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 (75, 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.
Source: Summa Theologica (New Advent)