Cartesian Ontology Necessitates Mind/Body Dualism

Descartes is usually credited with being the father of modern philosophy. Modern philosophy, in this sense, represents two different yet related ideas. First, Descartes inaugurated a radical methodological doubt which refused to give any sort of deference to the traditions of the past. This philosophical individualism is wholly distinct from everything that came before it. Aristotle, who was the paradigmatic philosopher for most of western history until the early modern period, would always consider the opinions of his predecessors before expounding his own particular doctrines. Even Plato’s Socrates, who was notorious for his criticism of popular Greek moral and political thought, did not offer a wholesale rejection of tradition as the only means of properly attaining truth. Doubt, then, is Descartes’, and it was this doubt that became the foundation for his entire philosophy and much of philosophy afterward. Second, modern philosophy, particularly the early modern era, represents not merely a methodological rejection of the past but also its theoretical rejection. In other words, Descartes popularly marks the end of the Aristotelian grip on the world of western thought. Descartes’ explicit rejection of core Aristotelian doctrines, and the Scholasticism that was built upon them, makes him the first truly modern philosopher. This accolade, however, was ill-gotten.

The rejection of a broadly Aristotelian philosophy birthed a plethora of errors and seemingly unsolvable philosophical problems, and of them, none are more famous, or infamous, than the mind/body problem. When discussing this topic, philosophers rightly regard Descartes as being the first philosopher to state the relation of the mind and the body in such a way as to make the recognition of the problem obvious. This, in turn, results in an almost irredeemably puzzling confusion about how the mind and the body interact. This, however, is not the real problem. Rather, it is Descartes’ ontology and how it is responsible for creating the mind/body problem instead of simply discovering it. If there is any type of solution to be found it is in a return to a more Scholastic metaphysics and the Aristotelian doctrines that provide for its foundation.
In his Second Meditation, Descartes attempts to understand who or what he essentially is. By placing all things in doubt, Descartes arrives at one conclusion of which he is indubitably sure: He exists. He demonstrates how this one thing cannot be reasonably doubted:

“Is it then the case that I too do not exist? But doubtless I did exist, if I persuaded myself of something. But there is some deceiver or other who is supremely powerful and supremely sly and who is always deliberately deceiving me. Then too there is no doubt that I exist, if he is deceiving me. And let him do his best at deception, he will never bring it about that I am nothing so long as I shall think that I am something. Thus, after everything has been most carefully weighed, it must be finally be established that this pronouncement “I am, I exist” is necessarily true every time I utter it or conceive it in my mind” (Meditations on First Philosophy, pg108).

Now that he has found something of which he can be absolutely certain, Descartes proceeds to apply this same method to other aspects of his existence in order to understand more completely who, exactly, he is. He entertains some possible answers; perhaps he can say that he is a man. But he can then ask what a man is, and, in his mind, to say that man is a rational animal is a confusing waste of time (notice that the term rational animal would be a standard Scholastic answer to the question “What is a man?” Descartes vehemently and abruptly rejects this approach immediately). Ultimately, he will not be satisfied by any answer that suggests that his identity has anything to do with the nature of bodies. Instead, Descartes concludes that he is essentially a thinking thing. His essence is therefore thought, which can have nothing to do with being a body or being within a body. Even sensation and imagination are reduced to thought. The things which are sensed or imagined could be dreams that have no reality but, nevertheless, he is sensing and imagining and this cannot be doubted. We can ask why Descartes is at such pains to eliminate the possibility that his essence has anything to do with his corporeality, and the implications of his answer will provide both philosophers and scientists with a conceptual sketch of the nature of matter and causation that remains largely uncontested to the present day.
For Descartes, the essence of matter is extension. It is from this principle that every other attribute of matter can be derived. “Thus extension in length, breadth, and depth constitute the nature of corporeal substance; and thought constitutes the nature of thinking substance” (The Principles of Philosophy, Part 1, 53, pg. 245). What Descartes has done here is conceptualize matter in terms of what is mathematically quantifiable. In this way, physics now becomes the true method of discovering the manner in which objects actually exist. The implication of this belief, then, is that what we experience through our sensation is decidedly not a part of the actual nature of the substances which we perceive:

“In this way we shall perceive that the nature of matter, or body in its universal aspect, does not consist in its being hard, or heavy, or colored, or affecting our senses in some other way, but solely in its being something extended in length, breadth, and depth. For as regards hardness, we do not know anything of it by sense, except that the portions of the hard bodies resist the motion of our hands when they come in contact with them…” (The Principles of Philosophy, Part II, 4, pg. 254).

Descartes is making two critical points in this passage. First, everything that we perceive in the world, despite our particular sensations, is really made up of some collection of uniform particles which are nothing other than their quantifiable dimensions. Second, the only way we can explain any changes in these objects, either as we perceive them or as they change in themselves, is through what Descartes calls motion. By motion, Descartes means local motion or movement from place to place. Strictly speaking, Descartes says that motion “…is the transference of one part of matter or one body from the vicinity of those bodies that are in immediate contact with it, and which are regarded as at rest, into the vicinity of others” (The Principles of Philosophy, Part II, 25, pg. 262). A stock example would be billiard balls striking each other; one ball moves until it hits another one, that one then moves while the first ball comes to rest.
In this brief outline we have the now common image of the physical world. Everything is simply matter in motion. At its most fundamental level, the world is nothing other than material particles colliding with one another continuously. This is the modern mechanistic picture of nature, and it owes much of its existence to the father of modern philosophy.
Notice, though, that nowhere in this picture can mind or soul be found. All material substances are essentially extension, but extension is seemingly unsatisfactory in explaining human existence. If the things which we experience in sensation, for instance, are not actually parts of material substances, then it logically follows that the human mind cannot also be material. Consider a piece of chalk. Upon looking at it, most people would conclude that it is white. According to Descartes, however, this certainly is not the case. Whiteness is not “in” the chalk because the chalk is only the sum of its moving quantifiable elements, none of which possess any color, taste, smell, etc. The whiteness of the chalk, then, can only be in the mind of the observer. But the mind of the observer cannot also be said to be material since we just said that it is the mind which perceives these different qualia (sensible qualities). Cartesian dualism is thus the only way out of this conceptual impasse.
Although this argument might appear to be utterly distinct from the one in the Meditations, it is actually a different way of saying the same thing. Going back to the Second Meditation, we see that Descartes decides that he is a thinking thing only after he has established, however roughly, his conception of matter as extension. This is the only reason why he is able to imagine that his body does not exist and that therefore he must be whatever it is that thinks, which is his soul:

“On the contrary, I was under the impression that I knew its [body] nature distinctly. Were I perhaps tempted to describe this nature such as I conceived in my mind, I would have described it as thus: by “body,” I understand all that is capable of being bounded by some shape, of being enclosed in a place, and of filling up space in such a way as to exclude any other body from it; of being perceived by touch, sight, hearing, taste, or smell; of being moved in several ways, not, of course, by itself, but by whatever else impinges upon it” (Meditations on first Philosophy, pg. 109).

It is his clear and distinct idea of matter as one thing and thought as another that enables him to separate them. His conceptual distinction comes first, his idea of himself as essentially a thinking thing comes afterward.
Descartes has to understand the soul in terms of a separate and distinct immaterial substance that somehow exerts causal force over the physical world of matter in motion. How does it do this? Centuries have passed and it still eludes us; most philosophers have now simply disregarded the immaterial soul in favor of materialism because Cartesian dualism just seems so untenable. The truth, though, is that the mind/body problem is necessitated by Descartes’ mechanistic conception of nature, i.e. understanding nature as matter in motion. Instead of accepting this description of the world while denying Descartes’ dualism, we should deny his conception of the material world as thoroughly inadequate.


  1. I think you have done an excellent job pointing out some problems in the Cartesian ontology; however, you did not define how precisely Aristotle's philosophy, or the philosophy of the Scholastics, is more sufficient in explaining the concept of mind and matter as extension.Simply positing the mind as problematic to define in materialist terms, which Descartes did, does not create the problem. It merely points out the problem. If you can show how Aristotle defines the contents of consciousness in a more sufficient way, I would like to read it.

  2. Perhaps you have more specific questions, so I will give a rough outline of what the Scholastic conception would be.

    Descartes conceives of man as essentially a thinking thing. On this account, the body is accidental to human existence. I should note that Descartes explains that he does not want to imply this, he wants to maintain that the connection between the mind and body is closer than a mere vessel and its inhabitant, but he doesn't explain how this follows from his premises. In Meditation 6 he says:

    "Nature likewise teaches me by these sensations of pain, hunger, thirst, etc., that I am not only lodged in my body as a pilot in a vessel, but that I am besides so intimately conjoined, and as it were intermixed with it, that my mind and body compose a certain unity. "

    This, however, does not get to the heart of the matter. Using our imagination, we think of two physical objects interacting with one another, a button being pushed or a lever being pulled, and we use this analogy to understand how the mind plays a causal role in our bodily actions. This picture is created by Descartes' conception of matter and his rejection of Aristotelian notions of causality. He even posits the point of interaction; the pineal gland in the brain.

    But this is a muddle of both causation and philosophical psychology. For Aquinas and Aristotle, mind (soul) and body are not two distinct substances but rather two principles of one substance: man. The analogy with physical objects interacting, then, is inaccurate.

    On Aristotle's account, the relationship between body and soul is a specific type of form/matter relationship with soul being form and body being matter. Their composite forms one thing, so it makes no more sense to ask if the soul and body are one than to ask if a basketball and its roundness are one.