Summa Theologica, Part I (Prima Pars) From the Complete...

Having treated of the spiritual and of the corporeal creature, we now proceed to treat of man, who is composed of a spiritual and corporeal substance. We shall treat first of the nature of man, and secondly of his origin. Now the theologian considers the nature of man in relation to the soul; but not in relation to the body, except in so far as the body has relation to the soul. Hence the first object of our consideration will be the soul. And since Dionysius (Ang. Hier. xi) says that three things are to be found in spiritual substances—essence, power, and operation—we shall treat first of what belongs to the essence of the soul; secondly, of what belongs to its power; thirdly, of what belongs to its operation.

Objection 1: It would seem that the soul is a body. For the soul is the moving principle of the body. Nor does it move unless moved. First, because seemingly nothing can move unless it is itself moved, since nothing gives what it has not; for instance, what is not hot does not give heat. Secondly, because if there be anything that moves and is not moved, it must be the cause of eternal, unchanging movement, as we find proved Phys. viii, 6; and this does not appear to be the case in the movement of an animal, which is caused by the soul. Therefore the soul is a mover moved. But every mover moved is a body. Therefore the soul is a body.

I answer that, To seek the nature of the soul, we must premise that the soul is defined as the first principle of life of those things which live: for we call living things "animate," [*i.e. having a soul] and those things which have no life, "inanimate." Now life is shown principally by two actions, knowledge and movement. The philosophers of old, not being able to rise above their imagination, supposed that the principle of these actions was something corporeal: for they asserted that only bodies were real things; and that what is not corporeal is nothing: hence they maintained that the soul is something corporeal. This opinion can be proved to be false in many ways; but we shall make use of only one proof, based on universal and certain principles, which shows clearly that the soul is not a body.

It is manifest that not every principle of vital action is a soul, for then the eye would be a soul, as it is a principle of vision; and the same might be applied to the other instruments of the soul: but it is the first principle of life, which we call the soul. Now, though a body may be a principle of life, as the heart is a principle of life in an animal, yet nothing corporeal can be the first principle of life. For it is clear that to be a principle of life, or to be a living thing, does not belong to a body as such; since, if that were the case, every body would be a living thing, or a principle of life. Therefore a body is competent to be a living thing or even a principle of life, as "such" a body. Now that it is actually such a body, it owes to some principle which is called its act. Therefore the soul, which is the first principle of life, is not a body, but the act of a body; thus heat, which is the principle of calefaction, is not a body, but an act of a body.

Reply Obj. 1: As everything which is in motion must be moved by something else, a process which cannot be prolonged indefinitely, we must allow that not every mover is moved. For, since to be moved is to pass from potentiality to actuality, the mover gives what it has to the thing moved, inasmuch as it causes it to be in act. But, as is shown in Phys. viii, 6, there is a mover which is altogether immovable, and not moved either essentially, or accidentally; and such a mover can cause an invariable movement. There is, however, another kind of mover, which, though not moved essentially, is moved accidentally; and for this reason it does not cause an invariable movement; such a mover, is the soul. There is, again, another mover, which is moved essentially—namely, the body. And because the philosophers of old believed that nothing existed but bodies, they maintained that every mover is moved; and that the soul is moved directly, and is a body.

Reply Obj. 2: The likeness of a thing known is not of necessity actually in the nature of the knower; but given a thing which knows potentially, and afterwards knows actually, the likeness of the thing known must be in the nature of the knower, not actually, but only potentially; thus color is not actually in the pupil of the eye, but only potentially. Hence it is necessary, not that the likeness of corporeal things should be actually in the nature of the soul, but that there be a potentiality in the soul for such a likeness. But the ancient philosophers omitted to distinguish between actuality and potentiality; and so they held that the soul must be a body in order to have knowledge of a body; and that it must be composed of the principles of which all bodies are formed in order to know all bodies.

Objection 1: It would seem that the human soul is not something subsistent. For that which subsists is said to be "this particular thing." Now "this particular thing" is said not of the soul, but of that which is composed of soul and body. Therefore the soul is not something subsistent.

Obj. 2: Further, everything subsistent operates. But the soul does not operate; for, as the Philosopher says (De Anima i, 4), "to say that the soul feels or understands is like saying that the soul weaves or builds." Therefore the soul is not subsistent.

Obj. 3: Further, if the soul were subsistent, it would have some operation apart from the body. But it has no operation apart from the body, not even that of understanding: for the act of understanding does not take place without a phantasm, which cannot exist apart from the body. Therefore the human soul is not something subsistent.

On the contrary, Augustine says (De Trin. x, 7): "Who understands that the nature of the soul is that of a substance and not that of a body, will see that those who maintain the corporeal nature of the soul, are led astray through associating with the soul those things without which they are unable to think of any nature—i.e. imaginary pictures of corporeal things." Therefore the nature of the human intellect is not only incorporeal, but it is also a substance, that is, something subsistent.

I answer that, It must necessarily be allowed that the principle of intellectual operation which we call the soul, is a principle both incorporeal and subsistent. For it is clear that by means of the intellect man can have knowledge of all corporeal things. Now whatever knows certain things cannot have any of them in its own nature; because that which is in it naturally would impede the knowledge of anything else. Thus we observe that a sick man's tongue being vitiated by a feverish and bitter humor, is insensible to anything sweet, and everything seems bitter to it. Therefore, if the intellectual principle contained the nature of a body it would be unable to know all bodies. Now every body has its own determinate nature. Therefore it is impossible for the intellectual principle to be a body. It is likewise impossible for it to understand by means of a bodily organ; since the determinate nature of that organ would impede knowledge of all bodies; as when a certain determinate color is not only in the pupil of the eye, but also in a glass vase, the liquid in the vase seems to be of that same color.

Therefore the intellectual principle which we call the mind or the intellect has an operation per se apart from the body. Now only that which subsists can have an operation per se. For nothing can operate but what is actual: for which reason we do not say that heat imparts heat, but that what is hot gives heat. We must conclude, therefore, that the human soul, which is called the intellect or the mind, is something incorporeal and subsistent.

Reply Obj. 1: "This particular thing" can be taken in two senses. Firstly, for anything subsistent; secondly, for that which subsists, and is complete in a specific nature. The former sense excludes the inherence of an accident or of a material form; the latter excludes also the imperfection of the part, so that a hand can be called "this particular thing" in the first sense, but not in the second. Therefore, as the human soul is a part of human nature, it can indeed be called "this particular thing," in the first sense, as being something subsistent; but not in the second, for in this sense, what is composed of body and soul is said to be "this particular thing."

Reply Obj. 2: Aristotle wrote those words as expressing not his own opinion, but the opinion of those who said that to understand is to be moved, as is clear from the context. Or we may reply that to operate per se belongs to what exists per se. But for a thing to exist per se, it suffices sometimes that it be not inherent, as an accident or a material form; even though it be part of something. Nevertheless, that is rightly said to subsist per se, which is neither inherent in the above sense, nor part of anything else. In this sense, the eye or the hand cannot be said to subsist per se; nor can it for that reason be said to operate per se. Hence the operation of the parts is through each part attributed to the whole. For we say that man sees with the eye, and feels with the hand, and not in the same sense as when we say that what is hot gives heat by its heat; for heat, strictly speaking, does not give heat. We may therefore say that the soul understands, as the eye sees; but it is more correct to say that man understands through the soul.

Reply Obj. 3: The body is necessary for the action of the intellect, not as its origin of action, but on the part of the object; for the phantasm is to the intellect what color is to the sight. Neither does such a dependence on the body prove the intellect to be non-subsistent; otherwise it would follow that an animal is non-subsistent, since it requires external objects of the senses in order to perform its act of perception. _______________________

Objection 1: It would seem that the souls of brute animals are subsistent. For man is of the same genus as other animals; and, as we have just shown (A. 2), the soul of man is subsistent. Therefore the souls of other animals are subsistent.

Obj. 2: Further, the relation of the sensitive faculty to sensible objects is like the relation of the intellectual faculty to intelligible objects. But the intellect, apart from the body, apprehends intelligible objects. Therefore the sensitive faculty, apart from the body, perceives sensible objects. Therefore, since the souls of brute animals are sensitive, it follows that they are subsistent; just as the human intellectual soul is subsistent.

I answer that, The ancient philosophers made no distinction between sense and intellect, and referred both to a corporeal principle, as has been said (A. 1). Plato, however, drew a distinction between intellect and sense; yet he referred both to an incorporeal principle, maintaining that sensing, just as understanding, belongs to the soul as such. From this it follows that even the souls of brute animals are subsistent. But Aristotle held that of the operations of the soul, understanding alone is performed without a corporeal organ. On the other hand, sensation and the consequent operations of the sensitive soul are evidently accompanied with change in the body; thus in the act of vision, the pupil of the eye is affected by a reflection of color: and so with the other senses. Hence it is clear that the sensitive soul has no per se operation of its own, and that every operation of the sensitive soul belongs to the composite. Wherefore we conclude that as the souls of brute animals have no per se operations they are not subsistent. For the operation of anything follows the mode of its being.