Aquinas: Soul and Body

The Philosophical System of Thomas Aquinas, by Maurice de Wulf, edited and adapted for the web by by Jonathan Dolhenty

X. Soul and Body

A. The substantial Ego

The subject matter of scholastic psychology is not mere consciousness, or any single human function, but the whole man, the ego with the manifold activities of which he is the source. Even organic operations of nutrition and locomotion were dealt with in psychology. All these functions arise from one single source: the human ego. It is the same ego that eats, digests, moves, knows, wills, or suffers. This is so true that the intense exercise of one function can hinder the exercise of others. Thus, when I am digesting my dinner, I find the work of thought more difficult.

The ego is a substance, in other words a reality which is capable of existing by itself, in the sense that it does not exist in something else (VIII, B). Moreover, the ego is an individual or complete substance. It is only the individual human being as a whole that exists. To such an individual we give the name of ‘person,’ in order to bring out the fact that in the human species the individual subject is endowed with reason. The definition of Boethius still holds good: persona est rationalis naturae individua substantia, an individual substance of a rational nature. The true and unique human reality is therefore this particular human substance, this individual human being, which in the ordinary course of things is this person. To speak of ‘collective personality,’ or of a personality which would include other persons as parts, is to weave a concept from mutually contradictory notions. Indeed the members of such a collective personality could not themselves be persons, since a person must be independent of all other beings. Moreover consciousness naturally protests against the compenetration of my ego with another. We need not add that such a compenetration would mean the destruction of the freedom of the individual. Already we can see why scholastic moral and social philosophy emphasizes the value of individual personality, the psychological foundations of which are here laid down.

How does Thomas Aquinas prove the substantial and individual nature of the ego? He does so in arguing from consciousness, which testifies to its existence and to its permanence. Consciousness directly grasps my substantial ego in and through my activities. In thinking, in taking decisions, in walking, I attain to my own existing substance. However, it is important to note, that consciousness reveals only the existence of the ego, and teaches us nothing concerning its inmost nature. It tells us that the ego exists, not in what it consists. The best proof of this is the disagreement amongst thinkers concerning the nature of the ego, of the soul, or of man in general.

The permanence of the ego, as witnessed by memory, furnishes another demonstration that it is really and truly an individual substance. At the present moment I realize that I am the same person that I was five years ago, in spite of my many changes and activities since then. This permanence is an indication of the fact that I exist in myself, by my own right, so to speak.

B. Plurality of faculties

In order to harmonize the unity of the ego on the one hand, and the varied character of its functionings on the other, Scholasticism attributes such activities as cannot be mutually identified, such as nutrition, movement, sense knowledge, knowledge by abstraction, will, to immediate sources known as ‘faculties’ (VIII, C). Thomas maintained that these faculties are really distinct from the ego. Doubtless, in the last analysis, it is the man who acts, but he acts by means of his faculties, which are deeply rooted in what may be called the substance of the man, but are at the same time distinct from it. Moreover, Thomas teaches that man’s faculties of action are not only distinct from his substance, but that they are also really distinct from each other, e.g., intelligence from will. He bases this teaching upon the fact that they mutually influence each other, and that one and the same thing cannot be the subject and object of an action.

This already shows us that the whole doctrine is the result not of an intuition but of a reasoning process. The classification of the proximate principles of human action or faculties reduces itself to a catalogue of those activities of the ego which cannot be identified with each other. It is not a psychological, but a metaphysical explanation. Consciousness tells us nothing about the faculties or energies of the ego, apart from their exercise. Apart from thought, the mind remains a mystery to itself forever. “The human intellect has within itself the power of understanding, but not of being understood except in so far as it is in a state of activity” [1]. There are no means of getting at the mind-in-itself, nor of saying beforehand, as Fichte did, what objects it is capable of attaining. Nor does the theory of faculties tells us anything more concerning the precise nature of the action. For instance, to know that vision is a faculty adds nothing to our understanding of the activity of sight itself, but it sheds light upon the internal constitution of the acting subject; from the specific differences of human activities, it becomes evident that manifold principles of action must exist in one subject. Critics of this theory must bear in mind the elementary principle that we must not demand from the theory of faculties what it does not profess to give.

The same reasoning process which informs us of the existence of faculties also teaches us that the ego is composed of a soul and a body.

C. Soul and body

The substantial ego, or human individual, is not a simple being, but one composed of a body and a soul. This leads us to the current definition of man: a ‘rational animal’ (definition by logical parts) or ‘a compound of body and soul’ (definition by real parts). Like the other living organisms — plant or animal, unicellular or higher organism — man is regarded as a compound made up of a body which plays the part of ‘matter’ and of a soul which acts as the ‘substantial form.’ If we recall what has been said in the previous part about matter and form, we shall understand the role of the soul and the body in man.

In the first place, since man really is a single whole, he is not a compound of two independent substances, as Plato and Augustine held, but one substance. It is true that the extended body and the soul are parts of man, and parts of a substantial kind, since neither the soul not the body exist in something else: but neither the soul nor the body alone is complete, or individual. Soul compenetrates body to the very essence of its being; they give themselves to each other, and thus form one unit.

This leads us to a second doctrine which is another application of the theory explained above. Since the human soul plays the role of substantial form, it confers on the whole individual man his specific character (IX, D). It is on account of his soul, which is higher in the scale of perfection that the vital principles of animals and plants, that the functions of man include the specific human powers of knowledge and will. Similarly, the functions of animals are wider than those of plants because of the specific differences of their vital principle, as the vital principle of the lion differs from that of the rose tree. And in general all living creatures are difference from and superior to inorganic bodies, such as a molecule of water or a loadstone, because they possess a form which is superior in perfection to any form found in the inorganic world. The human soul organizes its body from within and makes it its ow body, by continually influencing and compenetrating it, and, when death puts an end to this union, the body ceases to be human and becomes something else.

It is because of this organizing role that Aquinas holds fast to the unity of the human soul, and this is a third doctrine which we want to emphasize. The question of the unity or plurality of the soul was a subject of heated discussions. If the individual is one being, it can only possess in itself one organizing element which confers this unity, although this one principle, if it occupies a high place in the scale of beings, like the human soul, possesses many kinds of activity which are found separately in inferior beings. The single human soul embraces the vegetative powers of nutrition and reproduction, the animal powers of sense perception and appetition, and in addition the powers of rationality. Here, as everywhere else, the psychological thesis of unity of the soul is simply a particular application of the more general metaphysical doctrine of forms. There is a doctrinal solidarity throughout, and man takes his place in the vast harmony of the universe.

Finally — and this is a fourth application of the same general doctrine — the human body, which plays the role of matter, is the reason of the multiplicity of individual men within the human race. It is really the human body, as a product of generation, which is the principle of individuation; the precise reason why a man has such or such a soul, with its more or less perfect potentialities, is because he has such or such a body. The soul possesses the particular body for which it is fitted. It is true that the generation of a child is nothing but the becoming of a new substance, that its development comprises several stages specifically different in kind, and each more perfect than the one preceding, and that the immortal soul is created by God and united to the embryo only when the dispositions of the new organism are sufficiently perfect to require union with a human soul. But, although the spiritual and immortal soul is not a product of generation, nevertheless the parents in producing the body of their child assume the responsibility of fixing the potentialities of its whole being. The soul may be compared to wine, which varies in quantity according to the size of the cup.

D. Organic character of human operations

Since the body is everywhere penetrated by the soul, since flesh, muscles and nerves derive from it their qualification of human, we can easily understand that not only our organic life, but also our psychic life, is closely bound up with the organism. Sensations and sense desires, which man possesses in common with other animals, have their seat in the organism, and are in consequence extended and divisible. In the case of abstract and universal concepts, scientific judgments and reasoning, the willing of good in general, and the free choice of particular goods, the soul is still held to the organism, since a disease of the nerves is sufficient to prevent the use of reason and to diminish or destroy our liberty. But there is an important difference to note here. The normal condition of the body is only an external condition: it is not responsible for the existence of thought or of will in their very essence. The body does not ‘secrete’ them. Thought and will are superior to everything that is material.

Why? Because the human concept has the royal prerogative of extending its dominion over reality, in depriving it, by abstraction, of all that makes it merely corporeal, multiple, and tied to time and space. It transcends the corporeal. The most profound notions, such as those of being, cause, force, substance, have a representational content so far detached from the corporeal or sensible that there is no contradiction in extending them to reality which is non-corporeal, or suprasensible, if such are proved to exist.

E. Spirituality, Simplicity, Immortality

We have seen that abstract knowledge has a content independent of material existence. In consequence, the soul too — of which abstract knowledge is an activity — shares the same character of independence. The vital principle of man — the soul — transcends matter: it is immaterial or spiritual. If it were otherwise, the effect (thought) would exceed the power of the cause, the less would produce the more and this would lead to the identity of contradictories. To be spiritual consists only in being able to act and exist without depending intrinsically on a corporeal co-element or body. It is true that our rational soul depends indirectly on the organism inasmuch as the soul draws from the sense perceptions material for abstract knowledge, and therefore the human soul naturally tends to be united to a body. But such a dependence does not affect the very essence or nature of the soul which is of a superior kind. Whereas the vital principles of plants or animals are plunged in matter (immersa) the human soul can subsist without body, although the body could not be without the soul.

Being spiritual, the soul has no quantitative or material parts in it. Moreover, self-consciousness does not admit of internal composition, since it is a process by which our soul imposes its whole self upon itself (reditio completa). If one folds a corporeal thing, for example a sheet of paper, only a part covers another part, but the whole sheet cannot be completely folded upon itself. Thus, if the soul were composed of quantitative parts consciousness would be partially but not totally imposed upon itself. Simplicity means absence of composition. It is a perfection of course, since in every composed being, the parts are limits of the whole, but we grasp it by way of negation, because, as has been seen above (III, B), we have no proper knowledge of realities which go beyond the realm of sense perceptions.

Simplicity precludes the very conception of dissolution; the soul is not subject to death [2]. Only God could annihilate it. As the soul is naturally capable of surviving death, and as on the other hand it is naturally destined to inform or determine a body and to find in the senses the channels of its knowledge, a new union after death, with a body which will thereby become its own, does not involve any contradiction. Moreover, the intermediate state of the disembodied soul was regarded as provisional and incomplete.

In this way the chain of deductions unfolds itself, as did the great doctrines of Greek philosophy (spirituality, simplicity, immortality) which Aquinas regarded as truths accessible to human intelligence in virtue of its own powers. The arguments of Plato’s Phaedon are completed by the reasoning of the De Anima of Aristotle, and the De immortalitate of Augustine. The Schoolmen without exception continue the line of Spiritualist philosophers. Materialism, which confuses sensation and thought, and which puts human individuality at the mercy of ever-changing chemical combinations, like a rose tree which withers or a lamb which is slaughtered, has an implacable enemy in Scholasticism.

On account of the spirituality of his soul, man occupies a central position in the universe. He is a spirit, but one destined to display its life in a body. He is midway between merely corporeal things and pure spirits. He is, to use a comparison dear to the Middle Ages, a microcosm, for all the perfections of reality as a whole meet in him in a wonderful alloy.



  1. Summa. Theol., Ia, q.87, art. 1.

  2. There are other proofs which are used in favor of immortality, such as the universal desire of survival, universal belief in life after death, etc.

The late Dr. Jonathan Dolhenty was the Founder and President of The Center for Applied Philosophy and the Radical Academy, and is Honorary Philosophy Editor at The Moral Liberal. The Moral Liberal has adopted these projects beginning with a republishing and preserving of all of Dr. Dolhenty’s work.