle argues, Metaph. viii (Did. vii, 6), against Plato, that if the idea of an animal is distinct from the idea of a biped, then a biped animal is not absolutely one. For this reason, against those who hold that there are several souls in the body, he asks (De Anima i, 5), "what contains them?"---that is, what makes them one? It cannot be said that they are united by the one body; because rather does the soul contain the body and make it one, than the reverse.

Secondly, this is proved to be impossible by the manner in which one thing is predicated of another. Those things which are derived from various forms are predicated of one another, either accidentally, (if the forms are not ordered to one another, as when we say that something white is sweet), or essentially, in the second manner of essential predication, (if the forms are ordered one to another, the subject belonging to the definition of the predicate; as a surface is presupposed to color; so that if we say that a body with a surface is colored, we have the second manner of essential predication.) Therefore, if we have one form by which a thing is an animal, and another form by which it is a man, it follows either that one of these two things could not be predicated of the other, except accidentally, supposing these two forms not to be ordered to one another---or that one would be predicated of the other according to the second manner of essential predication, if one soul be presupposed to the other. But both of these consequences are clearly false: because "animal" is predicated of man essentially and not accidentally; and man is not part of the definition of an animal, but the other way about. Therefore of necessity by the same form a thing is animal and man; otherwise man would not really be the thing which is an animal, so that animal can be essentially predicated of man.

Thirdly, this is shown to be impossible by the fact that when one operation of the soul is intense it impedes another, which could never be the case unless the principle of action were essentially one.

We must therefore conclude that in man the sensitive soul, the intellectual soul, and the nutritive soul are numerically one soul. This can easily be explained, if we consider the differences of species and forms. For we observe that the species and forms of things differ from one another, as the perfect and imperfect; as in the order of things, the animate are more perfect than the inanimate, and animals more perfect than plants, and man than brute animals; and in each of these genera there are various degrees. For this reason Aristotle, Metaph. viii (Did. vii, 3), compares the species of things to numbers, which differ in species by the addition or subtraction of unity. And (De Anima ii, 3) he compares the various souls to the species of figures, one of which contains another; as a pentagon contains and exceeds a tetragon. Thus the intellectual soul contains virtually whatever belongs to the sensitive soul of brute animals, and to the nutritive souls of plants. Therefore, as a surface which is of a pentagonal shape, is not tetragonal by one shape, and pentagonal by another---since a tetragonal shape would be superfluous as contained in the pentagonal---so neither is Socrates a man by one soul, and animal by another; but by one and the same soul he is both animal and man.

Reply to Objection 1: The sensitive soul is incorruptible, not by reason of its being sensitive, but by reason of its being intellectual. When, therefore, a soul is sensitive only, it is corruptible; but when with sensibility it has also intellectuality, it is incorruptible. For although sensibility does not give incorruptibility, yet it cannot deprive intellectuality of its incorruptibility.

Reply to Objection 2: Not forms, but composites, are classified either generically or specifically. Now man is corruptible like other animals. And so the difference of corruptible and incorruptible which is on the part of the forms does not involve a generic difference between man and the other animals.

Reply to Objection 3: The embryo has first of all a soul which is merely sensitive, and when this is removed, it is supplanted by a more perfect soul, which is both sensitive and intellectual: as will be shown further on (Question [118], Article [2], ad 2).

Reply to Objection 4: We must not consider the diversity of natural things as proceeding from the various logical notions or intentions, which flow from our manner of understanding, because reason can apprehend one and the same thing in various ways. Therefore since, as we have said, the intellectual soul contains virtually what belongs to the sensitive soul, and something more, reason can consider separately what belongs to the power of the sensitive soul, as something imperfect and material. And because it observes that this is something common to man and to other animals, it forms thence the notion of the "genus"; while that wherein the intellectual soul exceeds the sensitive soul, it takes as formal and perfecting; thence it gathers the "difference" of man.

Objection 1: It would seem that in man there is another form besides the intellectual soul. For the Philosopher says (De Anima ii, 1), that "the soul is the act of a physical body which has life potentially." Therefore the soul is to the body as a form of matter. But the body has a substantial form by which it is a body. Therefore some other substantial form in the body precedes the soul.

Objection 2: Further, man moves himself as every animal does. Now everything that moves itself is divided into two parts, of which one moves, and the other is moved, as the Philosopher proves (Phys. viii, 5). But the part which moves is the soul. Therefore the other part must be such that it can be moved. But primary matter cannot be moved (Phys. v, 1), since it is a being only potentially; indeed everything that is moved is a body. Therefore in man and in every animal there must be another substantial form, by which the body is constituted.

Objection 3: Further, the order of forms depends on their relation to primary matter; for "before" and "after" apply by comparison to some beginning. Therefore if there were not in man some other substantial form besides the rational soul, and if this were to inhere immediately to primary matter; it would follow that it ranks among the most imperfect forms which inhere to matter immediately.

Objection 4: Further, the human body is a mixed body. Now mingling does not result from matter alone; for then we should have mere corruption. Therefore the forms of the elements must remain in a mixed body; and these are substantial forms. Therefore in the human body there are other substantial forms besides the intellectual soul.

On the contrary, Of one thing there is but one substantial being. But the substantial form gives substantial being. Therefore of one thing there is but one substantial form. But the soul is the substantial form of man. Therefore it is impossible for there to be in man another substantial form besides the intellectual soul.

I answer that, If we suppose that the intellectual soul is not united to the body as its form, but only as its motor, as the Platonists maintain, it would necessarily follow that in man there is another substantial form, by which the body is established in its being as movable by the soul. If, however, the intellectual soul be united to the body as its substantial form, as we have said above (Article [1]), it is impossible for another substantial form besides the intellectual soul to be found in man.

In order to make this evident, we must consider that the substantial form differs from the accidental form in this, that the accidental form does not make a thing to be "simply," but to be "such," as heat does not make a thing to be simply, but only to be hot. Therefore by the coming of the accidental form a thing is not said to be made or generated simply, but to be made such, or to be in some particular condition; and in like manner, when an accidental form is removed, a thing is said to be corrupted, not simply, but relatively. Now the substantial form gives being simply; therefore by its coming a thing is said to be generated simply; and by its removal to be corrupted simply. For this reason, the old natural philosophers, who held that primary matter was some actual being---for instance, fire or air, or something of that sort---maintained that nothing is generated simply, or corrupted simply; and stated that "every becoming is nothing but an alteration," as we read, Phys. i, 4. Therefore, if besides the intellectual soul there pre-existed in matter another substantial form by which the subject of the soul were made an actual being, it would follow that the soul does not give being simply; and consequently that it is not the substantial form: and so at the advent of the soul there would not be simple generation; nor at its removal simple corruption, all of which is clearly false.

Whence we must conclude, that there is no other substantial form in man besides the intellectual soul; and that the soul, as it virtually contains the sensitive and nutritive souls, so does it virtually contain all inferior forms, and itself alone does whatever the imperfect forms do in other things. The same is to be said of the sensitive soul in brute animals, and of the nutritive soul in plants, and universally of all more perfect forms with regard to the imperfect.

Reply to Objection 1: Aristotle does not say that the soul is the act of a body only, but "the act of a physical organic body which has life potentially"; and that this potentiality "does not reject the soul." Whence it is clear that when the soul is called the act, the soul itself is included; as when we say that heat is the act of what is hot, and light of what is lucid; not as though lucid and light were two separate things, but because a thing is made lucid by the light. In like manner, the soul is said to be the "act of a body," etc., because by the soul it is a body, and is organic, and has life potentially. Yet the first act is said to be in potentiality to the second act, which is operation; for such a potentiality "does not reject"---that is, does not exclude---the soul.

Reply to Objection 2: The soul does not move the body by its essence, as the form of the body, but by the motive power, the act of which presupposes the body to be already actualized by the soul: so that the soul by its motive power is the part which moves; and the animate body is the part moved.

Reply to Objection 3: We observe in matter various degrees of perfection, as existence, living, sensing, and understanding. Now what is added is always more perfect. Therefore that form which gives matter only the first degree of perfection is the most imperfect; while that form which gives the first, second, and third degree, and so on, is the most perfect: and yet it inheres to matter immediately.

Reply to Objection 4: Avicenna held that the substantial forms of the elements remain entire in the mixed body; and that the mixture is made by the contrary qualities of the elements being reduced to an average. But this is impossible, because the various forms of the elements must necessarily be in various parts of matter; for the distinction of which we must suppose dimensions, without which matter cannot be divisible. Now matter subject to dimension is not to be found except in a body. But various bodies cannot be in the same place. Whence it follows that elements in the mixed body would be distinct as to situation. And then there would not be a real mixture which is in respect of the whole; but only a mixture apparent to sense, by the juxtaposition of particles.

Averroes maintained that the forms of elements, by reason of their imperfection, are a medium between accidental and substantial forms, and so can be "more" or "less"; and therefore in the mixture they are modified and reduced to an average, so that one form emerges from them. But this is even still more impossible. For the substantial being of each thing consists in something indivisible, and every addition and subtraction varies the species, as in numbers, as stated in Metaph. viii (Did. vii, 3); and consequently it is impossible for any substantial form to receive "more" or "less." Nor is it less impossible for anything to be a medium between substance and accident.

Therefore we must say, in accordance with the Philosopher (De Gener. i, 10), that the forms of the elements remain in the mixed body, not actually but virtually. For the proper qualities of the elements remain, though modified; and in them is the power of the elementary forms. This quality of the mixture is the proper disposition for the substantial form of the mixed body; for instance, the form of a stone, or of any sort of soul.

Objection 1: It would seem that the intellectual soul is improperly united to such a body. For matter must be proportionate to the form. But the intellectual soul is incorruptible. Therefore it is not properly united to a corruptible body.

Objection 2: Further, the intellectual soul is a perfectly immaterial form; a proof whereof is its operation in which corporeal matter does not share. But the more subtle is the body, the less has it of matter. Therefore the soul should be united to a most subtle body, to fire, for instance, and not to a mixed body, still less to a terrestrial body.

Objection 3: Further, since the form is the principle of the species, one form cannot produce a variety of species. But the intellectual soul is one form. Therefore, it should not be united to a body which is composed of parts belonging to various species.

Objection 4: Further, what is susceptible of a more perfect form should itself be more perfect. But the intellectual soul is the most perfect of souls. Therefore since the bodies of other animals are naturally provided with a covering, for instance, with hair instead of clothes, and hoofs instead of shoes; and are, moreover, naturally provided with arms, as claws, teeth, and horns; it seems that the intellectual soul should not have been united to a body which is imperfect as being deprived of the above means of protection.

On the contrary, The Philosopher says (De Anima ii, 1), that "the soul is the act of a physical organic body having life potentially."

I answer that, Since the form is not for the matter, but rather the matter for the form, we must gather from the form the reason why the matter is such as it is; and not conversely. Now the intellectual soul, as we have seen above (Question [55], Article [2]) in the order of nature, holds the lowest place among intellectual substances; inasmuch as it is not naturally gifted with the knowledge of truth, as the angels are; but has to gather knowledge from individual things by way of the senses, as Dionysius says (Div. Nom. vii). But nature never fails in necessary things: therefore the intellectual soul had to be endowed not only with the power of understanding, but also with the power of feeling. Now the action of the senses is not performed without a corporeal instrument. Therefore it behooved the intellectual soul to be united to a body fitted to be a convenient organ of sense.

Now all the other senses are based on the sense of touch. But the organ of touch requires to be a medium between contraries, such as hot and cold, wet and dry, and the like, of which the sense of touch has the perception; thus it is in potentiality with regard to contraries, and is able to perceive them. Therefore the more the organ of touch is reduced to an equable complexion, the more sensitive will be the touch. But the intellectual soul has the power of sense in all its completeness; because what belongs to the inferior nature pre-exists more perfectly in the superior, as Dionysius says (Div. Nom. v). Therefore the body to which the intellectual soul is united should be a mixed body, above others reduced to the most equable complexion. For this reason among animals, man has the best sense of touch. And among men, those who have the best sense of touch have the best intelligence. A sign of which is that we observe "those who are refined in body are well endowed in mind," as stated in De Anima ii, 9.

Reply to Objection 1: Perhaps someone might attempt to answer this by saying that before sin the human body was incorruptible. This answer does not seem sufficient; because before sin the human body was immortal not by nature, but by a gift of Divine grace; otherwise its immortality would not be forfeited through sin, as neither was the immortality of the devil.

Therefore we answer otherwise by observing that in matter two conditions are to be found; one which is chosen in order that the matter be suitable to the form; the other which follows by force of the first disposition. The artisan, for instance, for the form of the saw chooses iron adapted for cutting through hard material; but that the teeth of the saw may become blunt and rusted, follows by force of the matter itself. So the intellectual soul requires a body of equable complexion, which, however, is corruptible by force of its matter. If, however, it be said that God could avoid this, we answer that in the formation of natural things we do not consider what God might do; but what is suitable to the nature of things, as Augustine says (Gen. ad lit. ii, 1). God, however, provided in this case by applying a remedy against death in the gift of grace.

Reply to Objection 2: A body is not necessary to the intellectual soul by reason of its intellectual operation considered as such; but on account of the sensitive power, which requires an organ of equable temperament. Therefore the intellectual soul had to be united to such a body, and not to a simple element, or to a mixed body, in which fire was in excess; because otherwise there could not be an equability of temperament. And this body of an equable temperament has a dignity of its own by reason of its being remote from contraries, thereby resembling in a way a heavenly body.

Reply to Objection 3: The parts of an animal, for instance, the eye, hand, flesh, and bones, and so forth, do not make the species; but the whole does, and therefore, properly speaking, we cannot say that these are of different species, but that they are of various dispositions. This is suitable to the intellectual soul, which, although it be one in its essence, yet on account of its perfection, is manifold in power: and therefore, for its various operations it requires various dispositions in the parts of the body to which it is united. For this reason we observe that there is a greater variety of parts in perfect than in imperfect animals; and in these a greater variety than in plants.

Reply to Objection 4: The intellectual soul as comprehending universals, has a power extending to the infinite; therefore it cannot be limited by nature to certain fixed natural notions, or even to certain fixed means whether of defence or of clothing, as is the case with other animals, the souls of which are endowed with knowledge and power in regard to fixed particular things. Instead of all these, man has by nature his reason and his hands, which are "the organs of organs" (De Anima iii), since by their means man can make for himself instruments of an infinite variety, and for any number of purposes.

Objection 1: It would seem that the intellectual soul is united to the body through the medium of accidental dispositions. For every form exists in its proper disposed matter. But dispositions to a form are accidents. Therefore we must presuppose accidents to be in matter before the substantial form; and therefore before the soul, since the soul is a substantial form.

Objection 2: Further, various forms of one species require various parts of matter. But various parts of matter are unintelligible without division in measurable quantities. Therefore we must suppose dimensions in matter before the substantial forms, which are many belonging to one species.

Objection 3: Further, what is spiritual is connected with what is corporeal by virtual contact. But the virtue of the soul is its power. Therefore it seems that the soul is united to the body by means of a power, which is an accident.

On the contrary, Accident is posterior to substance, both in the order of time and in the order of reason, as the Philosopher says, Metaph. vii (Did. vi, 1). Therefore it is unintelligible that any accidental form exist in matter before the soul, which is the substantial form.

I answer that, If the soul were united to the body, merely as a motor, there would be nothing to prevent the existence of certain dispositions mediating between the soul and the body; on the contrary, they would be necessary, for on the part of the soul would be required the power to move the body; and on the part of the body, a certain aptitude to be moved by the soul.

If, however, the intellectual soul is united to the body as the substantial form, as we have already said above (Article [1]), it is impossible for any accidental disposition to come between the body and the soul, or between any substantial form whatever and its matter. The reason is because since matter is in potentiality to all manner of acts in a certain order, what is absolutely first among the acts must be understood as being first in matter. Now the first among all acts is existence. Therefore, it is impossible for matter to be apprehended as hot, or as having quantity, before it is actual. But matter has actual existence by the substantial form, which makes it to exist absolutely, as we have said above (Article [4]). Wherefore it is impossible for any accidental dispositions to pre-exist in matter before the substantial form, and consequently before the soul.

Reply to Objection 1: As appears from what has been already said (Article [4]), the more perfect form virtually contains whatever belongs to the inferior forms; therefore while remaining one and the same, it perfects matter according to the various degrees of perfection. For the same essential form makes man an actual being, a body, a living being, an animal, and a man. Now it is clear that to every "genus" follow its own proper accidents. Therefore as matter is apprehended as perfected in its existence, before it is understood as corporeal, and so on; so those accidents which belong to existence are understood to exist before corporeity; and thus dispositions are understood in matter before the form, not as regards all its effects, but as regards the subsequent effect.

Reply to Objection 2: Dimensions of quantity are accidents consequent to the corporeity which belongs to the whole matter. Wherefore matter, once understood as corporeal and measurable, can be understood as distinct in its various parts, and as receptive of different forms according to the further degrees of perfection. For although it is essentially the same form which gives matter the various degrees of perfection, as we have said (ad 1), yet it is considered as different when brought under the observation of reason.

Reply to Objection 3: A spiritual substance which is united to a body as its motor only, is united thereto by power or virtue. But the intellectual soul is united by its very being to the body as a form; and yet it guides and moves the body by its power and virtue.

Objection 1: It seems that the soul is united to the animal body by means of a body. For Augustine says (Gen. ad lit. vii, 19), that "the soul administers the body by light," that is, by fire, "and by air, which is most akin to a spirit." But fire and air are bodies. Therefore the soul is united to the human body by means of a body.

Objection 2: Further, a link between two things seems to be that thing the removal of which involves the cessation of their union. But when breathing ceases, the soul is separated from the body. Therefore the breath, which is a subtle body, is the means of union between soul and body.

Objection 3: Further, things which are very distant from one another, are not united except by something between them. But the intellectual soul is very distant from the body, both because it is incorporeal, and because it is incorruptible. Therefore it seems to be united to the body by means of an incorruptible body, and such would be some heavenly light, which would harmonize the elements, and unite them together.

On the contrary, The Philosopher says (De Anima ii, 1): "We need not ask if the soul and body are one, as neither do we ask if wax and its shape are one." But the shape is united to the wax without a body intervening. Therefore also the soul is thus united to the body.

I answer that, If the soul, according to the Platonists, were united to the body merely as a motor, it would be right to say that some other bodies must intervene between the soul and body of man, or any animal whatever; for a motor naturally moves what is distant from it by means of something nearer.

If, however, the soul is united to the body as its form, as we have said (Article [1]), it is impossible for it to be united by means of another body. The reason of this is that a thing is one, according as it is a being. Now the form, through itself, makes a thing to be actual since it is itself essentially an act; nor does it give existence by means of something else. Wherefore the unity of a thing composed of matter and form, is by virtue of the form itself, which by reason of its very nature is united to matter as its act. Nor is there any other cause of union except the agent, which causes matter to be in act, as the Philosopher says, Metaph. viii (Did. vii, 6).

From this it is clear how false are the opinions of those who maintained the existence of some mediate bodies between the soul and body of man. Of these certain Platonists said that the intellectual soul has an incorruptible body naturally united to it, from which it is never separated, and by means of which it is united to the corruptible body of man. Others said that the soul is united to the body by means of a corporeal spirit. Others said it is united to the body by means of light, which, they say, is a body and of the nature of the fifth essence; so that the vegetative soul would be united to the body by means of the light of the sidereal heaven; the sensible soul, by means of the light of the crystal heaven; and the intellectual soul by means of the light of the empyrean heaven. Now all this is fictious and ridiculous: for light is not a body; and the fifth essence does not enter materially into the composition of a mixed body (since it is unchangeable), but only virtually: and lastly, because the soul is immediately united to the body as the form to matter.

Reply to Objection 1: Augustine speaks there of the soul as it moves the body; whence he uses the word "administration." It is true that it moves the grosser parts of the body by the more subtle parts. And the first instrument of the motive power is a kind of spirit, as the Philosopher says in De causa motus animalium (De mot. animal. x).

Reply to Objection 2: The union of soul and body ceases at the cessation of breath, not because this is the means of union, but because of the removal of that disposition by which the body is disposed for such a union. Nevertheless the breath is a means of moving, as the first instrument of motion.

Reply to Objection 3: The soul is indeed very distant from the body, if we consider the condition of each separately: so that if each had a separate existence, many means of connection would have to intervene. But inasmuch as the soul is the form of the body, it has not an existence apart from the existence of the body, but by its own existence is united to the body immediately. This is the case with every form which, if considered as an act, is very distant from matter, which is a being only in potentiality.

Objection 1: It would seem that the whole soul is not in each part of the body; for the Philosopher says in De causa motus animalium (De mot. animal. x): "It is not necessary for the soul to be in each part of the body; it suffices that it be in some principle of the body causing the other parts to live, for each part has a natural movement of its own."

Objection 2: Further, the soul is in the body of which it is the act. But it is the act of an organic body. Therefore it exists only in an organic body. But each part of the human body is not an organic body. Therefore the whole soul is not in each part.

Objection 3: Further, the Philosopher says (De Anima. ii, 1) that the relation of a part of the soul to a part of the body, such as the sight to the pupil of the eye, is the same as the relation of the soul to the whole body of an animal. If, therefore, the whole soul is in each part of the body, it follows that each part of the body is an animal.

Objection 4: Further, all the powers of the soul are rooted in the essence of the soul. If, therefore, the whole soul be in each part of the body, it follows that all the powers of the soul are in each part of the body; thus the sight will be in the ear, and hearing in the eye, and this is absurd.

Objection 5: Further, if the whole soul is in each part of the body, each part of the body is immediately dependent on the soul. Thus one part would not depend on another; nor would one part be nobler than another; which is clearly untrue. Therefore the soul is not in each part of the body.

On the contrary, Augustine says (De Trin. vi, 6), that "in each body the whole soul is in the whole body, and in each part is entire."

I answer that, As we have said, if the soul were united to the body merely as its motor, we might say that it is not in each part of the body, but only in one part through which it would move the others. But since the soul is united to the body as its form, it must necessarily be in the whole body, and in each part thereof. For it is not an accidental form, but the substantial form of the body. Now the substantial form perfects not only the whole, but each part of the whole. For since a whole consists of parts, a form of the whole which does not give existence to each of the parts of the body, is a form consisting in composition and order, such as the form of a house; and such a form is accidental. But the soul is a substantial form; and therefore it must be the form and the act, not only of the whole, but also of each part. Therefore, on the withdrawal of the soul, as we do not speak of an animal or a man unless equivocally, as we speak of a painted animal or a stone animal; so is it with the hand, the eye, the flesh and bones, as the Philosopher says (De Anima ii, 1). A proof of which is, that on the withdrawal of the soul, no part of the body retains its proper action; although that which retains its species, retains the action of the species. But act is in that which it actuates: wherefore the soul must be in the whole body, and in each part thereof.

That it is entire in each part thereof, may be concluded from this, that since a whole is that which is divided into parts, there are three kinds of totality, corresponding to three kinds of division. There is a whole which is divided into parts of quantity, as a whole line, or a whole body. There is also a whole which is divided into logical and essential parts: as a thing defined is divided into the parts of a definition, and a composite into matter and form. There is, further, a third kind of whole which is potential, divided into virtual parts. The first kind of totality does not apply to forms, except perhaps accidentally; and then only to those forms, which have an indifferent relationship to a quantitative whole and its parts; as whiteness, as far as its essence is concerned, is equally disposed to be in the whole surface and in each part of the surface; and, therefore, the surface being divided, the whiteness is accidentally divided. But a form which requires variety in the parts, such as a soul, and specially the soul of perfect animals, is not equally related to the whole and the parts: hence it is not divided accidentally when the whole is divided. So therefore quantitative totality cannot be attributed to the soul, either essentially or accidentally. But the second kind of totality, which depends on logical and essential perfection, properly and essentially belongs to forms: and likewise the virtual totality, because a form is the principle of operation.

Therefore if it be asked whether the whole whiteness is in the whole surface and in each part thereof, it is necessary to distinguish. If we mean quantitative totality which whiteness has accidentally, then the whole whiteness is not in each part of the surface. The same is to be said of totality of power: since the whiteness which is in the whole surface moves the sight more than the whiteness which is in a small part thereof. But if we mean totality of species and essence, then the whole whiteness is in each part of a surface.

Since, however, the soul has not quantitative totality, neither essentially, nor accidentally, as we have seen; it is enough to say that the whole soul is in each part of the body, by totality of perfection and of essence, but not by totality of power. For it is not in each part of the body, with regard to each of its powers; but with regard to sight, it is in the eye; and with regard to hearing, it is in the ear; and so forth. We must observe, however, that since the soul requires variety of parts, its relation to the whole is not the same as its relation to the parts; for to the whole it is compared primarily and essentially, as to its proper and proportionate perfectible; but to the parts, secondarily, inasmuch as they are ordained to the whole.

Reply to Objection 1: The Philosopher is speaking there of the motive power of the soul.

Reply to Objection 2: The soul is the act of an organic body, as of its primary and proportionate perfectible.

Reply to Objection 3: An animal is that which is composed of a soul and a whole body, which is the soul's primary and proportionate perfectible. Thus the soul is not in a part. Whence it does not follow that a part of an animal is an animal.

Reply to Objection 4: Some of the powers of the soul are in it according as it exceeds the entire capacity of the body, namely the intellect and the will; whence these powers are not said to be in any part of the body. Other powers are common to the soul and body; wherefore each of these powers need not be wherever the soul is, but only in that part of the body, which is adapted to the operation of such a power.

Reply to Objection 5: One part of the body is said to be nobler than another, on account of the various powers, of which the parts of the body are the organs. For that part which is the organ of a nobler power, is a nobler part of the body: as also is that part which serves the same power in a nobler manner.