Cartesian Dualism

Cara Spencer

From, archived at

I. Dualism and Materialism

In the Sixth Meditation of his Meditations on First Philosophy, Descartes argues for a position called substance dualism. Before you can evaluate his argument for that position, you need to know more about what the position is. Substance dualism is similar to a position that many people hold for religious reasons--that in addition to our physical bodies, each one of us also has an immaterial soul. This immaterial soul is not identical to any physical part of our body, indeed it is not physical at all, in that it is not made of physical matter. Each person’s immaterial soul is intimately related to his or her body, but it can continue to exist after the death or destruction of the body.

I said that substance dualism is similar to this position. How is it similar and how is it different? First, Descartes agrees that each person has an immaterial part, which I will call a mind, as well as a material part, which is the body. Descartes also thinks that a person’s mind can exist independently from that person’s body--that it is not automatically destroyed when the physical body dies or is destroyed. In these ways, Descartes’ position is similar to the position described above. It is different mainly in being specific about certain issues that the position I described above leaves unaddressed. First, Descartes thinks that any experiences a person has are in fact experienced by that person’s soul. (So right now, as you are reading this, you are having a complex experience--you are thinking about what it says, seeing the words on the screen in front of you, and you may also be partly occupied by other thoughts or feelings--that you feel tired or hungry for instance, or that you plan to call your mom later. According to Descartes, all of these experiences are had by your immaterial mind). Second, Descartes says that minds are substances. What does this mean? Philosophers use the term "substance" to refer to a particular thing that that has certain essential properties and other accidental properties. The essential properties of a substance are those that it must have if it is to continue existing. The accidental properties are those it can lose and still continue to exist. So for instance, Descartes says that one essential property of physical substance is extension. Physical substances can differ in their size and shape, but they all must have a size and a shape. A physical substance that has no size or shape is inconceivable. An accidental property of physical substances is being the color red. Some physical substances are red, some aren’t, and those that are red can be altered so that they are no longer red (say by being painted yellow), and still continue to exist. The essential property of mental substance, according to Descartes, is that it thinks. Another special feature of substances is that they can exist independently of anything else, in that it is metaphysically possible for a substance to exist on its own. This makes substances different from properties, which can only exist if they are properties of objects. Substances also can exist over time, and undergo change while still continuing to be the same substance. This distinguishes substances from events, which take place over time but do not undergo change, as well as from properties, which do not undergo change.

Some dualists agree with Descartes that minds are in some sense non-physical, but they reject Cartesian substance dualism in favor of property dualism. Most contemporary dualists defend a version of property dualism rather than substance dualism. According to property dualism, all substances are physical, so there is no mental substance. However, not all properties are physical. Some properties are irreducibly mental, and some physical substances (like human beings) can have these irreducibly mental properties as well as physical properties.

Materialists, on the other hand, think that all substances and all properties are physical. That does not mean that materialists deny that we have minds. Materialists of almost every stripe agree that we have rich mental lives, full of different kinds of experiences, thoughts, intentions, desires, beliefs, and sensations. They just think that all of these phenomena can be explained in purely physical terms, that having a belief, or an experience, or a sensation, is just a matter of having a physical body that has certain physical properties, or in which certain physical events take place. Materialists agree with dualists that we have minds but they disagree quite profoundly about what minds are.

Do materialists think that a person’s mind is a physical object, just like a person’s body is, (in an extended sense, perhaps) just a physical object? [NOTE: In philosophy, the word "object" is almost always used in this extended sense] Not exactly. Some materialists say that the mind is just the brain, but most take a more sophisticated view. On this more sophisticated view, having a mind is not like having a an arm or a leg, which is just having some (in principle) separable physical object be a part of you. Rather, having a mind is more like having a good time or having a difficult year. When you are having a good time or having a difficult year, there isn’t some object, a good time or a difficult year that you have, in the way that you have an arm or a leg or a bicycle. Having a good time, or having a difficult year, is a matter of having certain properties or participating in certain events. Similarly, having a mind is also a matter of having certain properties (such as believing that grass is green or feeling pain) or participating in certain events (such as deliberating about what to major in).

NOTE: Both materialists and property dualists claim that having a mind is a matter of having certain properties or participating in certain events. Property dualists differ from materialists in allowing that some of these properties or events are irreducibly mental. Materialists claim that all properties are physical properties, and all events are physical events.

ALSO NOTE: The distinction between properties and events is important metaphysically, but in this course we will not need to be too careful about it. So in the future I’m going to just talk about mental properties and physical properties, and pretty much what I say will also go for mental events and physical events.

An argument for substance dualism

Excerpt from Descartes' Sixth Meditation (originally published in Latin in 1641)

"First, because I know that anything that I clearly and distinctly understand is capable of being created by God so as to correspond exactly with my understanding of it. Hence the fact that I can clearly and distinctly understand one thing apart from another is enough to make me certain that the two things are distinct, since they are capable of being separated, at least by God. The question of what kind of power is required to bring about such a separation does not affect the judgement that the two things are distinct. Thus, simply by knowing that I exist and seeing at the same time that absolutely nothing else belongs to my nature or essence except that I am a thinking thing, I can infer correctly that my essence consists solely in the fact that I am a thinking thing. It is true that I may have (or, to anticipate, that I certainly have) a body that is very closely joined to me. But nevertheless, on the one hand I have a clear and distinct idea of myself, in so far as I am simply a thinking, non-extended thing; and on the other hand I have a distinct idea of body, in so far as this is simply an extended, non-thinking thing. And accordingly, it is certain that I am really distinct from my body, and can exist without it." (Translated by Cottingham, Stoothoff, and Murdoch, and excerpted in Rosenthal 26 L-R)

In this passage from the Sixth Meditation, Descartes offers a concise statement of his argument for dualism, which is often called the "Conceivability Argument" I've offered an interpretation of this argument below, but this interpretation is by no means set in stone–there are disagreements between interpreters of Descartes about what the best interpretation is.

III. Conceivability Argument for Dualism

1. All things I clearly and distinctly understand can be made by God to be exactly as I understand them.

2. If I can clearly and distinctly understand and object A existing separately from an object B, then God can make A exist separately from B (from 1)

3. If God can make A exist separately from B, then A and B really are distinct.

4. I can clearly and distinctly understand my mind existing separately from my body. (see subargument)

5. God can make my mind exist separately from my body. (from 4, 2)

C: My body and my mind really are distinct.(from 5,3)

Subargument for premise 4:

1. I know that my mind exists, and that it's essence is to be a thinking thing.

2. If my body exists, I know that it is an extended thing, since it is in the essence of bodies to be extended

3. I can conceive of something being a thinking thing without being an extended thing, and vice versa.

C: I can clearly and distinctly understand my mind existing separately from my body.

IV. Remarks on the Conceivability Argument: Those unfamiliar with the notion of a philosophical argument should see an online reading I have prepared on the subject. The parenthetical remarks following some of the premises are not parts of the premises. Rather, they briefly point to justifications of the premises. For instance, following premises 2 of the main argument is the parenthetical remark "(from 1)." I just mean to say there that the second premise follows directly from 1 (i.e. the first premise). Premise 5 and the conclusion of the main argument are inferences from two other premises.

1. There are many senses of the word "possible." Which sense does Descartes mean?

He means "contingent" in the philosopher's sense. The notions of contingency and necessity are discussed in Philosophy of Mind and Cognition (see the index and glossary) as well as on my online handout.

1. Why think that whatever is conceivable is also possible?

Why does Descartes think that what he can clearly and distinctly imagine or conceive of is any guide to how things really are? His answer seems to be that God is not a deceiver, so God would not give us rules of thought that could lead us into error. Is there any other reason to accept this purported link between conceivability and possibility? Perhaps. First, at least some of our modal beliefs are trustworthy. They have to be correct. For instance, we know that it might have rained today, or more people might have come to lecture today. It would be very counterintuitive to say that none of our modal beliefs were any guide to what really and truly is possible. Of course, this does not suggest that all of our modal beliefs are equally reliable.

2. Why does Descartes think that he can conceive of his mind existing without his body?

Descartes describes this conception vividly in the First and Second Meditations. (The Second Meditation is in your course packet.) There he says that he cannot rule out the possibility that he is not just a disembodied mind, being deceived by an evil demon into thinking that he has a body and lives in a physical world with other people, animals, and things. In this argument, Descartes is not relying on the claim that he cannot rule out such a possibility (i.e. that he cannot know that it is false). All he assumes here is that he can imagine this possibility.

3. Why accept that because we cannot conceive of ourselves as not thinking, we are not essentially embodied?

The argument again concerns the possible properties that we could have, and the possible properties that physical objects could have. So D. argues as follows. What is essential to the mental is not essential to the physical, and vice versa. But objects with different essences must be distinct. Therefore, a person's mind must be a distinct substance from his or her body.

V. Objections to Cartesian Dualism

A philosophical theory is supposed to be well-supported by argument, but it is also supposed to adequately explain otherwise puzzling phenomena. So we might object to a theory by finding some fault with the argument that is supposed to establish it, and we might also find fault with it because it fails to explain what it is supposed to explain. People have objected to dualism in both of these ways.

A. the conceivability premise (1) is false. We can imagine many situations we know to be impossible. For instance, consider the person who believes that Bill Bradley the former Knicks starter is not the same person as Bill Bradley the former candidate for the Democratic nomination for President. What this person believes, and so conceives of, is impossible. The conceivability premise is the subject of much current debate between contemporary dualists and materialists, and the debate is far from closed. (See Saul Kripke's Naming and Necessity (Cambridge, MA: Harvard University Press) 1980. And more recently David Chalmers, who defends property dualism, in The Conscious Mind (New York: Oxford University Press) 1996.)

B. Premise 4 is false, since we cannot really imagine ourselves as disembodied minds. Many have argued that the evil demon scenario that Descartes imagines is not really coherent, and thus that it does not represent a genuine metaphysical possibility even if the conceivability premise is true. Contemporary dualists typically respond that even if premise 4 of Descartes' argument is false, dualism can still be defended without relying on (4). These new arguments rely instead on the conceptual possibility of physical beings exactly like human being in every respect, except that they have no sensations. Again, the debate about the genuine possibility here is far from closed.

1. It is clear that the mind causally interacts with the body. Our thoughts often cause our actions, and physical states of the body often cause mental states. How does causal interaction take place between physical and mental substances? Substance dualists do not have a good answer. Materialists, who think that the mind is nothing over and above the brain and body, have at least the beginnings of an answer, in so far as they have some account of causation in general as a relation between physical events. Property dualists have a somewhat easier time answering this challenge. They can say, with David Chalmers, that mental states causally interact with physical states of the brain, body, and environment, but only as a matter of contingent physical law. They allow that it is possible that physical states of the brain, body, and environment do not causally interact with mental states. If this were true, it would show that dualism is contingently true. But even the property dualist has to explain how mental causation is different from physical causation.

2. Related to the first objection is a mystery about the mental substance Descartes believes in. What is it, and what are its essential properties? Has Descartes even given us the right kinds of reasons for believing in this mental substance?

Supervenience and materialism

The materialist thinks that all properties are physical properties. What does it mean to say that all properties are physical? One way is to say that all properties, including mental properties, are simply identical to physical properties. What do I mean by "identity" here? We are familiar with identity of objects. For instance, we know that Muhammad Ali is identical to Cassius Clay. That is, the very same person is named both "Muhammad Ali" and "Cassius Clay." What does property identity consist in? That is, what is it for a sentence of the form "A = B" to be true, where "A" and "B" designate properties rather than objects? Many philosophers suppose that it is both necessary and sufficient that every possible object that is an A is also a B, and every possible object that is a B is also an A. So for instance, a property identity like "Being triangular = Being trilateral" is true if and only if every possible triangular thing is also trilateral, and every possible trilateral thing is also triangular. Since this is the case, this property identity is true.

In this vein, some philosophers (Jack Smart, for one) have argued for a mental state-brain state identity theory, or more briefly a mind-brain identity theory. According to this theory, every mental state, such as feeling a toothache, believing that grass is green, or seeing a red square, is identical to a particular physical state of the brain. Which physical state a given mental state is identical to is a matter for neuroscience to discover, but the idea is that having a particular sort of experience, such as feeling a toothache, is just a matter of certain neurons firing in a certain part of the brain.

The mind-brain identity theory was once the received theory among materialists about the mind. Most materialists still subscribe to a kind of mind-brain identity theory, although not the one described above. Why has this simple view fallen out of favor? The simplest reason is that it allows no room for the multiple realizability of mental states. What does it mean to say that mental states are multiply realizable? Simply that different people, or different creatures, may be in the same mental states even though their brains are in different physical states. Why think that mental states are multiply realizable? There are two reasons. First, neuroscience has shown that the brain, unlike the other organs in the body, is remarkably plastic, in that its different parts can be adapted for different purposes. We know this because brain functions are typically localized to certain parts of the brain. One part controls motion, one part controls facial recognition, and another part controls visual processing. But individuals who have damage in the parts of the brain responsible for certain functions can still perform these functions, and studies of their brain activity reveals that they have adapted to use other parts of the brain to perform the functions typically performed in the damaged part. This shows that, for instance, the mental state of reading a book silently is not identical to a particular neural state, since different people who perform the very same act of reading silently will thereby be in different neural states. Another reason for thinking that mental states are not identical to states of the brain arises from the possibility of extraterrestrial intelligent life. If there were such life, then it is possible, indeed likely, that their brains would be very different from ours. But if they were intelligent, then that means that they experience some of the same mental states we do. So if they can have the same mental states as we do without having the same neural states, then neural states can’t be identical to mental states.

But these problems don’t force us to reject materialism altogether. It might be that all of the physical facts about the world--the distribution of physical properties--completely determine the distribution of mental properties. Once all of the physical facts about a world are fixed, all of the mental facts are also, and thereby, necesssarily, fixed. Another way of putting the same point: If two possible worlds are identical with regard to the physical facts that hold in that world, they are necessarily also identical with regard to the mental facts that hold in that world.

Many philosophers use the term "supervenience" to refer to the inverse of this determination relation. Roughly, if the physical facts determine the mental facts, then the mental facts supervene on the physical facts.

VI. Contemporary Dualism

Throughout most of the 20th century, dualism has fallen out of favor among Western philosophers and regular folks. (Just consider how few of you raised your hands when I asked, during the first lecture, whether any of you were believed in souls. Either some of you were being reticent or dualism is unpopular in our class.) Among philosophers, it is typically regarded as unscientific, incapable of offering a better explanation of the mind then materialism, and beset with a number of problems. But, as I have suggested above, dualism is enjoying a new popularity among contemporary philosophers. The new dualism is not the substance dualism of Descartes, according to which the mind is composed of a nonphysical substance. Rather, it is property (or attribute) dualism, the view that there are some irreducibly mental properties. Philosophers attracted to dualism typically are compelled by the conceivability of organisms that are physically identical to us but mentally quite different (who either have different mental lives or who lack mental lives of any kind), and by the explanatory inadequacy of materialism. See David Chalmers' The Conscious Mind and Thomas Nagel's "What is it like to be a bat?" We will return to some of their arguments later on this semester.

Page created by: Cara Spencer
Last Modified: August 26, 2003