For philosophers who find both a dualistic and a purely materialistic account of the human soul unacceptable, the Aristotelian-Thomistic conception of the soul as the substantial form of the living body may appear to be an intriguing alternative. However, even if one is not afraid of the prospect of committing oneself to an apparently "obsolete" metaphysics, developing such a commitment may not look to be a wise move after all, since upon closer inspection the doctrine may seem to be frustratingly obscure, if not directly self-contradictory.
In what follows I first present what may seem to be a fundamental problem with Aquinas's conception. Second, I will provide the solution that emerges from some crucial distinctions made by Aquinas in this context. The subsequent analysis of these distinctions will show how they fit into the larger context of Aquinas's general metaphysical, mereological and logical considerations, providing further clues as to how these considerations fit together in Aquinas's thought. In the concluding section of the paper I will argue that with the proper understanding of these conceptual connections, despite possible appearances to the contrary, Aquinas's conception does indeed offer a viable alternative to the modern dilemma of dualism vs. materialism.
In his recent book, Aquinas on Mind, Anthony Kenny calls our attention to the problem as follows:
"If we identify the human soul with Aristotelian substantial form, it is natural to identify the human body with prime matter. But body and soul are not at all the same pair of items as matter and form. This is a point on which Aquinas himself insists: the human soul is related to the human body not as form to matter, but as form to subject (S 1-2,50,1). A human being is not something that has a body; it is a body, a living body of a particular kind. The dead body of a human being is not a human body any longer -- or indeed any other kind of body, but rather, as it decomposes, an amalgam of many bodies. Human bodies, like any other material objects, are composed of matter and form; and it is the form of the human body, not the form of the matter of the human body, that is the human soul"
Despite the fact that one might object to the way in which Kenny poses the problem--unfortunately, the rather sloppily presented contrast between matter and subject is not quite supported by the passage he refers to, and Aquinas himself would not contrast the two in the way in which Kenny intends this contrast--, there is a genuine problem here.
For Aquinas does indeed say both that a human being is a human body, namely, a rational, sensitive, living body, and that a human being consists of a soul and a body. But these two claims are apparently incompatible. For according to the latter claim the body is an integral part of the whole human being consisting of body and soul. But then the whole human being cannot be this body, for no integral part can be the same as the whole of which it is only a part.
Furthermore, if the human soul is the substantial form of the human body, then, since what a substantial form informs is the Aristotelian prime matter, according to Aquinas, it seems that the human body has to be prime matter. However, the human body cannot be prime matter, since prime matter in itself cannot exist in actuality, whereas the human body obviously does exist in actuality. 
To be sure, at this point one might easily retort that the human body does exist in actuality precisely because it is actually informed by the soul. So the human body is prime matter actually informed by the soul. 
However, this quick riposte will not do. For if we were to identify the human body with the matter that the soul informs in the context of the claim that a human being is composed of body and soul, then we would also have to admit that the human body in this composition is that component which persists through a substantial change, such as death, since prime matter in the composition of a material substance is precisely that part which is the permanent subject of a substantial change, when it loses one substantial form and takes on another. But the human body does not persist through death, for when it ceases to be informed by the soul it ceases to be, since the dead body of the human being is not a human body, except equivocally, according to Aquinas. So the human body cannot be prime matter, which is the immediate and persistent subject of the substantial form of the body.
On the other hand, given Aquinas's theory of the unity of substantial forms, it seems that it cannot be anything else either. For according to this theory, a substantial form cannot have anything else as its subject but prime matter, since otherwise it would have to inform something that would already exist in actuality. But this is impossible, for something that exists in actuality already has its own substantial form, so it cannot take on any other form as its substantial form.
In fact, Aquinas's doctrine of the unity of substantial forms involves even further strange consequences in this regard. For according to this doctrine, the form on account of which a man is a body, his corporeity, is the same as that on account of which he is an animal, his animality, and this, in turn, is the same as that on account of which he is a human, his humanity. But Aquinas also argues that a man's humanity or quiddity is what he calls the "form of the whole" [forma totius], as opposed to the "form of the part" [forma partis], which he identifies as the soul, and that the form of the whole differs from the form of the part because the form of the whole contains both matter and form. So the form of the whole, the quiddity of man, contains the soul as its part, so it obviously cannot be the same as the soul. But if it is not the same as the soul, and yet it is a form of the human being, and it is clearly not an accidental form, then it seems that we have at least two substantial forms here, one of which is a part of the other, and which, besides the form of the part, also contains matter! At this point, perhaps, our confusion has reached its peak, so it is about time we set about clarifying the basic concepts involved in these considerations.
The question, then, is this: exactly how are we to understand Aquinas's claim that a human being essentially consists of body and soul, given his other claim that the soul is the one and only substantial form of the body?
To answer this question first we have to consider Aquinas's distinction, which he takes over from Avicenna, between several senses of the term `body'. In his De Ente et Essentia he writes as follows:
"The name `body' can be taken in several senses. For a body (1), insofar as it is in the genus of substance, is said to be a body (1) because it has such a nature that three dimensions can be designated in it; but the three designated dimensions themselves are the body (2) which is in the genus of quantity. But it happens that something that has some perfection also has a further perfection, as is obvious in the case of man, who has a sensitive nature, and beyond that also an intellective one. Likewise, to the perfection of having such a form that in the thing three dimensions can be designated another perfection can be added, such as life, or something like that. The name `body' (3), therefore, can signify something which has a form from which there follows the designability of three dimensions with precision, namely, so that from that form no further perfection would follow, but if something is added, then it is beyond the signification of `body' (3) in this sense. And in this sense the body (3) will be an integral and material part of an animal, for in this way the soul will be beyond the signification of the name `body' (3), and it will be superadded to the body itself, so that the animal will be constituted from these two, namely, from the soul and the body (3), as its parts. But the name `body' (1) can also be taken so that it should signify some thing which has a form on account of which three dimensions can be designated in it, whatever that form may be, whether it may give rise to some further perfection or not. And in this sense `body' (1) will be a genus of `animal', for an animal contains nothing which is not contained implicitly in a body (1). For the soul is not a form other than that on account of which in that thing three dimensions can be designated; and so when it was said that a body (1) is that which has such a form that three dimensions can be designated in it, it was understood so that whatever that form might be, whether animality or stoneness, or whatever else. And thus the form of animal is implicitly contained in the form of body (1), insofar as body (1) is its genus."
To understand this passage correctly, we have to recall that according to Aquinas concrete common names signify the forms or natures of things, however, they do not refer to, or, using the medieval technical term, supposit for (supponit pro), these forms in virtue of their signification, but rather to the things themselves that have these forms in actuality. What we can use to refer to a form itself is the abstract name corresponding to the concrete name. But then, if it turns out that we use a name in different senses, that is, with various significations, this means that the same name in its different senses signifies different forms in the same thing, and thus, in the corresponding different senses, the corresponding abstract term will refer to those different forms. For example, if someone is a bachelor both in the sense of holding a bachelor's degree and in the sense of being unmarried, then his bachelorhood in the first sense is certainly not the same as his bachelorhood in the second sense, which is clearly shown by the fact that if he gets married he loses the latter, but not the former. In fact, this example also shows that the forms signified [formae significatae] by concrete terms and referred to by their abstract counterparts do not even have to be forms in the strict metaphysical sense of being some determinations of some real, whether substantial or accidental, act of being of their supposita. For, obviously, the forms signified by the term `bachelor' in both senses are some beings of reason: in the first sense the form signified is a relation of reason connecting the bachelor in question to some academic institution and its regulations, while in the second sense it is the privation of a relation of reason connecting him to his spouse, insofar as these relations are recognized by the relevant members of society.
In the same way, while the term `body', insofar as it is the genus of all bodies, signifies the substantial form of all bodies, referred to by the term `corporeity' in the first sense, the same term in the sense in which it is in the genus of quantity signifies an accidental form of the same bodies, namely, their corporeity in the second sense, that is, their dimensions extending them in space. But it is neither the first, nor the second sense of the term `body' distinguished by Aquinas that is relevant to the claim that a human being, or indeed, any living being, is composed of body and soul. For the sense of the term `body' relevant here, as Aquinas characterizes it, is clearly the sense in which a lifeless body, say a stone, is said to be a body, with the strict implication of lacking life. But since no living body can be a body in this sense, the corporeity of a living body in this sense is obviously not the substantial form of a living body. So while the forms signified by the term `body' in the first and the third senses distinguished by Aquinas coincide in lifeless bodies, since in these bodies the term in both of these senses signifies their substantial form, the negative implication of the third sense of the same term, which it does not have in the first sense, prevents it from signifying the substantial form of living beings in this sense; therefore, in this sense it cannot refer to the whole living being, but only to a part of it.
So with this distinction at hand we can give an acceptable answer to the question of how Aquinas can claim both that a human being is a body and that he or she has a body, as his or her integral part. For a human being is a body in the first sense, while it has a body in the third of the three senses distinguished here, and thus no inconsistency is involved in these two claims.
However, this solution still does not answer the further doubts raised above. For it is still not clear how the corporeity signified in a human being by the term `body' in its first sense is related to the corporeity signified in the same human being by the same term in its third sense, and how these are related to the soul of the same human being.
Before going into the details of this issue, however, we should recall the simple truth that there is more than one way to slice a cake. That is to say, the division of any integral whole into its integral parts will always depend on how we distinguish the parts in the whole.
Nevertheless, we must also keep in mind that the apparent arbitrariness involved in distinguishing the parts of something according to our criteria does not make these parts "unreal". For example, if we take the hapless Socrates and distinguish his left and right or upper and lower parts, in this process we get parts no less real than by distinguishing his members or organs, the only difference being that while in the former cases we distinguished his parts on the basis of their spatial orientation, in the latter we distinguished them on the basis of their function. To be sure, we may find some divisions to be more natural than others, in that they better "cut at the joints" of some whole. But that has rather to do with the relative unity of the parts in constituting the absolute unity of the whole, or vice versa, than with the reality or non-reality of the parts. As St. Thomas reminds us:
"... nothing prevents some things from being many in some respect and being one in another. Indeed, all sorts of things that are many are one in some respect, as Dionysius says in the last chapter of On Divine Names. But we have to be aware of the difference that some things are many absolutely, and one in some respect, while the case is the reverse with others. Now something is said to be one in the same way as it is said to be a being. But a being absolutely speaking is a substance, while a being in some respect is an accident, or even [only] a being of reason. So whatever is one in substance, is one absolutely speaking, yet many in some respect. For example, a whole in the genus of substance, composed of its several integral or essential parts, is one absolutely speaking, for the whole is a being and a substance absolutely speaking, while the parts are beings and substances in the whole. Those things, however, which are diverse in substance, and one by accident, are diverse absolutely speaking, and one in some respect, as many humans are one people, or many stones are one heap; and this is the unity of composition or order. Likewise, many individuals that are one in genus or species are many absolutely speaking, and one with respect to something, for to be one in genus or species is to be one with respect to reason. [For example,] in the genus of natural things, some whole is composed from matter and form, as man from body and soul, who is one natural being, although he has a multitude of parts [...]"
So, although it is up to us to assign our criteria of distinguishing the integral parts making up some integral whole, which is, again, marked off by us as being the whole constituted by those parts, there will nevertheless be some absolute standard according to which the mereological constitution of the whole is not dependent on us, namely, the ontological status of the parts so distinguished and of the whole thus marked off. That is to say, even if we are absolutely free to regard a heap of stones as one, and an individual stone as a part of this one, nevertheless it is obvious that the unity of the individual stones is not of the same kind as the unity of the heap. For the heap is not a being in the same sense as the stones are, since precisely in that sense in which a stone is one being the heap is not one being, but rather it is several beings. Again, we are absolutely free to regard one half of one of these stones as one part of this one stone and the other half as the other part of the same stone, yet, it is clear that the unity of each of its halves is not the same as the unity of the stone, for in the sense in which one half of it is one being, the stone is not one being, but two beings, whereas in the sense in which the stone is one being, its halves are not even beings at all. For the stone is actually a being in its own right, while neither of its halves is actually a being in its own right; it only can be a being in its own right if the stone is actually cut into those two halves. But as the stone is actually undivided, it is one substance actually, while its two halves are two substances only potentially. And since only that thing is one entity in the absolute, unqualified sense which is a being in the absolute, unqualified sense, and only what is actually a substance is a being in the absolute, unqualified sense, only the stone is actually one in the absolute, unqualified sense. And so, despite the fact that we could distinguish in the stone two halves, and we can say that it is made up of those two halves, this will not make the stone into two beings or two entities.
Furthermore, when we divide a stone into two parts, we can do so in a number of different ways, since obviously such a division need not result in two equal halves:
So clearly, if we divide it into two halves, or into one third and two thirds, or one quarter and three quarters, we are able to mark out these parts even before actually dividing it, on the basis of how much of the quantity of the whole we conceive of as belonging to the one part, and how much as belonging to the other. But then, in a similar manner, we can distinguish in the same thing not only parts of its quantity on this basis, but also any sorts of other parts, on the basis of how much of whatever we conceive in the thing we conceive as belonging to the one part and how much as belonging to the other.
Now, what does all this mean concerning the composition of man from body and soul? First of all, it is clear that the term `body' in the first sense, in which it is the genus of all bodies, since it is predicable of a whole human being and not only of some part of him or her, signifies the unique substantial form of any human being, and so what it signifies in human beings, their corporeity, coincides with their rational soul. In this sense, therefore, we do not distinguish the body from the soul as a part from another part, but as the whole from one of its parts. But this part of this whole, namely, the soul, is not distinguished from the other parts on the basis of dividing the quantity of this whole. Rather, the distinction is made on the basis of the different perfections, indicating the different modes of existence that we conceive in this whole, namely, the spatio-temporal, material mode of existence which this body has in common with all bodies, as opposed to the mode of existence which enables this body to perform several sorts of vital functions, that is, life, which it has in common with all living beings. But once we have distinguished these two modes of existence, namely, material, spatio-temporal existence on the one hand, and life on the other, we can obviously use different names, or the same names in different senses, to signify the substantial forms on account of which a thing has one of these modes of existence, or the other, or both in its own unique act of substantial being. So if we distinguish corporeity as that substantial form on account of which whatever has it exists in a material, spatio-temporal manner, whether the thing in question is alive or not, then the corporeity thus distinguished will clearly coincide in all living bodies with their soul, conceived as that substantial form on account of which whatever has this form is alive, whether it is a body or not. Therefore, in this non-exclusive sense, both the corporeity thus conceived and the soul thus conceived are nothing but the form of the whole, that is, the essence or quiddity of a living body. But if we conceive of corporeity as that on account of which whatever has it exists in a spatio-temporal manner, but is not alive, the corporeity thus conceived cannot coincide with the substantial form of a living body, so this conception of corporeity can mark out only some part of the essence of a living body. Also, if we conceive of the soul as that on account of which whatever has it is alive but is not a body, the concept of soul thus conceived can mark out only some part of a living body, in which both material existence and life are united in its single act of substantial existence, its spatio-temporal, material life.
Of course, spatio-temporality and life in themselves are not incompatible, which is shown by the manifest existence of living bodies. However, they do not entail each other either, as is shown by the manifest existence of lifeless bodies as well as by the at least conceivable existence of living immaterial substances. Therefore, it should come as no surprise that we can form both the non-exclusive and the exclusive concepts of those substantial forms on account of which any substance has either life or spatio-temporality or both. Accordingly, the concepts of `body' and `soul' in the relevant non-exclusive and exclusive senses can indeed be properly characterized by the following entailments and denials of entailments, just as St. Thomas suggested:
1. x is a body1 ® x is spatio-temporal
2. ~ (x is a body1® x is not alive)
3. x is a body2 ® x is spatio-temporal & x is not alive
4. y is a soul1 ® (x has y ® x is alive)
5. ~ (y is a soul1 ® (x has y ® x is not spatio-temporal))
6. y is a soul2 ® (x has y ® x is alive & x is not spatio-temporal)
Therefore, the essence of a living body, which can be referred to as a corporeity, in the first, non-exclusive sense, has to consist of a corporeity in the second, exclusive sense, and of a soul also in the second, exclusive sense, as its integral parts:
7. corporeity1 (of a living body1) = corporeity2 (of the same living body1) + soul2 (of the same living body1)
By contrast, the corporeity of a non-living body will be the same essence in both senses:
8. corporeity1 (of a non-living body1) = corporeity2 (of the same non-living body1),
But then a living body1 will have to consist of a body2, and a soul2:
9. living body1 = soul2 (of the same living body1) + body2 (of the same living body1)
In fact, this is precisely how Cajetan interprets St. Thomas's above-quoted remarks in his commentary on the De Ente et Essentia:
"As of now, it seems to me that it must be said that man is composed from soul and body, and is a third thing not only as composed from two things, but also as from two parts which are a whole in reality. I take body not in so far as it is the genus, but in so far as it signifies a part, and take soul in its exclusive meaning, as defined in II de Anima. Thus viewed, body means a composite of matter and a corporeal perfection taken exclusively. Soul means the perfection of life exclusively.
I prove my thesis thus. Body differs really from soul, and not as a whole differs from a part; therefore it differs as a part from a part. The added point is proved: the whole includes, at least confusedly, the part, but the body viewed in this way excludes the soul. Thus the body is included in the definition of the soul as a subject supporting the soul, as St. Thomas says there. The first proposition is evident in itself and conceded by all. The consequence draws its force from an adequate division. For if the soul and the body differ really, the body must differ really from the soul as a whole from the part or as a part from a part. Man, therefore, is composed from body and soul as from parts that are really distinct, which was our thesis."
So far, so good, one might say, but can all this "word-magic" solve the genuine philosophical problem of body and mind? For even if by using these "rubber-band" concepts of body and soul one may save the consistency of what Aquinas says in various contexts, making them comprise more in one context and less in another, body and soul are still claimed to be really distinct parts of a human being in the above-described exclusive senses of these terms. But then, if the body and the soul are really distinct entities, with one belonging to the spatio-temporal physical world and the other belonging to some alleged spiritual realm, then we immediately seem to face here the problem of "mysterious" interaction vs. causal closure, all too familiar from the woes of Cartesian dualism.
To this, we have first to reply that since a philosophical problem is some conceptual conflict, or rather a bundle of conceptual conflicts within a broad conceptual framework, there is no such thing as "the genuine philosophical problem" of anything apart from the conceptual framework that gives rise to it. So, if we find that despite superficial appearances to the contrary St. Thomas's conception of body and soul is sufficiently different from the concepts figuring in the modern problem of body and mind, we may well find that in his conceptual framework the familiar problem, or rather the familiar bundle of problems, need not and does not arise at all. Indeed, what lies at the bottom of all the familiar problems is the assumption that body and soul are two distinct entities of radically different natures, having entirely distinct causal powers rooted in these distinct natures, on account of which they are accessible by us for observation in radically different ways.
If we understand it properly, however, we can easily realize that the real distinction of body and soul in the Thomistic-Aristotelian framework means nothing like this. In the first place, that body and soul, in the exclusive senses of these terms, are distinct parts of the same entity does not mean that they are distinct entities in the sense in which subsistent entities are distinct from one another. As St. Thomas often repeats, unum convertitur cum ente: there is one entity, absolutely speaking, whenever there is a being having one act of existence, even if the being in question is composed of several parts. But body and soul, as distinguished in the exclusive senses of these terms, have the same unique act of substantial existence, namely, the life of a living body; therefore, body and soul are one being, one entity, absolutely speaking, not two entities. Since causal powers and the corresponding actions belong to the beings which perform those actions by means of those powers, if the body and the soul are one being, then no question of their inter-action can arise on the basis of their distinct causal powers rooted in their radically distinct natures. For this question can properly be raised only concerning distinct beings each having a substantial act of being of its own, founding their distinct causal powers and the corresponding actions.
However, at this point it may seem that by laying so much emphasis on their substantial unity this position does not leave any room for the real distinction of body and soul, or indeed, for a non-materialistic conception of the soul. For if soul and body are one entity, namely, a living body, having all their powers and actions in common in the whole they constitute, then their distinction is apparently a merely conceptual one: the concepts of body and soul provide us merely with different aspects for considering the same, essentially material entity.
But this objection is based on a radical misunderstanding of what it means for essential parts of the same entity to be distinct from one another, and yet to constitute the same one entity. For even if, for example, Socrates is an ensouled body, a unitary substance with one act of substantial being, his essential parts, his body and soul in the exclusive senses of these terms, are strictly distinct parts in the unitary whole insofar as the one part is that which accounts for one distinct sort of perfections of the whole Socrates, namely, spatio-temporality and whatever that entails; whereas the other part is that which accounts for another sort of perfections, namely, human life, and whatever that entails. But since these are obviously distinct perfections, whose distinction is given regardless of the intellect's consideration, the parts of the whole accounting for these perfections, each for its own sort, have to be parts that are really distinct, again, regardless of the intellect's consideration.
Nevertheless, while we maintain their real distinction in this way, we also have to realize that body and soul can be distinct only as distinct parts of the same substantially one whole. Indeed, they cannot possibly be distinct in the same way as the whole they constitute is distinct from other wholes of the same kind. For the whole they constitute is a complete substance, which therefore is a complete being in the primary, unqualified sense of the term `being', having its own unique substantial act of existence. But then, no part of this one whole can have the same act of being in the same way, for otherwise they would not be parts in the whole, but whole beings in the same unqualified sense of the term as the original whole. Therefore, the essential parts of the whole, since they are essential, share the same act of being as the whole; nevertheless, since they are parts, they can have this existence only in the sense in which a part in a whole can. As St. Thomas says:
[...] existence [esse] is said to be the act of a being [ens] insofar as being, that is, that by which something is denominated a being in the nature of things. And being in this sense is attributed only to the things themselves which are contained in the ten categories, whence 'being' [ens] predicated on account of such [an act of] existence [esse] is divided by the ten categories. But this [act of] existence [esse] is attributed to something in two senses. In one sense as to that which [quod] properly and truly has being, or exists. And thus it is attributed only to a per se subsisting substance; whence that which truly exists is said to be a substance in bk. 1. of the Physics. All those [things], however, which do not subsist per se, but in others and with others, whether they are accidents or substantial forms or any sorts of parts, do not have existence [esse] so that they themselves would truly exist, but existence [esse] is attributed to them in another sense, namely, as to something by which [quo] something exists; as whiteness is said to exist, not that it itself would subsist in itself, but because it is by [this whiteness] that something has it that it is white. [...]
What this means, then, is that in line with St. Thomas's general conception of the analogy of being, the whole and its essential parts, while they are denominated beings on account of the same substantial act of existence, they are not denominated beings in the same sense. For the whole substance is denominated a being in the primary, unqualified sense of being, in the sense in which only a complete, self-subsistent entity can be called a being, existing on its own. The essential parts of this being, namely, its matter and substantial form, however, can be called beings only in some derivative sense of the term. For the form can be called a being only in a secondary sense, because, insofar as it is a form, it can be said to exist only in a secondary sense. And this is so because for a form to exist in this secondary sense is nothing but for it to inform that which exists in the primary sense, namely, the primary substance. But this is just another way of saying that for the form to exist is nothing but for the thing to exist, or to have existence, in respect of the form, which makes it clear that the sense in which existence is attributed to the form is obtained by adding some qualification to the sense in which existence is attributed to the substance which is said to exist in the primary, unqualified sense. Obviously, similar considerations apply to the body, in the exclusive sense of the term, insofar as it is the other essential part of a living being.
But then, if we recognize the analogical character of the predication of the notion of being with respect to the whole and with respect to its essential parts, it should come as no surprise that body and soul in the exclusive senses of these terms are said to be one being, in the primary sense of the term, yet they can be said to be two beings in the derivative sense in which distinct parts of a whole can be said to be beings. But since having power and action can attach properly only to a being in the primary sense, the problem of interaction between body and soul still cannot arise, for both the actions and the corresponding powers will still belong only to the unified whole, and not to either of the parts. This has to be the case, at least, unless there is some action which can properly be said to belong only to one of them, in which case that part will also have to be regarded as a being not only in the sense in a which a part is a being, but also in the sense in which the whole is.
But this is precisely the point Aquinas makes with respect to the unique case of the human soul in his proof for its immortality:
"We have to say that the human soul is entirely incorruptible. For a clear understanding of which we should consider that what per se belongs to something cannot be removed from it, just as from a man it cannot be removed that he is an animal, nor from a number that it is either even or odd. It is clear, however, that being per se belongs to form, for everything has being in virtue of its proper form; whence being can in no way be separated from form. Therefore, things composed of matter and form are corrupted by losing the form to which being per se belongs. But the form itself cannot be corrupted per se, but it is corrupted per accidens, insofar as the composite thing that exists by the form loses its being, provided the form is such that it is not a thing that has being, but it is only that by which the composite thing has being. If, therefore, there is a form which is a thing that has being, then it is necessary for that form to be incorruptible. For being is not separated from something that has being, except by its form getting separated from it; therefore, if that which has being is the form itself, then it is impossible that being should be separated from it. It is manifest, however, that the principle by which a man understands is a form that has being in itself, and [that it does not have this being] only as that by which something [else] exists. For understanding, as the Philosopher proves in bk. 3. of the De Anima is not an act performed by some bodily organ."
Now, clearly, if understanding is the act of the intellective soul alone (which is a claim to be established by a separate argument, but that need not concern us here), then this means that the soul has some action and the corresponding power, which only a subsistent being has. But then, if the soul is a subsistent entity, this means that it is a being not only in the derivative sense in which a substantial form is a being, but also in the unqualified sense in which a subsistent entity is a being.
As we could see in the foregoing considerations, body and soul are one being in the absolute, unqualified sense, not two beings united in some mysterious interaction with one another. Nevertheless, despite the fact that this one being, the whole human being, is a material substance, if Aquinas's claim that understanding is the act of the soul alone is true, then the form of this being has some act of its own which denominates the whole only through this part to which alone it can belong. But then, since this form has an activity of its own, it is a form which has the being of the whole not only in the sense in which any other form insofar as a form has being, but also in the same sense in which the whole has it. Therefore, it could be destroyed also only in the sense in which the whole is destroyed, namely, by losing its substantial form, but since it is a form, that is precisely the sense in which it cannot be destroyed. Hence, it is incorruptible. But then, if we really understand how these claims fit together, we can clearly see that Saint Thomas's conception does indeed manage to steer its way safely between the Scylla of dualism and the Charybdis of materialism.
Department of Philosophy, University of Notre Dame
Institute of Philosophy, Budapest, Hungary
Aquinas's works are referred to by their standard abbreviations.
Cajetan, T. 1964, Commentary on Being and Essence, tr. L. H. Kendzierski and F. C. Wade, S. J., Marquette University Press: Milwaukee, Wis.
Henry, D. P. 1991, Medieval Mereology, Bochumer Studien zur Philosophie, Band 16, B. R. Gruener, Amsterdam-Philadelphia
Kenny, A. 1994, Aquinas on Mind, New York: Routledge
Klima, G. 1984, "Libellus pro Sapiente: A Criticism of Allan Bäck's Argument against St. Thomas Aquinas's Theory of the Incarnation", The New Scholasticism, 58, pp. 207-219.
Klima, G. 1993, "The Changing Role of Entia Rationis in Medieval Philosophy: A Comparative Study with a Reconstruction", Synthese, 96; No. 1, pp. 25-59.
Klima, G. 1996, "The Semantic Principles Underlying Saint Thomas Aquinas's Metaphysics of Being", Medieval Philosophy and Theology, 5, pp. 87-141.
 Kenny, 1995, 28.
 For Aquinas the contrast between (prime) matter and subject in the strict sense is the contrast between that which is informed by a substantial form and that which is informed by an accidental form. See c. 1. of his De Principiis Naturae. However, apparently, Kenny rather intends to distinguish here between what in another place Aquinas calls subiectum informe and subiectum formatum; cf. text quoted in n. 6. But this is not the issue in the text referred to by Kenny.
 Cf. in Meta lb. 5, lc. 21, n. 1099. For detailed discussion of the notions of integral part and whole, as opposed to other kinds of parts and wholes distinguished by medieval philosophers, see D. P. Henry, 1991; for a discussion of Aquinas's views in particular see especially pp. 218-328. However, in general, for the purposes of the present discussion the concept of integral part can be defined as follows: a is an integral part of A if and only if a A and there is some b such that a + b = A. For example, a slice of a cake is an integral part of it, since the slice is not the cake, yet there is something, namely, the rest of the cake, such that the slice and the rest together are the cake. (Here, if someone has worries concerning the possible situation in which the slice is actually cut out, and thus the separated part and the rest do not make up the original cake, we can take `the rest' to include not only the remaining quantitative part, but also its continuity with the slice. Cf. what St. Thomas says about the issue in Meta lb. 5. lc. 21. esp. nn. 1104-1108.) Likewise, the tone C is an integral part of the chord C major, for the tone is not the chord, yet there is something, namely, the tones E and G together, such that the tone C and the tones E and G together are the chord C major, that is, C + E + G = C major. As these examples show, the `+' symbol in the above definition is used as a collective nominal conjunction, which is in fact the general logical notion that has the familiar arithmetical operation only as its special case, as restricted to numerals. However, in general, if `a' and `b' are any two names, then `a + b' is another name, the name of the integral whole having a and b as its integral parts, and the truth of `P(a + b)' does not imply either `Pa' or `Pb'. For example, while two and three are five, neither two nor three are five, and even if Plato and Socrates are men, neither of them is men, although, of course, each of them is a man, etc.
 " ... dicimus quod essentia animae rationalis immediate unitur corpori sicut forma materiae, et figura cerae, ut in 2 De Anima dicitur. Sciendum ergo, quod convenientia potest attendi dupliciter: aut secundum proprietates naturae; et sic anima et corpus multum distant: aut secundum proportionem potentiae ad actum; et sic anima et corpus maxime conveniunt. Et ista convenientia exigitur ad hoc ut aliquid uniatur alteri immediate ut forma; alias nec accidens subjecto nec aliqua forma materiae uniretur; cum accidens et subjectum etiam sint in diversis generibus, et materia sit potentia, et forma sit actus." 2SN d. 1, q. 2, a. 4, ad 3-um
 "Sed dicebat quod corpus humanum ipsum esse corporis habet per animam.- Sed contra, Philosophus dicit in II De Anima, quod anima est actus corporis physici organici. Hoc igitur quod comparatur ad animam ut materia ad actum, est iam corpus physicum organicum: quod non potest esse nisi per aliquam formam, qua constituatur in genere corporis. Habet igitur corpus humanum suum esse praeter esse animae." QDA a. 1, obj. 15.
 "Ad decimumquintum dicendum quod in definitionibus formarum aliquando ponitur subiectum ut informe, sicut cum dicitur: motus est actus existentis in potentia. Aliquando autem ponitur subiectum formatum, sicut cum dicitur: motus est actus mobilis, lumen est actus lucidi. Et hoc modo dicitur anima actus corporis organici physici, quia anima facit ipsum esse corpus organicum, sicut lumen facit aliquid esse lucidum." QDA a. 1, ad 15-um.
 Cf. De Princ. c. 1.
 Cf. QDA a. 9. In corp.; ST1 q. 76, aa. 6, 7; 2SN d. 1, q. 2, a. 4; SCG lb. 2, c. 71; etc.
 4SN d. 44, q. 1, a. 1, ad 2-um; SCG 4, 81; QDL 2, q. 2, a. 2; in Meta lb. 7, lc. 9.
 Cf. 1SN d. 25, q. 1, a. 1, ad 2-um; SCG lb. 4, c. 81.
 EE c. 2. In the translation I marked each occurrence of the term `body', indicating in each particular case in which of the three senses distinguished here the term is being used.
 To be sure, there are concrete terms which refer to the forms they signify, but they do so not on account of their signification, but on account of the simplicity of the thing they signify. Cf. 3SN d. 11, q. 1, a. 4; 1SN d. 33, q. 1, a. 2, ad 2. For further details on Aquinas's semantic theory, see Klima, 1996.
 As St. Thomas wrote: "...dicendum est quod illud a quo aliquid denominatur non oportet quod sit semper forma secundum rei naturam, sed sufficit quod significetur per modum formae, grammatice loquendo. Denominatur enim homo ab actione et ab indumento, et ab aliis huiusmodi, quae realiter non sunt formae." QDP, q. 7, a. 10, ad 8.
 Concerning the semantic role and ontological status of beings of reason in Aquinas, see Klima, 1993.
 "Et dicitur corpus mathematicum, corpus consideratum secundum dimensiones quantitativas tantum, et hoc est corpus in genere quantitatis ..." 2SN d. 30, q. 2, a. 2.
 Cf. here what Aristotle and Thomas say about the different kinds of unity in Meta lb. 5. lc. 7 and 8. See also Klima, G. (2000) “Aquinas on One and Many”, Documenti e Studi sulla Tradizione Filosofica Medievale, 11(2000), pp. 195-215.
 ST1-2 q. 17, a. 4.
 Cf. e.g. in Meta lb. 5, lc. 21, n. 1102; in Meta lb. 7, lc. 16, nn. 1632-1633.
 Cf.: "Non enim anima est alia forma ab illa, per quam in re illa poterant designari tres dimensiones; et ideo, cum dicebatur quod corpus est quod habet talem formam, ex qua possunt designari tres dimensiones in eo, intelligebatur: quaecumque forma esset, sive animalitas sive lapideitas sive quaecumque alia." EE c. 2.
 Cf. text quoted in n. 6.
 Cajetan, 1964, p. 120.
 "Nihil autem potest per se operari, nisi quod per se subsistit. Non enim est operari nisi entis in actu, unde eo modo aliquid operatur, quo est. Propter quod non dicimus quod calor calefacit, sed calidum." ST1 q. 75, a. 2.
 QDL 9. q. 2, a. 2, in corp.
 QDA a. 14, co.
 Since in this context we are only concerned with what it means to say that the soul is both a form and a subsistent substance, and not with proving its truth, we should only consider under what conditions it would be true. To be sure, I also think Aquinas's argument for the claim that understanding is the act of the intellect alone can indeed be shown to work, but the proof of this claim is a different issue, to be dealt with in another paper.
 And this is why we can truly say that a man understands, and not only that a human soul understands, despite the fact that the act of understanding is an act inherent only in the soul, and not in the body-soul composite. Cf.: "... et operationes partium attribuuntur toti per partes. Dicimus enim quod homo videt per oculum, et palpat per manum, aliter quam calidum calefacit per calorem, quia calor nullo modo calefacit, proprie loquendo. Potest igitur dici quod anima intelligit, sicut oculus videt, sed magis proprie dicitur quod homo intelligat per animam." ST1 q. 75, a. 2, ad 2-um. Cf. also De Unitate Intellectus, c. 3, n. 69. For a discussion of Aquinas's general rule concerning the proper denomination of a whole by some property of its part see Klima, 1984.