Commentaria in Summam contra Gentiles, Liber II

The reasons having been set forth by which some proved that an intellectual substance cannot be the form of a body, Saint Thomas now disputes against the opinions of those who think in this way¹. Concerning this he does two things. First, indeed, he disputes against the opinions of those who judged that the intellective soul, on account of which this difficulty was raised, is an immaterial substance; second, against the opinions of those who judged it to be some material disposition, chapter LXII.

On the first point he does two things², in accordance with his dispute against two opinions of Plato. First, indeed, he inquires whether the intellective soul is united to the body only as its mover. Second, whether it is distinct within us from the vegetative and sensitive soul, in the chapter that follows.

I. As to the first, then: since Plato held that the intellectual soul is not the substantial form of the human body, in order to preserve the nature of man, he said that it is united to the body only through contact of power, as Gregory of Nyssa relates in his book which he composed on the Soul³. For he said that the soul is in the body as a sailor is in a ship. Wishing to argue against this position, Saint Thomas first proves that the intellectual soul is not united to the body merely as a mover; second, that it is united to it as form⁴.

As to the first, he argues first as follows⁵ It follows from this position that man is not one simply, and consequently not a being simply either, but a being per accidens: for from the contact of a power there does not result something one simply, as was shown above; yet man results from soul and body. But this is unfitting. Therefore, and so on.

II. If it is said, as Plato held, that man is not composed of soul and body, but is the soul itself using the body (just as Peter is not something compounded of man and clothing, but a man using clothing): against this it is argued in two ways.

First. It would follow that man and animal would not be any sensible material things: because the soul is not anything sensible and material. But this is false. Therefore, etc.

Second. There are certain operations common to soul and body, such as fearing, becoming angry, sensing, and the like, since they occur by way of a transmutation of some determinate part of the body. Therefore, etc. The consequence is proved: because it is impossible that things diverse in esse have one operation as it proceeds from the agent, although it can be one on the side of what is acted upon. For since action follows form and power, those whose forms and powers are diverse must also have diverse actions.

III. If it is said, according to Plato's opinion, that the aforementioned operations are common to soul and body, the soul indeed as mover, but the body as moved; and that it is not unfitting for mover and moved, although they differ in being, to share one and the same act — this is argued against.

Then, because the sensitive soul is not related in sensing as an agent and mover of the body, but as that by which the patient is acted upon by sensible things—which cannot be diverse in esse from the patient. For things lacking sense are not acted upon by sensible things in that mode of passivity which is required for sensing.

Then, because on this account the sensitive soul will have some operation proper to itself: for although motion is a common act of the mover and the moved, yet to produce motion is one operation, and to receive motion is another. And thus it will follow that it will have its own subsistence. And consequently it will not be destroyed when the body is destroyed, but will be immortal. Which seems improbable, as will be shown below: although it does not disagree with Plato's opinion.

2. It must be noted, with respect to the first objection against the solution given, that although sense is a passive power, and therefore it is necessary that to sense be a kind of being acted upon and being moved by external sensible things, as is said in II De Anima; and consequently that in sensing the sense is not disposed as mover but as a moved power, and as that by which the organ is moved by a spiritual motion: nevertheless, the sense is not called a passive power, nor is sensing said to be moved and acted upon, as if the sense had no act which would be an operation. For it is manifestly false that this is so, since sensation is a certain operation of the sense by which it judges of the sensible. But, since a power can be compared to its object in two ways—namely, either according as it exists absolutely in nature, or according as it is already united to the power through its similitude—something is properly said to be a passive or active power by comparison to the object considered in nature, not however as it is considered as united to the power through its similitude. For if the power is transmutative of the object itself, it is called an active power, for it is compared to the object as agent to patient: just as the heating power of fire is an active power, because it is transmutative of what is heatable. But if the power itself is transmutable by the object, then it is passive: because it is compared to the object as patient to agent. From the fact, however, that a power is compared to its object as it is in nature as patient to agent, it follows that the similitude of the object received in the power is compared to the power itself as act and form to subject. And because it is not unfitting for a subject and its act to concur together in one productive operation, nothing prevents some power from being passive, and yet at the same time concurring with the similitude of the object received in it toward some operation: just as what is heatable, together with the form of heat caused in it by something hot, can give heat.

Sense, therefore, is a passive potency by which the organ is acted upon by the sensible through the reception of the sensible species; and yet it is the principle of the operation of sensing. And so, when the Philosopher says that sensing occurs in being moved by sensibles, this does not mean that to be moved by sensibles is essentially to sense: for sensing is not to receive (except as something antecedent), but to operate, because it is to judge about a sensible thing. Rather, the meaning is that sensing, if considered under the aspect of transmutation, belongs not from the fact that the sense or its organ transmutes itself or the object, but from the fact that the sense and its organ are transmuted by sensibles. For sense cannot sense unless it receives within itself a likeness of the object: which is to be moved and to be acted upon.

3. In the second objection, where it is said that to cause motion and to receive motion are different operations, it must be noted that operation is not taken here for the act produced by that of which it is called an operation, since to receive motion is not an act produced by the mobile thing; rather, in accordance with what is said in Book III of this work, chapter XXII, operation is taken as meaning that the ultimate means by which each thing is ordered to its end is called its operation; according to which mode, being heated and being cut are said to be the operation of things heatable and cuttable.

IV. Concerning the foundation of the principal argument ⁶, namely that it is impossible for the operation of diverse things to be one insofar as it proceeds from the agent, although it can be one on the side of the thing acted upon, a doubt arises. For if action, as was determined above ⁷, posits nothing absolute in the agent, but is only the very motion received in the patient; if the action is one on the side of the thing acted upon, it must also be one insofar as it proceeds from the agent.

It is answered by maintaining that action posits nothing absolute in the agent; for although materially it signifies only the motion or change received in the patient, formally, however, it connotes a respect of origin in the agent. And therefore where several agents concur toward one effect, although the motion is one, there are nevertheless several respects of origin in the several agents. Hence it can be said that there is one action materially, but many formally. And thus the example of St. Thomas is understood, when he says that, with many men drawing a ship, on the part of those drawing there are many actions, because there are diverse impulses toward drawing: for they are many formally, on account of the diverse impulses, that is, the diverse applications of powers to drawing, in which is signified that there are diverse respects of origin in those drawing the ship, just as their powers are diverse; although materially the action is one. St. Thomas therefore understands the assumed proposition of the formal unity of the action, which results from the unity of the respect imported by the action: but not of its material unity, from which the objection proceeds.

V. Second principal argument ⁸. It follows that the body and its parts do not receive their species from the soul. Therefore, when the soul departs, the body and its parts will remain of the same species. But that this is false is evident: since then no proper operation would be present in any of the parts, which is what follows upon the species. Therefore the soul is not united to the body merely as its mover. — The inference is proved: because a movable thing does not formally receive its species from its mover as such.

Third. A body will not have being through the soul: because what is moved does not have being through its mover (that is, through one which is a mover only). Therefore neither will it live through the soul: because for living things, to live is to be. This is false. Therefore etc.

Fourth. It follows that in the union of the soul to the body there will be no generation, nor in its separation any corruption. And consequently death will not be a corruption of the animal. — The consequence is proved. Because the mobile is not generated by the application of the mover to it, nor is it corrupted by its separation: since the mobile does not depend on the mover as such with respect to being, but only with respect to being moved.

Fifth. It will be within the power of the soul to separate itself from the body whenever it wills, and to be united to it again whenever it wills. That this is false is evident. — The consequence is proved. For, according to Plato, the soul moves the body as a self-mover. But a self-mover has in itself both to move and not to move. If, therefore, it is united to the body only as mover, it will be able to be united and not united to it as it wills.

VI. As to the second ⁹, Saint Thomas proves that the soul is united to the body as its form.

And it is argued first as follows. Through the soul the body becomes actually a being from being in potency. Therefore, etc. — The antecedent is proved. Because the seed before animation is living only in potency, but through the soul it becomes actually living. — The consequence is also proved. Because that by which something becomes from being in potency to being actually is its form and act.

2. Concerning the proof of the antecedent, a doubt arises. For, as is held in Physics II ¹⁰ and Metaphysics V ¹¹, the seed stands in generation as efficient cause. But matter and the efficient cause do not coincide in one and the same thing numerically. Therefore the seed is not the matter of the living being. The opposite of this the Angelic Doctor seems to will, when he says that before animation the seed lives only in potency, but through the soul becomes living in act.

To make this evident, it must be known that the opinion of Aristotle, as is clear from ¹², was that the semen of an animal does not contribute to generation as matter, but only as agent, insofar as in it there is the power of the generator. Hence also in Physics II and Metaphysics V he places semen in the genus of efficient cause. — Others, however, hold that semen contributes to generation as matter, from which through many transmutations the body is formed.

Following, therefore, the opinion of Aristotle, it cannot be understood that what is said here concerns seed severed from the generator; rather, by the name of seed one must understand the menstrual blood from which the body of a perfect animal is generated, or whatever else stands as the proximate matter of the generation of a living thing.

But if we wish to follow the opinion of others, we may say, according to the teaching of Saint Thomas, Potentia, q. III, a. 9 and 11, in the replies to the objections, and below in this Second Part, chapter LXXXIX, that the semen of an animal contributes both effectively and materially to generation, in respect of the different things found within it. For there is in the semen a corporeal substance, from which, through many transformations, the body is formed: and in this respect the semen stands as matter. There is also contained within it a spirit, in which there resides a formative power as in its proper subject, and which is not destroyed but remains even after generation is complete: and in this respect it contributes effectively to generation. Therefore Saint Thomas, if we wish to follow this opinion, speaks of the semen with regard to its corporeal substance, but not with regard to the spirit contained within it. And thus it is clear that it does not follow that matter and agent coincide in one and the same thing numerically.

VII. Second. To live and to sense are attributed to the soul as to the principle of life and of sense. Therefore the soul is the form of the body. — The consequence is proved: because, since esse and operating belong to a composite, they are attributed to two things, one of which is related to the other as form to matter. Therefore, if living is attributed to the soul as to principle, but to the body as to matter, the soul itself will be the form of the body.

Third. A part of the soul is related to a part of the body as its form and act: for sight is the form of the eye. Therefore the soul is the form and act of the body. — The consequence is proved. Because the whole sensitive soul is related to the whole body just as a part to a part.

Note that Saint Thomas is speaking of a potential part of the soul, not an integral or essential one: for the soul does not have such parts, at least the soul of the more perfect animals.

Note also that the eye can be considered in two ways. In one way, materially, that is, with respect to its substance. And in this way sight is not its form and act (I speak of the form constitutive of the eye), but the very substance of the soul. — In another way, formally, as it is the organ and instrument of the soul by which the animal sees. And in this way sight is the form constitutive of the eye: for the eye is apt for seeing on account of the visual power. Hence, if the visual power itself is absent, it cannot be an instrument for seeing, and it is no longer formally an eye, although it can remain materially an eye. — Saint Thomas, therefore, speaks here of the eye in the second way, not the first.