What is The Human Soul

The most concise and authoritative statement of Aquinas’s theory of human nature comes in questions 75–89 of the first part of the Summa Theologiae, known as the Treatise on Human Nature. 3 That discussion begins with a rapid argument for the twin that human beings have a soul and that this soul is not a body. These look like giant, contentious claims to come so quickly at the start. Still, Aquinas is quick here for a reason: one of the claims is simply a matter of terminological stipulation, and the other is such a big question that it can scarcely be adjudicated within the context of a discussion of human nature. What is supposed to be confirmed by a stipulation is that human beings have a soul. Following Aristotle, Aquinas holds that “the soul is said to be the first principle of life in the things that are alive around us.” 4 This means that “soul” is simply a convenient catchphrase for the sort of thing that biologists investigate to this day — the fundamental (“first”) explanation (“principle”) of life in the natural world. If Aquinas were here assuming that there is just one kind of explanation shared by all living things or even that within a single item, there is just one fundamental explanatory principle; then he would be saying something controversial. But these are further issues that, as we will see, he takes up later. For now, we have just the stipulation that “soul” will be used not in any speculative, supernatural sense but the down-to-earth biological sense recommended by Aristotle. Too large to be treated adequately within a theory of human nature is the further thesis that the human soul is not a body. This is not the claim it is likely to seem at first glance. Aquinas does not suppose from the start the truth of dualism in its popular, bastardized form — the idea that the soul is not made of material, human stuff, and so must be made of some other, more ethereal stuff. This is a thesis that medieval authors entertained, but they did so about the celestial realm. Like all of his contemporaries, Aquinas took the heavens to be made of an imperishable sort of stuff utterly unlike the things in our familiar material realm — not composed of any of the four elements, then, but of some quintessence. This is an idea that has tempted cosmologists ever since Aristotle, holding sway until In the seventeenth century, Galileo went out of fashion and now came back into fashion with our modern talk of dark matter. However, as far as our natures are concerned, it has never seemed very credible on serious reflection to suppose that we are composed of some dark stuff of our own, invisible but yet constituting our essence. This way of understanding dualism, indeed, is one that only an opponent of the theory is likely to find very appealing. Historically, the advocate of dualism has generally wanted to say that what makes human beings unique is that we contain some amazing, ghostly stuff. Our nature is partially constituted by something that is not stuff at all but is an entirely different explanatory principle. In the Aristotelian tradition, this principle is known as a form. Aquinas’s fundamental thesis about human nature is that we are not just bodies but bodies animated by a certain kind of form, a soul. This is, however, not a result that is specific to human beings; it is instead an instance of Aquinas’s general embrace of Aristotle’s hylomorphic metaphysics — that is, the thesis that human substances, in general, are form–matter composites. Matter by itself — “prime matter” — cannot exist at all without form: “in itself, it can never exist, because given that by its nature it has no form, it has no actual existence, since actual existence comes only through form, whereas it is solely in potentiality.” 5 On this understanding of the hylomorphic framework, everything that exists has form. So the fact that human nature consists not just of a body but of an informed body is just an application of a broader metaphysical thesis. As we have seen, to call this form a soul is simply an application of the stipulative point that, in the case of living things, that fundamentally makes them alive is what will get called their soul. Hence, all things have formed. And all living things have souls. Nothing could be more critical to an overall evaluation of Aquinas’s theory of human nature than a just appraisal of its background hylomorphism. Considered most broadly, the appeal to form represents a rejection of the reductive approach of much of early ancient philosophy.
The mainline of pre-Socratic thought, culminating in the atomism of Democritus, approached philosophical explanation as an exercise in finding the right sort of material stuff to serve as the building blocks of nature. According to Aquinas, “the first of those who philosophized about the natures of things held that only bodies exist. They claimed that the first principles of things are certain corporeal elements, either one or many.” 6 On the more straight line of thought pursued by Plato and then, in a different way, by Aristotle, the explanation requires appeal not just to matter but to form. In one shape or another, this idea would hold sway throughout late antiquity and the Middle Ages, first dominating Islamic philosophy and then Christian, all the way until the seventeenth century, when Descartes and others suddenly shook it off and turned back to the reductive approaches of old. Famously, Descartes drew a line between the human case and others, treating the rest of the natural world as simply bodies in motion while ascribing to human beings alone an explanatory principle of another kind, an immortal soul. This is an instance — indeed the exemplary instance — of Aquinas’s supernatural approach only partly embraces. In his view, instead, human beings have a form just as all things have a form and have a soul just as all living things have a soul. Descartes, however, sees things quite differently. From his perspective, Aquinas and others are best seen not as applying their overarching naturalism to the human case, but as overgeneralizing from the human case to the rest of nature. That is, Descartes regards the appeal to form as inherently supernatural in character, and so treats talk of forms and souls throughout nature as a sort of misguided obscurantism that makes it impossible to give a naturalistic explanation of anything. 7 However much we may regard hylomorphic explanation as appropriate for the natural world in general, Aquinas thinks that its application to human beings must be handled carefully, inasmuch as we are indeed a special case. For even while he begins the Treatise on Human Nature by situating human beings within the rest of nature, as creatures composed of matter and form, he immediately turns to show that human beings are special, in virtue of having a form that can exist apart from matter. Here is where, as he sees it, the naturalistic approach runs out. Obviously, this is a result that Aquinas needs as a Christian, since if human beings are to survive death it is minimally required that their souls survive death, which means that these souls must survive the destruction of their bodies. Over the course of his career, Aquinas makes various attempts to prove that the human soul can exist apart from its body, and something should be said about these arguments. The fi rst thing to consider, however, is whether it is even coherent to treat the human soul both as a form and as independent from matter. One bad reason for suspicion is an overly crude conception of what a form is. To be sure, if one thinks of a form on the model of a shape, then it will look just preposterous to suppose that the human soul can exist apart from its body. It is indeed hard to see how anyone could think that a shape can exist apart from some sort of stuff that has that shape. Aquinas, however, as will become progressively more clear, does not think of souls as anything like shapes. A moment’s refl ection will make this obvious. For even if it is natural to motivate the hylomorphic framework by appealing to a case like a statue, where the matter is the bronze and the form is the shape, the human case must clearly be quite different. A statue, perhaps, can be roughly understood as nothing more than a certain sort of stuff having a certain sort of shape — though even here the clever student will see the potential for diffi culties. But a human being is more than a certain sort of stuff so shaped. That will not take account, most obviously, of what distinguishes a living body from a corpse. The reductive materialist must disagree. If human beings are just so many molecules organized in such and such a way, then the difference between a living human being and a corpse just will consist in either a difference in molecules or a difference in how those molecules are arranged. But from Aquinas’s perspective one can have all the right material and still not have a human being, not because the materials have the wrong spatial alignment — the wrong shape — but because they are lacking some further explanatory principle, a soul. Forms, for Aquinas, are not mere shapes, but are causal principles in the natural world. They are indeed the primary causal principles in nature, actualizing matter that would otherwise be characterless and inert, if it could exist at all. 8 A soul without a body is therefore not to be conceived on the model of a free-fl oating shape. Souls are causal agents, powers. A soul is a form of a special kind, a substantial rather than an accidental form, which is to say that it is the kind of form that defi nes a substance as what it is. Setting aside until later the precise role of a substantial form, and considering forms in general as causal powers in nature, it may look as if all such forms should be able to exist apart from matter. This is precisely the sort of result one fi nds mocked in Descartes and other seventeenth-century critics of scholasticism. Aquinas, however, thinks the human soul is a special case — the only case where it is naturally possible for a form to exist without the matter it informs. 9 Only our substantial forms are such that “they do not exist in matter in such a way that their existence depends on matter.” 10 This is certainly a good reason to be suspicious about the human soul’s alleged separability. Why should it be a special case, in a way so nicely calibrated to accommodate the Church’s teachings on human immortality? The key idea, for Aquinas, is that the human soul is the sort of causal agent that can operate independently of the matter it inheres in. Whereas every other form in the natural world can act only in virtue of informing a body of the proper sort, the human soul does not require a body. More specifi cally, the human soul can think without a body. This is the fundamental premise on which the whole of Aquinas’s theory of human nature rests. Let us call it the Independent Operation Premise (IOP) and state it in Aquinas’s own words: IOP: “The intellectual principle, which we call mind or intellect, has an operation of its own, which the body has no share in.” 11 Before turning to the arguments for IOP, we should be clear about what it entails. By itself, clearly, IOP does not show that the “intellectual principle” (the soul that is ultimately responsible for intellectual cognition) is immortal. Moreover, IOP does not even show that the soul has the possibility of existing apart from the body. To get those further results, Aquinas argues, fi rst, that a thing’s manner of operation tracks its manner of existence, so that whether or not a thing can operate apart from other things shows whether or not it can exist apart from other things. This shows, as Aquinas thinks of it, that the human soul is a substance, because to be a substance just is to be the sort of thing that can exist without inhering in something else. 12 Of course, not all substances are incorruptible, so to get the further result that the human soul will naturally continue to exist even apart from its body, Aquinas further argues that whereas form–matter composites are always corruptible, substances that are pure forms are by nature such that, once created, it is impossible for them naturally to cease to exist. 13 The supplementary principles just mentioned are perhaps just as doubtful as is IOP itself, but even so it seems right to keep our focus on that fundamental premise. For if Aquinas can establish that the human soul has an operation of its own, independent of the body, then he will have dealt a fatal blow to the sort of reductive materialism that, then as now, looms as the main adversary to a view like Aquinas’s. For we would then know that “human soul” is not just a convenient catchphrase for whatever it is that explains human life, but that it in fact picks out an independent causal principle within us, irreducible to any material description. Admittedly, that alone does not show that the soul can exist apart from the body, but it takes the decisive first step.