8. Body and Soul

Philosophers nowadays will want to know how this account of substance places Aquinas on the question of the relation of body and soul with respect to Dualism and Physicalism. Not easily. Aquinas maintains that the soul is capable of existing apart from the living body after the death of the body, because the soul is incorruptible. This might suggest that he is a kind of Substance Dualist, the soul being one substance and the body another, with the soul “interacting” as it were with the other substance, the body. However this picture fails to recognize the Aristotelian terms of the account that Aquinas provides of soul and body. Thomas knows and accepts Aristotle’s assertion in De anima II.1 that it is as pointless to ask whether soul and body are one as it is to ask whether the seal and the wax are one–they are.

The soul is indeed capable of existence apart from the body at death. This incorruptibility results from the actualities of understanding and willing that are not the actualities of any bodily organ, but of the human animal as such distinguished by the rational form. However, Thomas merely concludes from this fact that the soul is a “particular thing” and thus a subsistent after the death of the body. He argues that what belongs to the notion of “this particular thing” is only that it be a subsistent, and not that it be a substance complete in a nature. A subsistent is something with an operation of its own, existing either on its own or in another as an integral part, but not in the way either accidental or material forms exist in another. Existing on its own is not distinctive of substances alone. A chair is a particular thing, and thus a subsistent. But on Aquinas’ account it is not a substance; it is rather an accidental unity of other subsistents which may or may not be substances. A hand has an operation distinctive of it as an integral part of a living body, an operation different from the operation of the stomach; it is a particular thing and also a subsistent. (Summa Theologiae Ia.75.2 ad1; also Quaestiones Disputate de Anima 2.) And yet being an integral and functional part of a substance, it does not have the complete nature of a substance.

A substance, on the other hand, is something that is both subsistent and complete in a nature—a nature being an intrinsic principle of movement and change in the subject. A human soul is a constitutive element of the nature of a human substance. It is the formal principle of a human substance. It is what is specified when we say what the substance is. But it is incomplete. What it is for a soul to be is to be the form of some substance. In that sense it is a principle of a substance, ‘principle’ being a technical term that refers back to the first entry, arche, in Aristotle’s philosophical lexicon in the Metaphysics, as well as Thomas’ commentary on it, and Thomas’ On the Principles of Nature. As the principle of a nature, its nature is to be the formal element of a complete substance. Consequently, it doesn’t have its own nature and is not a substance in its own right, even if it is capable of subsisting apart from the living body. It is because it is naturally incomplete as subsisting apart from the body that Thomas sees this state as unnatural for it, and an intimation of, but not an argument for, the resurrection of the body.

Question Ia.75 of the Summa Theologiae is the best place to look for Thomas’ discussion of the subsistent reality of the human soul, although the Quaestiones Disputatae De Anima and the commentary on Aristotle’s De anima provide important supplementary material to that discussion. Thomas begins 75 by pointing out that his concern is the concern of a theologian, and that the theologian is concerned with human nature primarily in relation to the soul. He is concerned with the body only in its relation to the soul. The body of the question is filled with philosophical argument, and yet its order and point is theological. That theological order and point, however, can lead to certain philosophical distortions concerning the soul if one isn’t careful. So Thomas is very careful.

Considered as a substantial form of a material body, the soul exists in a living being as the substantial form of an animal. Here it is important to clarify. ‘Immaterial’ can be said in two ways of forms. In the first way, any form as such is immaterial because it is not a material principle. It is distinguished as a principle of actuality in a being from the material principle which is a principle of potentiality and change in corporeal beings. In that sense, any substantial form whatsoever will be immaterial, including the substantial form of an oak tree or the substantial form of a dog. And so also is the substantial form of the human immaterial in that sense. Aquinas is explicit about this when he proves that the human soul is immaterial in Summa TheologiaeIa.75.5. It is immaterial in just the way in which any form whatsoever is immaterial. But in the second way, ‘immaterial’ is said of subsistent forms—forms that subsist without matter like angels or spiritual substances in general.

In 75.1 Thomas had argued against the ancient materialists, that the soul is not a body; it is incorporeal. In 75.2 he proved that the human soul is a subsistent because it has an activity that pertains to it without the use of a corporeal organ, namely, the activity of understanding in intellect. But then immediately in 75.3 he proved that the souls of other animals are not subsistent, because they do not have an operation that does not employ a corporeal organ. The souls of other animals are incorporeal in the sense of 75.1, but they are not subsistent as in 75.2 In 75.4, Thomas proves that the soul is not the man. Socrates, the man, has vital activities that are the activities of a living animal, like sensation, nutrition, reproduction, and so on, activities that are not distinctive activities of the soul itself as intellect is in the human case. Since these are activities of Socrates and not activities of the soul, Socrates and the soul are not identical. And so Socrates, if anything, is a living animal just like the other animals. Tacitly this leaves open the possibility that there might be an animal soul for Socrates that is not identical to the intellectual soul, and as shown in 75.3 that this animal soul of Socrates would not be subsistent. This possibility of two souls in Socrates, an animal soul and an intellectual soul will only be excluded later in question 76. In 75.5 Thomas proves that the human intellectual soul is immaterial just like the souls of other animals. But in conjunction with the result of 75.2, now we have a soul that is an immaterial subsistent, where in conjunction with 75.3 the souls of other animals are not immaterial subsistents.

In 75.6, relying upon all that has gone before, Thomas argues that the human soul is a subsistent that is incorporeal, and thus does not cease to exist as a result of the death of the body. This result shows the soul to be a subsistent form that can exist without out matter. And so it is now seen to be an immaterial subsistent in the second sense described above, not just the first sense. Now ‘immaterial’ characterizes its mode of existence, not just the negative fact that it is immaterial like all other forms are immaterial.

So the difference between the human intellectual soul and the souls of other animals is that while both are immaterial in the first sense, the sense of not being material principles, the intellectual soul is an immaterial subsistent in the second sense while the souls of other animals are not immaterial subsistents. And it is the second sense of ‘immaterial’ that gives us a key for understanding what Thomas means by a “material form,” particularly a material substantial form. A material form is a form that is not an immaterial subsistent; it exists either as an accident in a corporeal subject or as a substantial form in a corporeal subject, and does not subsist. So the substantial forms of bodies, particularly the souls of living bodies, are in general material forms with the exception of the intellectual soul. The souls of other animals are immaterial in the first sense and material with regard to the second sense, while the human soul is both immaterial in the first sense and immaterial in the second sense.

Confirmation of this distinction of senses of ‘immaterial’ comes when in the very last article of the question, 75.7, Thomas asked whether the human intellectual soul is an angel. In 75.6 Thomas used the result of 75.5 and 75.2 to prove that the human soul is incorruptible, where something is corruptible if it can cease to exist through corruption, that is, by the separation of form from matter as we see it in the death of living things. The souls of other animals are notdirectly generated and do not directly corrupt. It is the living animal that corrupts. But their souls can be said to corrupt with the animal. (Quaestiones Disputatae De Anima 2) However, the human soul, because it is a subsistent immaterial form, does not corrupt with the death of the human being. So when all these results are put together the intellectual soul is an incorporeal, immaterial, incorruptible subsistent, an immaterial form in the second sense, which looks an awful lot like an angel, since angels are also incorporeal, immaterial, incorruptible subsistents, and immaterial forms in the second sense. This is the potentially distorting view of the theologian–to see the human intellectual soul as something like an angel, and we are reminded of Shakespeare, “how like an angel.” Notice, however, that the potential distortion is premised on the soundness of the philosophical arguments that have been employed throughout the question by the theologian, driven by his primary interest in the soul.

In 75.7, Thomas argues that the intelletual soul is not of the same species as an angel, because it is a substantial form of an animal. Angels are complete in their natures as incorporeal, immaterial, incorruptible subsistent forms—they are thus substances properly speaking. But Thomas had insisted all along that the soul is incomplete in its nature, even as it is an incorporeal, immaterial, incorruptible subsistent form—it is not a substance properly speaking. Still, the soul can be called substance by analogy, insofar as it is the formal principle of a substance. In English it might be better to call it “substantial” rather than “substance.” And in that regard, it cannot be considered as forming the basis for a kind of substance dualism in Thomas.

The argument of 75.7 leads naturally to the subject of the next major question, Ia.76, on the union of soul and body. We’ve already seen that Thomas, following Aristotle, thinks asking questions about the union of soul and body makes little sense for the philosopher. But because of the potentially distorting view of the theologian, the latter in a sense is forced to do so; the theologian has to ask philosophical questions the philosopher need not ask, in order to avoid a distorted view of the soul. So in question 76 Thomas argues for the complete unity of soul with body against various alternative positions to be found among his contemporary theological interlocutors. Thus question 75, proceeding as it does from the theological perspective, gives rise to philosophical aporiae to be solved in question 76. And just as it was the theologian’s use of philosophical arguments in 75 that threatened a distorted view, it is the theologian’s use of philosophical arguments in 76 that solves the aporiae, and avoids the distortion. Apart from anything else Thomas does in the two questions, taken together they provide an exemplar of the use of philosophy within theology, not just to advance certain theological positions but to assist the theologian in avoiding error given the exclusivity of his theological perspective. Thomas fulfills what he himself had said is one of the roles of philosophy within theology in the first question of the Summa.

There are at least three important results of Ia.75-76 for thinking about human nature. In the first place, in 76.3-4 Thomas argues against the pluralist position on the Plurality of Substantial Forms. It might be tempting to think of the human substantial form as a kind of layering of quasi substantial forms or as composed out of them. One substantial form for the corporeality of the body, perhaps one to account for the vegetative activities of the human being, yet another for the animal activities, and then a final one for the intellectual activities of the human being. Recall that 75.4, in arguing that Socrates is not identical to the intellectual soul, tacitly leaves open the possibility that Socrates might have a soul as principle for these vital animal activities in addition to the intellectual soul with which he is not identical. However, Thomas decisively rejects this plurality on the basis of the manifest unity of the human being in his acts. If there were multiple substantial forms there would be no unity to being human—multiple substantial forms implies multiple substances and multiple beings. And yet the human being is one, a single substantial unity manifested in his or her acts. Here Thomas is relying upon the substantial unity that is obvious to the philosopher to reject a kind of substance plurality, not just soul-body dualism.

In particular he relies upon the fact that it is Socrates himself who engages in intellectual activity. Again, in 75.4 he had rejected the view that Socrates is identical with his soul because Socrates engages in vital animal activities that do not belong to the soul as such. However, what he did not claim in 75.4 is that the activity of intellect that characterizes the soul is not one of Socrates activities. In fact, now in 76 he claims it is Socrates’ activity. Socrates has vital activities that do not belong to the soul alone, and yet the activity that belongs to the soul alone, understanding, is one of Socrates’ activities. But the soul is the principle of activity in living things. Thus the animal soul (and for similar reasons the vegetative soul) is identical in Socrates with the rational soul. There is no plurality of substantial forms because of the unity of Socrates’ activities, including both animal activities and reason. Neither is the human soul composed of any quasi-substantial forms.

This is the second striking result of 76. Socrates and his soul, while not being identical, are subjects of the same activity—not subjects of the same type of activity, but subjects of the sametoken instance of an activity. In 75, the soul as a subsistent with its own operation of understanding was said to be the subject of existence (esse) per se. In the case of other animals it is the animal itself, the living substance, that is the subject of the act of existence, and both soul and body have existence through the substance. Here in the human case, the soul is said to be the subject of the act of existence because it has its own operation. Of course, Socrates is a substance with operations that pertain to him, animal activities, but also the operation of intellect; it is Socrates who thinks in virtue of his intellect. So he too is the subject of the act of existence. And yet the operation in virtue of which the soul is the subject of the act of existence, intellectual activity, is the operation in virtue of which Socrates is the subject of the act of existence, again, not the same type of operation but the same token of operation. So Socrates and his soul have the same act of existence. The principle for drawing this latter conclusion is that the operation of a subject follows from the act of existing of that subject, as the actuality of a power follows from the actuality of the being. (Quaestiones Disputatae De Anima 2)

So Socrates, as a living animal substance, is not identical to his soul. Anima mea non est egoThomas asserts in his Commentary on St. Paul’s First Letter to the Corinthians. “I am not my soul.” And yet Socrates and his soul share as subjects the very same act of existence. It is because Socrates’ soul’s act of existence is Socrates’ act of existence that the soul’s intellectual operation is Socrates’ intellectual operation. It is also because of this sharing in the act of existence, that the soul can be the substantial form of the living human animal. Because the soul is a substantial form, it is not complete in its nature, and cannot be a spiritual substance like an angel, properly speaking. Thus the soul receives its act of existence as the soul of a human being, and cannot pre-exist the human being whose soul it is. And yet, as Thomas argued in 75.6, the soul is not subject to the corruptibility of the human being who is subject to death, and can thus survive the death of the human being.

The third significant result is that the soul is not composed from its powers as if a unified collection of them. Thomas will often speak of the “parts of the soul,” the appetitive part or the rational part, for example. However, this way of speaking is for the purposes of classifying the powers. It does not signal actual ontological parts of the soul. As the first act of a body, the soul is, like all act, ontologically simple, undivided, and un-composed. The powers “flow” from the essence of the soul as necessary accidents of the substance. And Thomas tells us they are formally related to the soul as their principle in what Aristotle calls in the Posterior Analytics the second mode of per se predication—that mode in which the subject of a predication enters into the definition of the predicate, were one to define the predicate. From this it follows that if the human soul is incorruptible, the powers of Socrates that are powers of corporeal organs cease to exist with the death of Socrates. And yet the power of intellect as a power of the soul without a corporeal organ remains incorruptible with the human soul. However, Thomas is clear in denying that only the intellect survives the death of the human; one cannot have a free floating incorruptible power in existence without the subject of the power in existence. The soul is other than the powers, and thus the intellectual power is incorruptible as a power of an incorruptible soul as its subject, and even the other powers remain “in” the soul as “in their principle.”

All of this emphasis upon the unity of the human being comes out clearly in Aquinas’ understanding of the mode of human activity as acting knowingly and willingly. Such acting knowingly and willingly is expressed as the rational activity of an animal, that is, as animal activity distinguished formally as rational. Rationality is the distinctive form that intelligence takes in human beings as animals. Rationality involves the back and forth of argument moving from one thing known to another, and advancing in knowledge by such movement. Thus, for Thomas, while angels and God can be said to be intelligent, they are not rational.

This movement in understanding is necessary for human beings because as animals they only ever have a partial grasp of the natures of things, insofar as their knowledge depends upon always incomplete and partial sensible experience of the world. But it is sense experience, as well as the self movement that springs from it, that places human beings within the genus animal. So human understanding and willing is intrinsically bound up with the sensate activity of an animal; as a result, rational is the form that it takes in that animal. Reason does not cause eating as something separate from it, and as an efficient cause; on the contrary, human eating is not adequately described formally unless it is described as rational eating. To fail to eat rationally is not a failure in its cause, but in the eating itself. And the human animal is not adequately described except as a rational animal, rational providing not another substance or expression of a fissure between soul or mind and body, but the fully adequate description of the human substance. Reason does not distinguish us from animals; it distinguishes us as animals. So according to Aquinas, while it is true that the activities of intellect and will are not the actualities of any physical organs, they are nonetheless the activities of the living human animal. It is Socrates the animal who knows and wills, not his mind interacting with his body.

Another consequence of this insistence on Aquinas’ part is that it is inadequate and inaccurate to speak of activities we share in common with other kinds of creatures. To be sure, there are descriptions that apply equally to what we do and what other animals do, for example the description “eating” or the description “reproducing.” But these are generic descriptions that do not adequately capture the human act as opposed to the act of a horse or dog, until the human acts are specified formally as rational. So the goods that are the objects of human powers are not specified adequately by such generic descriptions as pursuing eating, reproducing, friendship, etc., as if human beings and other animals pursue the same goods, only humans bring reason to bear upon those identical type goods.

All of this might lead one to think then that, not being a dualist, Aquinas must be a physicalist, there being only two broad possible positions. Now, the difficulties of providing an adequate account of just what Physicalism is are well known. But suppose we take a minimal characterization of Physicalism as involving the claim that there is some privileged physical science or set of physical sciences, using the term ‘physical’ merely nominally and sociologically as we use it of certain sciences today, that ideally will provide a fully adequate account of all that exists and the fundamental characteristics of reality. Then Aquinas cannot be understood to be a physicalist, since the result of his analysis of perception and thought was to say that these activities are “immaterial,” which was to say, not adequately captured by the kinds of physical descriptions that do adequately account for much of the being and change we observe in the world. There are actually many variations on Dualism and Physicalism in play in recent philosophy. However, the difficulty of placing Aquinas in the broad outlines of that setting ought now to be clear. And without an actual demonstration that Aquinas’ view is incoherent, one lasting contribution of his thought is to show that the supposed exclusive disjunction between Physicalism and Dualism is inadequate. He poses to us a challenge to think more broadly and deeply about human existence than such an easy dichotomy allows.