The Aristotelian/Thomistic account of the soul is part and parcel of Natural Philosophy. It makes use, therefore, of the notions of matter and form, potency and act. Aristotle defines the soul as the act of a natural body with the capacity for life; and as the first act of a natural organic body. Soul is thus the formal cause of the animal, the efficient cause of its motions, as well as its final cause. The body cannot be the principle that accounts for life, since a body, when deprived of life, is still a body, but not alive. The body is matter to the soul, and soul is form or act to the potentiality of the body. Moreover, the matter, i.e. the constituents that make up the body, are constantly changing while the animal persists. The animal’s form or functional organization, i.e. organization of material parts by which an animal accomplishes its vital functions, remains the same. This form is the animal’s soul.
There is a hierarchy of vital functions, and thus of different kinds of souls. First of all, there is the vegetative soul which accounts for the functions of nutrition and reproduction. Plants have only this kind of soul. Next, there is the sensitive soul, by which higher animals perceive and respond to their environment. This kind of soul, for some animals, also includes the power of local motion. Finally, there is the rational soul, by which humans are able to use speech and have abstract thoughts. In all of the higher kinds of organisms, the functions that were performed by lower kinds of souls are performed by the higher. Thus, there is only one soul in any particular animal even though it is has the same vegetative capacities as plants. The vegetative functions, which are performed by a plant’s soul without sensitive functions, are also performed by the sensitive soul. Likewise, the rational soul is the principle also of sensitive and vegetative functions of human beings. Thus there is a hierarchy of souls and of vital functions, such that the higher souls subsume the lower, but the lower vital functions are necessary for there to be higher ones. The higher are never found without the lower, but the lower are found without the higher. Moreover, there is an interaction between the capacities that characterize higher and lower souls: a lion uses sight to find food, and moves toward the lamb it spies, which it then eats and digests so that it may chase other prey.
Aristotle, thus, opposes Platonic or Cartesian dualism. Body and soul together make up one substance. A major problem that Aristotle and Aquinas see with dualism is that it cannot explain why the soul, if it essentially different from and superior to the body, should be united to the body. For Aristotle and Aquinas, however, it is for the good of the soul (or rather, it is for the good of the composite which has its vital activities in virtue of its soul) that the soul is united to the body; a body is necessary for a soul to exercise all vital capacities, since (almost) all vital functions are the functions of body and soul together. The sensitive soul requires a body, since the acts of sensation, of seeing, for example, require bodily organs. Similarly, the act of intellection, which is proper to humans alone, requires sensation, and sensation in turn requires a body. Thus, if human beings are to exercise their proper functions, they necessarily must have a body.
Aquinas, interestingly, appeals to personal experience in his claim that a person is not his soul, or his intellect, alone, as Plato and Descartes claim. A man cannot be merely a mind without a body
because 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, and therefore the body must be some part of man.Summa Theologiae Ia 76, 1.
If a man were just a mind, essentially unrelated to the body, he would not directly experience things that happen to the body, as he clearly does when he senses. Therefore, the human soul is in essence the substantial form of a human body, and body and soul together make up one substance.
The Immateriality of the Intellect
Nevertheless, Aquinas believes too, that the soul of man is a subsistent spiritual reality. He argues that because man is able to know all bodily natures by means of his intellect, his intellect cannot have in itself a bodily nature. Having in itself a bodily nature would prevent the reception, and thus the knowledge, of any other bodily nature, since, for Aquinas, one knows by receiving the forms of what one knows into one’s intellect. Thus, if the intellect had a bodily nature, it would not be able to receive the forms of these things; but since it does receive these forms, it lacks any bodily nature.
Therefore, the intellectual principle, which we call the mind or the intellect, has an operation in which the body does not share. Now only that which subsists in itself can have an operation in itself. … We must conclude, therefore, that the human soul, which is called intellect or mind, is something incorporeal and subsistent.S.T. Ia, 75, 2.
Spiritual Soul as Form of the Body
This creates a tension for Aquinas. On the one hand he believes that the human soul is the form of the body, the principle by which the body lives, and the principle in virtue of which bodily activities, i.e. sensation, take place. And such activities, being the direct experience of man, implies that man is composed of body and soul. Nevertheless, man also has activities which do NOT involve the body, i.e. intellection. (See Summa Contra Gentiles, II, 56) Thus, he believes that the soul exists of itself, separate from the body. It is difficult to reconcile these two positions (the soul is the form of a body, the soul exists of itself without need of the body), since every other soul that is the form of a body CANNOT exist without that body, e.g. the souls of animals. (S.T. Ia, 75, 3) Some charge that this tension is so great as to render Aquinas’ account of the soul incoherent.
Aquinas’ answer is that the soul has its own act of existence which it communicates to the body, but that, without the body, it is not a complete substance (since it has an essential relation to the body). (S.C.G II, 68) Consequently, without the body it cannot exercise any of its natural activities. Thus, the rational soul can exist without the body, but it cannot do anything in, what is for it, an unnatural state. The separated soul, then, needs God either to reunite it with its body, or infuse it with knowledge, both of which would be supernatural gifts.