Since the rise and dominance of metaphysical naturalism in both science and philosophy, many academics have rejected the traditional Judeo-Christian understanding of the soul. To be sure, substance dualism–the view that both immaterial and material substances exist–is not a popular position amongst contemporary philosophers of mind. However, substance dualisms’ fall from grace is not entirely due to naturalistic philosophy. Another reason for its failure has been its persistent association with Cartesian dualism.
The linkage between substance dualism and Cartesian dualism in contemporary philosophy of mind is unfortunate for two reasons: (1) most philosophers believe Descartes arguments have been soundly refuted, and (2) the Cartesian form of dualism exhibits significant conceptual difficulties. It seems, then, that it is not substance dualism, per se, that modern philosophers find repugnant, but its widespread Cartesian formulation.
Thankfully, while Christians are committed to some form of substance dualism, they are not necessarily committed to a Cartesian view. As Eleonore Stump explains, Cartesian dualism is hardly the only game in town for orthodox Christians:
As a matter of historical fact . . . it is not true that a Cartesian sort of dualism has been the view traditional espoused by all major monotheisms. Aquinas, whose views surely represent one major strand of one major monotheism, is familiar with an account very like Cartesian dualism, which he associates with Plato; and he rejects it emphatically.
Unbeknownst to many, Aquinas proposed a form of substance dualism significantly different from both Plato and Descartes; one which naturalistic philosophers may find harder to refute.
This paper will introduce Thomistic dualism, compare and contrast it with the Cartesian view, and offer several reasons why Christian philosophers should favor this form of substance dualism above the Cartesian model. It will accomplish this by: (1) outlining Descartes understanding of the mind and the body and posing two formidable difficulties facing it, and (2) outlining Thomistic dualism and explaining how it better addresses the problems facing the Cartesian view.
Descartes and Cartesian Dualism
Cartesian dualism, as it is most commonly formulated, goes something like this: the mind, being a nonphysical object, is a completely different sort of thing than the body, which is a physical object, “located in space” and comprised of, “atoms familiar to chemistry.” Unlike the body, the mind is completely immaterial, lacking an exact special location, and unable to be seen or touched. Although the mind and the body are two completely different substances, they stand in a causal relationship with one another; each having a distinctive impact on the other.
To understand this causal relationship, one can imagine the mind and body working together in a way much like a scientist controlling a space probe:
Your body is like a probe, sent by NASA to explore a distant planet. The probe sends pictures back to mission control, where scientists decide what the probe should do next. Instructions are sent back to the probe which responds accordingly. The probe itself is entirely unintelligent. Similarly, information about the world is communicated by the body to the mind; the mind decides on a course of action and communicates the decision back to the body. The body itself makes no decisions.
As it stands, the body is not autonomous; depending upon the direction of the mind to accomplish anything.
Descartes arrived at these conclusions about the nature of the mind and the brain through a prolonged introspective process which he recorded in his seminal work: Meditations on First Philosophy. During his meditations, Descartes came to the startling conclusion that he could imagine himself without a body. Conversely, he also realized it was equally impossible for him to doubt that he had a mind: “If I try to doubt I have a mind, I will discover myself with thoughts like ‘I doubt I have a mind’, and so must admit that I have a mind—for the activity of doubting is mental.” Based upon these fundamental introspections, Descartes developed his theory of substance dualism.
As his thinking evolved, Descartes came to the disquieting conclusion that the mind constituted the total essence of the human being. As he explains, “I know I exist, and meanwhile notice nothing clearly to pertain to my nature or essence, except this alone, that I am a thinking thing, I rightly conclude that my whole essence consists in this one thing, that I am a thinking thing.” Descartes refusal to give the body a place in the essence of a human being was the natural outgrowth of his epistemology; namely, his near complete rejection of empiricism. For Descartes, the senses could not be trusted, and therefore, the only true knowledge one could have about the world was that which could be arrived at by pure reason. As a rationalist, the only aspect of his own humanity that he could be totally certain about, was the unavoidable fact that he was a “thinking thing.”
Seeing as how the mind and the body constitute two “ontologically distinct substances”, and that the, “mind alone constitutes the essence of the individual and the body in no way partakes of this essence,” Descartes concluded that the mind could exist completely independent from his body. This is made clear enough in his own words,
Because on the one hand I have a clear and distinct idea of myself, in so far as I am only a thinking thing, not extended, and on the other hand a distinct idea of body in so far as it is only an extended thing, not thinking, it is certain that I am really distinct from my body and can exist without it.
Unsurprisingly, this radical distinction between the mind and body is the source of significant challenges for Cartesian dualists.
To begin with, from a philosophical and scientific perspective, Cartesian dualists fail to provide an adequate account of the, “union of mind and body,” that is, to explain how two ontologically distinct and self-contained substances are capable of working together in a causally integrated way. Exactly, how is it that the mind (which is immaterial) can have a direct casual affect on the brain (which is material)? While, this objection does not constitute a defeater for Cartesian dualism, it does constitute a significant conceptual hurdle; one which is responsible for it’s rejection by contemporary philosophers.
Secondly, from a theological and anthropological perspective, Cartesian dualism fails to account for the importance or value of the physical body. Why is it, that God embodied our souls in the physical realm at all? Why is it that God insists on resurrecting our bodies in the last days? Why did God come down as the incarnate word of God—as the physical man Jesus Christ? Furthermore, why is it wrong, morally, to harm or disparage the human body? These, and other questions, are exceedingly difficult to answer under the Cartesian model.
It is for this reason that the author recommends the following Thomistic form of substance dualism.
Aquinas and Thomistic Dualism
In order to understand Aquinas’ view of the soul, one must first have a basic grasp of Medieval metaphysics. Like Aristotle, Aquinas and other scholastic philosophers distinguished between two basic dimensions of physical entities: matter and form. To understand Aquinas’ distinction between matter and form, one must strip away any modern conceptions of matter they might have.
To begin with, one should not think of matter in a physical sense; that is, “as an independently existing raw material yet to be formed.” Rather, one must think of matter in terms of “potentiality”; as Aquinas explains, “Matter is that which is not as such a ‘particular thing,’ but is in mere potency to become a ‘particular’ thing.” In other words, matter (or ‘prime matter’ as Aquinas termed it) on its own has no existence; it simply has the potential of being actualized into an existent entity. In this sense, prime matter is, “that which is common to all material things and which is formed into different entities by the second element—the substantial form.”
This leads naturally into the second dimension of all physical entities: the substantial form. The substantial form is the, “inherent principle which makes the existent entity [a physical object] what it is.” Or, put another way, the substantial form constitutes the essence of and actualization of a physical entity. One could very well imagine the substantial form as being the “information”, “blue-print”, or “code” underlining all reality and bringing physical existence into being.
When combined with prime matter, the substantial form “becomes the existent [individual] entity,” or substance. Hence, the substantial form acts as the grounding of all physical being; and as such, any physical thing that exists, or has being, has form. This basic metaphysical truth can be most easily remembered in Aquinas’ famous dictum: “form gives existence to matter.”
Seeing as how form is the essence of all existing entities, and provides grounding of being, it follows that physical entities are not the only objects which have form. According to Aquinas, immaterial entities (such as God or Angels) do as well–he referrers to this as the subsistent form. Subsistent form applies only to immaterial entities which have no physical instantiation—in this sense, it can be understood as, “form existing on its own.”
Aquinas’ motivation for adopting this view was derived from Augustine, who believed that, “being is a matter of having order, species, and mode.” In other words, anything that has being or existence must have some distinguishable order, configuration, or characteristics which make it distinctive from other entities; that is, it must have an essence. This is why immaterial entities, like angels, must also have form.
Now that these basic metaphysical principles have been elucidated, the reader is in a position to grasp Aquinas’ understanding of the human soul.
Aquinas and the Human Soul
To begin with, Aquinas understood the soul to be the substantial form of the body. He maintained this view because human beings are a part of physical reality; as such, like any other physical entity, they require a substantial form in order to instantiate or actualize their physical existence. As the reader will recall, “a substantial material form is the configurational state of a material object that makes that object a member of the kind or species to which it belongs and gives it the causal powers characteristic of things of that kind.” Accordingly, the soul, acting as the substantial form of the body, is what configures and differentiates and gives life to a unique human individual.
J. P. Moreland explains it this way: “the soul is an individuated essence that makes the body a human body and that diffuses, informs, animates, develops, unifies and grounds the biological functions of its body.” As such, the human person “must be defined as a deep unity of form and matter.” Accordingly, the combination of body and soul, like matter and form, constitutes one complete substance.
However, the soul, unlike the substantial form of other physical objects, may persist upon the death of the body, and unlike other material things is capable of existing without a physical instantiation. For this reason, Aquinas also identified the soul as being a subsistent form. In this sense, the human soul is truly unique, in that it has its foot in both the material and immaterial realms. Stump elucidates this paradoxical reality:
The human soul . . . is a configured configurer. On the one hand, like an angel, it is able to exist and function on its own, apart from matter. On the other hand, the human soul is not, as Plato thought, a spiritual substance moving the body which is also a substance in its own right; rather, the human soul is the substantial form constituting the material substance that a human being is, and it configures matter, as material forms do.
Hence, the soul, as both the substantial and subsistent form, contains the information that constitutes human essence; and even after we die, and the particles of our body dissipate, the information, that constitutes our essence, our identity, continues to exist.
However, it is important to note that when the soul is separated from the body this constitutes an abnormality; that is, the existence of the soul apart from the body is an incomplete one. As Aquinas explains, “since the soul is a part of human nature, it does not have perfection of its nature except in union with the body . . . and so, although the soul can exist and intellectively cognize when it is separated from the body, nonetheless it does not have the perfection of its nature.”
Facing the Challenges of Cartesian Dualism
The reader may recall that Thomistic dualism was offered up as a happy alternative to Cartesian dualism in light of the significant challenges facing Descartes theory. This final section will briefly demonstrate how Thomistic dualism faces up to the considerable challenges facing the Cartesian model and why Christian philosophers should favor this form of substance dualism above others.
The first hurdle facing Cartesian dualism was its inability to define the union between the soul and the body–more pointedly, it’s failure to explain how two ontologically distinct and self-contained substances are capable of working together in a causally integrated way. While this is a formidable problem for Cartesian dualism; the Thomistic dualist has less to be concerned about. This is because the Thomistic dualist, unlike the Cartesian, is not postulating the existence of two individual substances; rather they are postulating one complete being whose nature is comprised of both material and immaterial form.
As Aqunias says, “We must not think . . . of the soul and body as though the body had its own form making it a body, to which a soul is super-added, making it a living body; but rather that the body gets its being and its life from the soul.”  Under the Thomistic view, the union of body and soul is deep and well defined; hence, the conceptual problem of explaining the causal relationship between soul and body is much less significant.
Regarding the second hurdle, concerning the theological and anthropological significance of the body, Thomistic dualism is completely unfazed. Unlike the Cartesian, the Thomistic dualist holds that the physical body is part of the nature of man. Under the Thomistic model it’s considered normal for a soul to be united to a body; and, while the soul can subsist without a body, this is considered abnormal. Accordingly, it’s easy to see why God places such importance and value on the human body; why God sent his son to exist as a physical man; and why God intends to reunite our souls with a new glorified body at the resurrection.
While Cartesian dualism is viewed by many philosophers as being the only option for someone interested in substance dualism; it is certainly not the only game in town. As this paper demonstrates, Thomistic dualism offers a dynamic and arguably superior alternative to the Cartesian view. Unlike Descartes’ model, Aquinas’ conception of the body and soul is comprehensive and avoids the major challenges often thrown at substance dualism; by providing a clearer picture of the soul’s unity with the body and rendering the body intrinsically valuable. Consequentially, Thomistic dualism is more likely to stand up against the materialistic explanations of the mind which currently dominate western thinking; and place substance dualism back on the table as a viable option.
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Aquinas, Thomas. Thomas Aquinas: Selected Writings. Translated by Ralph McInernny. New York: Penguin Books, 1998.
Graham, George. Philosophy of Mind: An Introduction. New York: Blackwell Publishers, 2002.
Moreland, J. P. and Scott B. Rae. Body & Soul: Human Nature & the Crisis in Ethics. Downers Grove: IVP Academic, 2000.
———. P. and Stan Wallace. “Aquinas versus Locke and Descartes on the Human Person and End-of-Life Ethics.” International Philosophical Quarterly XXXV, no. 3 (Fall 1995). http://www.afterall.net/papers/490580.
Moyal, George J. D., ed. Descartes: Critical Assessments Volume III. New York: Routledge, 1991.
Ravenscroft, Ian. Philosophy of Mind: A Beginner’s Guide. New York: Oxford University Press, 2005.
Stump, Eleonore. Aquinas. New York: Routledge Taylor & Francis Group, 2003.
Eleonore Stump, Aquinas (New York: Routledge Taylor & Francis Group, 2003), 191. Emphasis mine.
Ian Ravenscroft, Philosophy of Mind: A Beginner’s Guide (New York: Oxford University Press, 2005), 9.
George Graham, Philosophy of Mind: An Introduction (New York: Blackwell Publishers, 2002), 151.
George J. D. Moyal, ed., Descartes: Critical Assessments Volume III (New York: Routledge, 1991), 150.
J. P. Moreland and Stan Wallace, “Aquinas versus Locke and Descartes on the Human Person and End-of-Life Ethics,” International Philosophical Quarterly XXXV, no. 3 (Fall 1995), http://www.afterall.net/papers/490580.
Moyal, Descartes: Critical Assessments Volume III, 150.
 It is the opinion of the author that this is the primary reason why Thomistic dualism is completely overlooked in contemporary discussions in the philosophy of mind.
 Moreland, International Philosophical Quarterly.
 Thomas Aquinas, Thomas Aquinas: Selected Writings, trans. Ralph McInernny (New York: Penguin Books, 1998), 19.
Stump, Aquinas, 198.
 Ibid., 200.
 Ibid., 197.
 J. P. Moreland and Scott B. Rae, Body & Soul: Human Nature & the Crisis in Ethics (Downers Grove: IVP Academic, 2000), 202.
 Moreland, International Philosophical Quarterly.
 This point can be confusing because Thomistic dualism is classified as a form of substance dualism. The truth be told, Thomistic dualism is a bit strange; it doesn’t fit into any category neatly. It is not a materialistic reductionist theory which reduces the mind to the brain and it also doesn’t fit well within the confounds of property dualism (the postulation that mental states emerge from brain states). Seeing as how Thomistic dualism still distinguishes between material and immaterial entities, it makes more sense to classify it as a form of substance dualism.
 Stump, Aquinas, 200.
 Ibid., 200-201.
 Ibid., 201.
 Moreland, International Philosophical Quarterly.</I>