For one who thinks that there is nothing more to reality than what can be detected and explained by the empirical sciences, discourse about the soul, especially the human soul, remains at best a quaint reminder of outmoded views of nature, at worst an absurd hindrance to our knowledge of the world. In such a view, the title of this essay needs an apostrophe so that we might discuss the material makeup of what has traditionally been called a “soul.” My title, however uses “matter” as a verb, not as a noun.
For materialists, souls simply do not matter. Nevertheless, despite the sustained efforts of thinkers such as Paul Churchland, Daniel Dennett, and countless others, there continues to be a lingering sense that there is more to living beings than simply their material components. What this “more” might be remains a subject of contention.
Most discussions about souls focus on human souls, but human souls are only one kind of soul. The word “soul” (psyche in Greek, anima in Latin) traditionally refers to the key feature of living things that makes them be the living things that they are. Thus, for example, Aristotle and Thomas Aquinas speak of the souls of vegetables, of animals, and of human beings. Each kind of living thing has its own proper source of being alive, its own proper kind of soul. Indeed, each natural substance, whether living or not, has its own proper informing principle, its own source for being the kind of thing that it is. To speak of souls is to recognize a fundamental difference between the non-living and the living and, accordingly, to recognize a distinguishing source or principle for the living precisely as living.
In discussing human souls, those in the tradition of Platonic philosophy affirm a kind of dualism—that is, the view that human beings are composed of two distinct substances, body and soul. For many Christian authors, both ancient and contemporary, some version of “dualism” seems not only philosophically compelling, but theologically required in order to be consistent with the doctrine of the immortality of the soul. Among contemporary philosophers of mind, there are many who defend a dualistic understanding of human nature, with no reference to religious belief.
It is really dualism that is the object of criticism by those scientists and philosophers, working within a materialist understanding of the world, when they call into question the intelligibility of a “soul.” Dualism, however, is not the only understanding of human nature that employs the notion of a soul. As we shall see, Aristotle and Thomas Aquinas are not dualists, although they both affirm the existence of the soul.
Empirical Science and Natural Philosophy
In a new book, The Soul Fallacy: What Science Shows We Gain by Letting Go of Our Soul Beliefs, Julien Mussolino, professor of psychology and member of the Center of Cognitive Sciences at Rutgers University, offers a sustained criticism of those whom he calls the “New Dualists.” His focus is on the human soul and, in particular, on its “detachability” from the body. The soul, understood in such terms, is for Mussolino “a scientific claim.” He writes:
Because belief in an immaterial soul represents a cluster of scientific hypotheses about physics, biology, and the sciences of the mind, determining whether we have souls is as objective a quest as answering questions about the origin of species or the age of the universe.
He has little difficulty affirming that “scientists have discovered that there is no credible evidence for the existence of the soul, that soul claims fly in the face of what we know about modern science, and that there is overwhelming evidence supporting materialism.” Mussolino notes that the “soul” he is rejecting is an immaterial entity in human beings, a particular version of a dualistic conception of human nature.
What counts as “scientific evidence” depends, of course, on what one includes under the category of science. Questions about souls—in the broad sense of the animating principles of living things—are located not in the individual empirical sciences themselves, but in a more general science of nature known as natural philosophy. It is a mistake to restrict our understanding of nature to what the empirical sciences alone tell us about the world. Rational discourse embraces a wider arena of knowledge. The philosophical disciplines, including metaphysics, further extend our understanding of reality; and all these do not require an appeal to religious faith. Christian faith does make claims that human souls are immortal and in some way separable at death, and that in the resurrection the human being, body and soul, exists in a new way. Thus, for Christians, dualism has always had a seductive appeal.
One of the many conceptual errors Mussolino and others make is to think that a commitment to the empirical sciences as sources of truth necessarily means a commitment to a materialist natural philosophy. Any discussion of souls, or really of any other subject concerning the nature of things, does require knowledge disclosed in the empirical sciences. Knowledge so gained, however, needs to be brought together in more general reflections about nature and human nature. It is really not possible to avoid conclusions about the world that go beyond the discoveries of individual sciences. Mussolino’s own claim that human knowledge is limited to only what the empirical sciences disclose is itself a philosophical claim, but a claim that denies the role of any philosophy of nature.
Mussolino’s approach fails to be persuasive partly because he engages only the very limited notion of the “soul” provided by a dualistic understanding of human nature. In that understanding, the “soul” is viewed as an independent entity, existing somehow in union with another independent entity, the body. But there are other, more philosophically rigorous accounts of human nature and human souls. Even if Mussolino’s criticism of what he takes human souls to be were true, his arguments do not really challenge the views of thinkers such as Aristotle and Thomas Aquinas.
The Natural Philosophy of Souls
We need to locate philosophical and theological claims about human souls in the wider context of the souls of all living things. We need to understand souls in the even wider context of the principles of all natural things, living and non-living.
Each natural substance has an actualizing principle that makes it be the substance that it is. Such a principle, known in the Aristotelian tradition as a form, is the source of the actuality and unity of what exists. Materialist and mechanist philosophies of nature tend to view things as mere conglomerations of material parts: nothing more than parts contiguous with other parts. Thus, they cannot really account for the unity of the whole entity. As recent proponents of theories of emergence in biology recognize, however, there is more to natural substances than their components. These biologists speak of “emergent properties” that are not reducible to the material components of a thing. For some, consciousness is an emergent property, not identical with the neural structure of the brain. The traditional notion of a form or principle of actuality can help us to understand such properties.
To use an expression from Ed Feser, the soul is “that by which” a living thing is the unified living thing that it is. The soul is not a “what;” it is “that by which.” A “that by which” is a real principle that is a necessary source of the actuality of a thing. Water, for example, has a proper form, its actualizing principle, that is distinct (but does not exist separately) from its material components. That there must be such actualizing principles follows from the need to account for the unity of each natural substance. In order for a natural substance to be, to exist, it must have a unifying, actualizing principle. And to reiterate, the term traditionally used for the form, the actualizing principle of living things, is “soul.”
Soul is not some outer shell or structure. It is not an independent substance. Rather, it is an intrinsic determining principle—an expression of the fact that the whole is a new reality, not reducible to its material components. Soul is not something added to, inside, or united to a physical thing. Soul is what makes a living being the kind of living thing it is, and a human soul makes one a human being.
Consciousness, Mind, Will, and Soul
It is easy, but wrong, to identify the human soul with consciousness, mind, or will. A human being, the one substance that is the unity of body and soul, is conscious. It is not the soul that is conscious, nor is it our consciousness. A human being is conscious, and consciousness is a feature of the kind of living thing a human being is. It is not that the soul makes a human being conscious; a soul is the actualizing principle of the entire human being precisely as a human being. As an actualizing principle, the soul is the source of the whole reality, the actuality, of that of which it is the actualizing principle.
Similarly, the soul is not our mind. Human beings think. The mind is the faculty or power that enables us to think, but thinking is a human activity. That human beings have the ability to think, an activity that is both material and immaterial, flows from what it means to be a human being. The human soul is the actualizing principle of a human being; it is not some internal thinking thing in us.
An error here is to think of the soul as an efficient cause of something (for example, thinking) rather than as a formal cause, a cause of the actuality of the whole and hence a cause of the actuality of all the features of an individual thing. As we saw in Mussolino’s account, since empirical science can show no evidence for an invisible efficient cause in human beings, there is no good reason to suppose the existence of the human soul if we conceive of it as an efficient cause. For most philosophers since Descartes, there are only two kinds of causes: efficient and material. Hence, with only two options, the soul, if it exists, has to be a kind of mover, something that pushes or pulls. The distinction between efficient cause and formal cause is often lost in contemporary discussions about nature, but it is an important feature for any sound philosophy of nature.
Our ability to choose is another feature of what it means to be human, and it too involves material and immaterial dimensions. But the human soul is not our free will; it is not something within us that chooses. Again, human beings choose and they are able freely to choose because human beings are the kind of things that they are. It is not a part of us that chooses, any more than it is a part of us that thinks, or a part of us that is conscious. The human soul is not some hidden “I” inside us.
The Incorruptibility of the Human Soul
Thomas Aquinas thought that we could know by reason alone that each human soul is neither corruptible nor destructible and therefore possesses a kind of natural immortality. His argument in metaphysics is very complex, and it rests on his understanding of the special kind of informing principle that a human soul is. He thought that since certain features of being human, such as thinking, were not directly the activity of a particular organ (e.g., the brain) there was a sense in which the informing principle, the soul, had a kind of independence from the human being that it informed. The term he used was “subsistent form.” As the source of a human being’s rationality, a power or faculty that has an operation or function ultimately independent of matter, the human soul is really separable from the body.
Thomas was always alert to avoid a kind of dualism that posited two distinct substances, body and soul, in man. Yet he also recognized that the human soul was a very special case. He did not think that his understanding of the human soul as incorruptible contradicted the more general claims in the philosophy of nature about the soul as the informing principle of living things.
A materialist natural philosophy that denies the reality of formal and immaterial features of the world is an impoverished view of nature, including human nature. In any complete analysis of what it means to be a living thing and, in particular, a human being, souls matter. Without souls, there are no living things.
William E. Carroll is Thomas Aquinas Fellow in Theology and Science at Blackfriars Hall, Oxford, and a member of the Faculty of Theology and Religion of the University of Oxford.