1 Substance Dualism in Descartes

Paul Richard Blum


René Descartes (1596-1650) was a French philosopher who is often studied as the first great philosopher in the era of “modern philosophy.” He is the most famous proponent of a view called “substance dualism,” which states that the mind and the body are two different substances. While the body is material (corporeal), the mind is immaterial (incorporeal). This view leaves room for human souls, which are usually understood as immaterial. Descartes argued on the basis of the Christian views that souls are immaterial and can exist separate from the body, but he emphasized that the mind alone is immaterial, whereas the other traditional functions of the souls can be explained as corporeal operations. His view and arguments were so influential that after him many philosophers referred to substance dualism under Descartes’ name as “Cartesian dualism.” In his explanation of the mind, the soul, and the ability of humans to understand the world around them through the powers of their minds, Descartes remains one of the most influential figures not just in modern philosophy, but throughout the history of philosophy. Even in the contemporary era, philosophers such as Gilbert Ryle (1900-1976) found worth in writing about and arguing against Descartes’ views to set up their own theories. Ryle questioned whether the mind and body are in fact distinct and argued that they would not communicate with each other if they were. Ryle states:

Body and mind are ordinarily harnessed together….[T]he things and events which belong to the physical world…are external, while the workings of [a person’s] own mind are internal….[This results in the] partly metaphorical representation of the bifurcation of a person’s two lives. (1945, 11-16)

Ryle stated that, if Descartes’ theory were correct, the mind would be a mere “ghost in a machine,” inactive and unable to cause actions in the body (the machine). Ryle did not term Decartes’ theory “substance dualism” but “Descartes’ myth.” Descartes’ arguments for substance dualism and the immaterial nature of the mind and soul are therefore paramount to any investigation of the philosophy of mind, and are still being debated in present-day theories. On the other hand, with his interpretation of what he calls passions (most operations of a living body), he also provides incentives for a non-dualistic physicalism of the mind.

The Traditional Concept of Substance

Descartes’ philosophy of mind was a response to the erosion of the traditional Aristotelian concept of substance after the Middle Ages. According to the Aristotelian view, any substance is composed of matter that is determined by the form that is its essence. So every living thing is a body conjoined with its soul (namely, what makes it alive as such or such thing). In other words, an animal is an animate body. The soul of a dog makes that bundle of flesh and bones a dog. The peculiar case of human beings is that this soul is also an intellect: the rational mind. In that case then, the soul (and certainly the mind) is something other than body; it is non-material (or incorporeal) because it forms and enlivens the material body. So the question arises: is the soul (or at least the human mind) something that exists on its own?

In the traditional Aristotelian approach, the form of a ship (what makes it look like a ship and makes the ship body float on water) is nothing separate from the ship, except that we can have a concept of it even if there is no ship around. But what about the form of a plant or an animal? The form of plants and animals is their soul. When they are destroyed, their form that makes them alive (with growth, movement, and senses) is gone. With human beings, that might be different: the mind may survive the death of the body. Some ancient thinkers argued that the mind or the soul survives death and enters another body, be that a person or a beast: the transmigration of souls or reincarnation. The Christian theory of humans teaches that the soul of an individual is created at the same time as the person; however, it lives on after the death of the person: the human intellect is immaterial and immortal. This is why some Christians venerate saints, and why some occultists invoke deceased persons for conversation.

The essence of things (whether an artifact like a ship or the souls of plants, animals, and humans) was termed the thing’s “substantial form.” Forms make and express the substance of things. The thing’s substantial form makes a thing what it is, and makes it possible to conceive of it and to know it.

This is where Descartes starts his theory of substances. In a letter to Henricus Regius (1598-1679), Descartes states that he does not reject substantial forms but finds them “unnecessary in setting out my explanations” (AT III492, CSM III 205).[1] He clearly sees them as a mere explanatory tool that may be replaced by a better one. Instead, Descartes suggests any material thing is only an aggregate of qualities and properties. He argues, in the same letter, against the habit to apply “substantial form” when defining the human being. He warns that to speak of substantial form both for humans and material things carries the risk to misunderstand the soul as something corporeal and material. Instead, he suggests limiting the term “substantial form” to the immaterial human soul alone in order to emphasize that the soul’s nature is “quite different” from the essence of things that “emerge from the potentiality of matter.” He says that “[T]his difference in nature opens the easiest route to demonstrating [the soul’s] non-materiality and immortality” (AT III 503, 505; CSM III 208). In order to elevate the soul to a level above bodily things, he downplays non-human things to mere upshots of matter. This letter shows that Descartes’ primary concerns are with method more than with facts and that he aims at separating material fields of knowledge from the soul.

The Immaterial Nature of the Soul

Descartes attempts to reconcile having an immaterial soul within a largely scientific (and physicalist) framework. This leads to some surprising turns within his theory that are quite different from previous theories on substances. Ultimately, Descartes’ view is dualist because, although he renders all earthly substances material (and understandable to science), one thing remains that is a true immaterial substance with an essence: the human soul. Animals and human bodies, because they are parts of the physical world, are not strictly substances with essences; they are more properly aggregates. He argues from what we can know (epistemology) instead of what there is (metaphysics), and this method directs his views on substances.

From the very beginning of his research, Descartes aimed at exploring the competence of thought in ascertaining knowledge, and in doing so he wrote Rules for the Direction of the Mind in search for assurance in science. This view would later be called “rationalism” because he prioritized the functions of intellect, imagination, sense perception, and memory. Rationalism influenced a long line of philosophers from the modern era throughout the contemporary era in philosophy. He later recommended a reduction of human knowledge from simple concepts and propositions. This method, as expounded in Rule XII, relies on the human mind as a “power.” He states:

As for the objects of knowledge, it is enough if we examine the following three questions: What presents itself to us spontaneously? How can one thing be known on the basis of something else? What conclusions can be drawn from each of these?

Notice his emphasis on the understanding of objective knowledge. The question is not “What is it?” but “How does it appear to me?” and “How does it connect with what I know?” Investigating the nature of the mind is of primary importance. Knowledge of objects themselves takes a back seat to the inner workings of the mind.

Descartes describes the intellect as “the power through which we know things in the strict sense [that] is purely spiritual, and is … distinct from the whole body.” To explain this power is difficult; Descartes explains that “nothing quite like this power is to be found in corporeal things.” It is the intellect that applies itself to seeing, touching, and so on; and only it can “act on its own,” that is, to understand. Although it may appear to be a trifle, Descartes does not make positive claims here, but buffers everything with “it is said” (dicitur): the mind “is said” to see, touch, imagine, or understand. What counts is that this mental power can both receive sense data and refer to themes that have nothing corporeal at all (AT X 410-417, CSM I 39-43).

In his last work, The Passions of the Soul, Descartes focuses on those activities that are not thoughts in the abstract sense but “passions”: “those perceptions, sensations or emotions of the soul which we refer particularly to it” (AT XI 349, CSM I 338f., art. 27). The body has a number of functions (movement, for instance); and the soul has two basic functions that are kinds of thought, namely, volition and perceptions. Volitions are activities, whereas perceptions are passive motions that do not originate from the soul itself (AT XI 349, CSM I 338f., art. 17). If a person desires something or resolves to do something, that is an activity of the soul; if a person sees or hears something, that impression does not come from inside but from outside—the soul is affected rather than active. This soul is not a member of the body; therefore, it has the surprising property not to have any location in the body, but to be “really joined to the whole body” precisely for being non-local, not extended, and immaterial. On the one hand, Descartes is reiterating the traditional Aristotelian understanding of ensoulment (the soul as shorthand for the life of animated things); on the other hand, he is enforcing the concept of body as a whole organism: since the soul is conjoined with the body as a whole, body and soul together appear to be an organism. The organism is an ensemble (“assemblage”) of material function (AT XI 351, CSM I 339, art. 30). A strictly physicalist and non-dualist explanation of sensations and passions is lurking in the background. Under a physicalist (i.e. materialist) view, everything (including the mind) can be explained physically; there is no need to refer to anything outside physics. The stakes are high for a philosophy of mind because conceiving of the body as an organism might lead to explaining all psychical movements as mere functioning of body parts. Descartes moves boldly in this direction.

The questions he answers in this treatise, The Passions of the Soul, before classifying and explaining the six basic passions, are: How are these corporeal passions conveyed to the mind and how does the mind impact bodily functions due to emotions? To answer these questions, Descartes employs the Stoic concept of animal spirits. According to the Stoic theory, a tenuous body, located in the brain, links the mind with corporeal operations. This view was en vogue in the early seventeenth century, for instance in Tommaso Campanella (1568-1639) (1999). Descartes’ animal spirits are “a certain very fine air or wind” that shuttle between the brain and the body parts (AT XI 332, CSM I 330, art. 7; Sepper 2016, 26-28). They must be like little messengers that travel between body parts and mind and seem to understand both languages of the body and mind. They are called “spirits” but are expressly described as very fine bodies coming from the blood. In order to make that plausible, an example Descartes gives will help.

Wonder is a sudden surprise of the soul….It has two causes: first, an impression in the brain, which represents the object as something unusual and consequently worthy of special consideration; and secondly, a movement of the spirits, which the impression disposes both to flow with great force to the place in the brain where it is located so as to strengthen and preserve it there, and also to pass into the muscles which serve to keep the sense organs fixed in the same orientation so that they will continue to maintain the impression in the way in which they formed it. (AT XI 380f., CSM I 353, art. 70)

But how do those minute spirits work to communicate with the mind? Descartes points to the pineal gland, which was the only part in the brain that he knew of that did not come in pairs. This gland, however, is not where Descartes claims the soul resides; the soul itself has no location at all and is tied to the body as a whole. Rather, the fine spirits that fill the cavities of the brain use the gland to unite images and other sense impressions; and it is here where the mind “exercises its functions more particularly than in the other parts of the body” (AT XI 353f., CSM I 339f., art. 30f). The animal spirits mediate between body and mind.

We are left with an apparently strictly physicalist explanation of a great deal of mental activity in a strongly dualist conception of mind.[2] For the soul is a substance and it is of a totally different nature than body. Moreover, the traditionally so termed “lower faculties” of the soul (growth, movement, and sensations), which are equally present in animals, are removed from the definition of the human soul and ascribed to the body as an organism. Thinking (beyond the corporeal) is now the only the activity of the soul. Traditionally, thinking had been the privilege of the intellectual part of the soul. In Descartes, soul now means “rational mind.” In his work on the Passions, Descartes explicitly refers back to his anatomy and physiology of blood circulation in his earlier Discourse on Method, where he also relies on animal spirits when presenting his research project of natural science (AT VI 54, CSM I 138, part 5). Hence The Passions of the Soul does not in principle deviate from the program of the Discourse.

In Part 5 of the Discourse, Descartes explicitly separates functions that are commonly attributed to the mind from the soul proper. Even speech can be found in animals as long as it is nothing but an indicator of some passions and, hence, can be imitated by machines.[3] While these functions can be compared with a clockwork, the soul cannot be reduced to matter (AT VI 58f., CSM I 140f). The human and the animal bodies are like robots that perform activities, including sense perception and communication. The mind comes in addition to that machine. Hence Gilbert Ryle’s criticism that the mind is a mere “ghost in the machine.”

What we find in the Discourse is the encounter of Descartes the scientist with Descartes the philosopher of knowledge. The early Rules had investigated the order of thinking for the sake of reliable interpretations of reality; the late Passions executed that in a paradigmatic way and showed to what extent methodical thinking can achieve scientific knowledge of one of the most insecure areas of research, human emotions. The Discourse links both efforts. It stresses method.

On the Way to Substance Dualism

Descartes entertained a notion of body, and of matter in general, that escapes the traditional terminology of substances. Descartes’ famous cogito ergo sum, often translated as “I think therefore I am,” identifies thinking as the essence of every thing that thinks. What is important for the notion of substance is that the content of what that thing is deliberately remains open. In a letter, Descartes claims that nothing material can be assuredly known to exist, whereas “the soul is a being or substance which is not at all corporeal, whose nature is solely to think” (AT I 353, CSM III 55). Descartes wavers between using terms such as “being,” “substance,” and “nature” (estre, substance, nature), which indicates that he is not committed to the professional philosophical terminology and concepts of his time. There is an incorporeal substance that exists by way of performing the thinking, and that is all that the mind can know.

Descartes’ method approaches something like substance dualism in his further development of his theories. In the Meditations on First Philosophy he elaborates on the mental experiment of reducing the soul to mere thought. The major purpose of this text is to prove that the soul is immaterial (if not immortal). The reduction of soul to mind yields the certainty of “I am, I exist,” which is necessarily true, whenever it is mentally conceived (AT VII 25, CSM II 17; 2nd med.). Once again we see the mind guaranteeing its own existence. After contrasting this existence with that of corporeal particulars and objects, Descartes pronounces that “I am, then, in the strict sense only a thing that thinks” (AT VII 27, CSM II 18). In the sixth meditation, Descartes distinguishes material objects from mind and stresses:

I have a clear and distinct idea of myself, in so far as I am simply a thinking, non-extended thing (res cogitans, non extensa); and on the other hand I have a distinct idea of body, in so far as this is simply an extended, non-thinking thing (res extensa, non cogitans). (AT VII 78, CSM II 54)

This talk of thinking thing vs. extended thing (res cogitans vs. res extensa) suggests a clear dualism of mind and body. They are mutually exclusive substances that appear to make up the world. At this point, the fourth objection in the Meditations, raised by Antoine Arnauld (1612-1694), should be taken into account. Arnauld surmises that Descartes is either siding with Platonists who hold that the soul is the only constituent of a human being and that uses the body as a tool, or he is offering a traditional abstraction as geometers do who abstract figures from complex reality (AT VII 203f., CSM II 143). Platonists tend to deny dignity of material things and see all reality as results of spirit; geometers deal with mere abstractions (as anyone knows who tries to draw a perfect circle). In both cases the dualism would be dissolved. In reply, Descartes admits that this interpretation is possible but insists that the real distinction of mind from body is the result of attentive meditation (AT VII 228f., CSM II 160f).

Reshaping the Concept of Substance

As pointed out repeatedly, Descartes is working with and around a traditional philosophical terminology while trying to escape it. Therefore, it is worth seeing how he defines “substance” in his Principles of Philosophy. One interpretation is that substance means “independent existence” and hence applies only to God who is defined as perfect and not dependent on anything. However, in the material world we learn about substances through the properties that appear to us. We don’t see a lake as a substance; what we see is the shiny surface of water, surrounded by a shore, which leads us to perceiving the lake. The “principal attributes” of body and mind are notably extension and thinking, respectively (AT VIII 24f., CSM I 210f., sections 51-53). Descartes was careful not to jump to conclusions about the actual existence of material substances separate from their attributes. Hence he uses the imprecise word “thing” when referring to himself as essentially a thinking thing. The Latin term is res. Like “thing” in modern English, res has no ontological claim whatsoever, that is, when we say “thing” we avoid explaining what we mean and whether it is real. It is the “something” that language can point out without saying what it is.[4]

We may conclude that Descartes was aware of the temptation to present mind and body as competing and cooperating substances and he tried to escape the dualism, not only because any dualism is in need of some mediation, as the involvement of animal spirits proves, but also and foremost because of its explanatory deficits. On the one hand, his view appears to embrace the dualism that comes with inherited language (for instance from Platonism and Aristotelianism). On the other hand, if the philosophical problem of mind is that of understanding human knowledge, then understanding must be accessible to material beings and not within the realm of the immaterial. Therefore, Ryle was right to believe that Descartes fundamentally missed the task of understanding the mind.

To summarize the main points of the role of Descartes at the origin of modern philosophy of mind and specifically of substance dualism: Descartes aimed initially at proving that the human soul is immaterial (as Christian doctrine teaches); for that purpose he emphasized the certainty of rational thinking and its independence from body and material objects. This led him to the (still debated) question of how the mind can work with the body in the process of sense perceptions, feelings, etc. His response engaged the theory of “animal spirits,” tenuous bodies that shuttle between the mind and the organs. As a consequence, he explained great deal of intellectual functions (perceptions, emotions, etc.) in purely physical terms. At the same time he underlined the immateriality of thinking. In traditional philosophical terminology, this amounted to the theory of two totally distinct substances: mind and body. However, it should be noted that Descartes undermined the concept of substance and reduced it to something deliberately vague. Therefore, philosophers who cling to the notion of substance as a reality will find substance dualism in Descartes; others, who focus on his attempts at explaining mental operations like perceptions and feelings in corporeal terms, will find him to be a proponent of physicalism.


Adam, Charles and Paul Tannery, eds. 1964-1976. Oeuvres de Descartes. Paris: Vrin.

Campanella, Tommaso. 1999. Compendio di filosofia della natura, eds. Germana Ernst and Paolo Ponzio, sect. 61, 222. Santarcangelo di Romagna: Rusconi.

Cottingham, John G., Robert Stoothoff, Dugald Murdoch, and Anthony Kenny, trans. 1985-1991. The Philosophical Writings of Descartes, 3 vols. Cambridge: Cambridge University Press.

Ryle, Gilbert. 1949. The Concept of Mind. London/New York: Hutchinson’s University Library: 11-16.

Sepper, Denis L. 2016. “Animal Spirits.” In The Cambridge Descartes Lexicon, ed. Lawrence Nolan, 26-28. New York: Cambridge University Press.

Further Reading

Ariew, Roger. 2011. Descartes among the Scholastics. Leiden/Boston: Brill.

Cottingham, John. 1992. “Cartesian Dualism: Theology, Metaphysics, and Science.” In The Cambridge Companion to Descartes, ed. John Cottingham, 236-57. Cambridge: Cambridge University Press.

Hassing, Richard F. 2015. Cartesian Psychophysics and the Whole Nature of Man: On Descartes’s Passions of the Soul. Lanham: Lexington Books.

Markie, Peter. 1992. “The Cogito and Its Importance.” In The Cambridge Companion to Descartes, ed. John Cottingham, 140-73. Cambridge: Cambridge University Press.

Ruler, Han van. 1999. “‘Something, I Know Not What’. The Concept of Substance in Early Modern Thought.” In Between Demonstration and Imagination: Essays in the History of Science and Philosophy Presented to John D. North, eds. Lodi Nauta and Arjo Vanderjagt, 365-93. Leiden: Brill.

Specht, Rainer. 1966. Commercium mentis et corporis. Über Kausalvorstellungen im Cartesianismus. Stuttgart-Bad Cannstatt: Frommann-Holzboog.

Voss, Stephen. 1993. “Simplicity and the Seat of the Soul.” In Essays on the Philosophy and Science of René Descartes, ed. Stephen Voss, 128-41. New York: Oxford University Press.

  1. Descartes’ works are cited by the standard French edition C. Adam and P. Tannery (eds.), Oeuvres de Descartes. Paris: Vrin, 1964-1976, “AT” with volume and page number; the standard English translation J. G. Cottingham, R. Stoothoff, D. Murdoch, and A. Kenny (trs.), The Philosophical Writings of Descartes, 3 vols. Cambridge: Cambridge University Press, 1985-1991, “CSM” with volume and page number.
  2. Cf. the “Fifth Responses” in the Meditations, AT VII 230, CSM II 161.
  3. It sounds like an anticipation of John Searle’s “Chinese Room”: exchanging signs does not entail thinking (See Chapter 3).
  4. It is worth noting, perhaps, that the Latin version of the famous statement in the Discourse “From this I knew I was a substance …” modified “substance” by adding “any some thing or substance.” Thus the author signaled that he was departing from traditional understanding of substance to a generic “something” (AT VI 558: “rem quondam sive substantiam”).


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