Descartes on the separateness of mind and body

Jeff Speaks

August 23, 2006

1 The method of doubt
2 What cannot be doubted
3 Why the mind cannot be identical to any body
4 The nature of the mind/body distinction
5 Descartes’ view of the relationship between mind and body
6 Varieties of dualism

1 The method of doubt

In the selection from the Meditations on First Philosophy that we read, Descartes argues that the mind is something distinct from any body. But the Meditations begins with a discussion of a topic seemingly far removed from the nature of the mind: the question of whether we can be certain of the truth of any of our opinions. What we have to see is how this question is related to questions about the relationship between mind and body.

Descartes begins (1.5) by noting that his opinions up to this point have been based on his senses, but that we cannot be certain that our senses do not deceive us. This is in part because we cannot be certain that what we think of as our sensations of the world are not a dream:

“How often have I dreamt that I was in these familiar circumstances, that I was dressed, and occupied this place by the fire, when I was lying undressed in bed? At the present moment ...I look upon this paper with eyes wide awake; ...but I cannot forget that, at other times I have been deceived in sleep by similar illusions; and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished ...” (1.7)

So, Descartes argues, there seems to be some sense in which I am less than certain about the existence of the bodies I seem to be perceiving. It seems to me that there is a computer monitor in front of me right now; but, because “there exist no certain marks by which the state of waking can ever be distinguished from sleep” I cannot be certain that I am not dreaming of a computer monitor rather than seeing one.

Descartes uses the figure of an ‘evil demon’ to make much the same point:

“ I will suppose, then, not that Deity, who is sovereignly good and the fountain of truth, but that some malignant demon, who is at once exceedingly potent and deceitful, has employed all his artifice to deceive me; I will suppose that the sky, the air, the earth, colors, figures, sounds, and all external things, are nothing better than the illusions of dreams, by means of which this being has laid snares for my credulity; I will consider myself as without hands, eyes, flesh, blood, or any of the senses, and as falsely believing that I am possessed of these ...” (1.12)

The point of this, for our purposes, is not whether it is plausible or reasonable to believe that we are constantly being deceived by an evil demon; we can assume that this is not a reasonable thing to believe. Rather, the important point is that, by reflecting on scenarios like dreaming and being deceived by an evil demon, it seems possible to doubt whether any of the external, physical things which we seem to perceive really do exist.

2 What cannot be doubted

At the beginning of the second Meditation, Descartes wonders whether there is anything whose existence cannot be doubted:

“I suppose, accordingly, that all the things which I see are false (fictitious); I believe that none of those objects which my fallacious memory represents ever existed; I suppose that I possess no senses; I believe that body, figure, extension, motion, and place are merely fictions of my mind. What is there, then, that can be esteemed true ? Perhaps this only, that there is absolutely nothing certain.” (2.2)

But he quickly finds that this is not the case; even though he can doubt the existence of any external thing, he cannot doubt his own existence:

“But I had the persuasion that there was absolutely nothing in the world, that there was no sky and no earth, neither minds nor bodies; was I not, therefore, at the same time, persuaded that I did not exist? Far from it; I assuredly existed, since I was persuaded. But there is I know not what being, who is possessed at once of the highest power and the deepest cunning, who is constantly employing all his ingenuity in deceiving me. Doubtless, then, I exist, since I am deceived; and, let him deceive me as he may, he can never bring it about that I am nothing, so long as I shall be conscious that I am something. So that it must, in fine, be maintained, all things being maturely and carefully considered, that this proposition (pronunciatum ) I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.” (2.3)

The same line of reasoning appears to carry over to particular episodes of thinking. Just as an evil demon cannot deceive me about my own existence, he cannot deceive me about the fact that I am being deceived.

3 Why the mind cannot be identical to any body

So far, we seem to have two results: that it is possible to doubt whether any external, physical things exist, but that it is not possible to doubt that oneself, or one’s own mental episodes, exist. At this point, you might ask: so what? What does this show about the relationship between the mind and the body?

Descartes is most explicit about this in paragraph 9 of Meditation 6:

“And, firstly, because I know that all which I clearly and distinctly conceive can be produced by God exactly as I conceive it, it is sufficient that I am able clearly and distinctly to conceive one thing apart from another, in order to be certain that the one is different from the other, seeing they may at least be made to exist separately, by the omnipotence of God; and it matters not by what power this separation is made, in order to be compelled to judge them different; and, therefore, merely because I know with certitude that I exist, and because, in the meantime, I do not observe that aught necessarily belongs to my nature or essence beyond my being a thinking thing, I rightly conclude that my essence consists only in my being a thinking thing or a substance whose whole essence or nature is merely thinking]. And although I may, or rather, as I will shortly say, although I certainly do possess a body with which I am very closely conjoined; nevertheless, because, on the one hand, I have a clear and distinct idea of myself, in as far as I am only a thinking and unextended thing, and as, on the other hand, I possess a distinct idea of body, in as far as it is only an extended and unthinking thing, it is certain that I, that is, my mind, by which I am what I am], is entirely and truly distinct from my body, and may exist without it.”

So see how this argument works, it helps to break it down into steps. First, Descartes says that if he can “clearly and distinctly” conceive some state of affairs, then God could create that state of affairs. So, if he can clearly and distinctly conceive some state of affairs, then that state of affairs is possible. The distinction between possible and impossible situations, and contingent and necessary truths.

So, if Descartes is right, we can show that it is possible that x and y are distinct things by clearly and distinctly conceiving of them as distinct. What he wants to show is that it is possible that mind and body are distinct; so what he needs to show is that he can clearly and distinctly conceive of mind and body as distinct. But, in a sense, he has already shown this. In Meditation 1, Descartes doubted the existence of material bodies; so, he was conceiving of bodies not existing. But, in Meditation 2, he found that he could not doubt his own existence. So, in this method of doubt, he was conceiving of his mind as existing, but of bodies as not existing. So he was conceiving of his mind as distinct from his body. So, if the above is correct, it follows that it is possible that his mind is distinct from his body.

But what we want to know is not whether it is possible for one’s mind to be distinct from one’s body; what we want to know is whether minds really are distinct from bodies. How can we get from one thesis to the other? The necessity of identity as bridging this gap in the argument.

We can think of Descartes’ argument for the distinctness of mind and body as breaking down into steps as follows:



If I can clearly and distinctly conceive of such and such being the case, God could make such and such the case.


If God could make such and such the case, then such and such is possible.


If I can clearly and distinctly conceive of such and such being the case, then such and such is possible. (1,2)


I can clearly and distinctly conceive of the mind existing without the body.


I can clearly and distinctly conceive of a case there the mind ⁄= the body. (4)


It is possible that the mind ⁄= the body. (3,5)


If a = b, then necessarily a = b.


The mind ⁄= the body.

Is this argument valid? Is it sound?

Can you see how to run a parallel argument to show that particular mental events — like certain thoughts, or pains — are not identical to any material bodies, or physical events?

4 The nature of the mind/body distinction

So we know that Descartes thinks that the mind is something other than the body; but what, exactly, does that mean? One way to answer this question is to get clearer on what Descartes thinks bodies are. Descartes often speaks of bodies as extended; part of what he means is expressed in the following passage:

“By body I understand all that can be terminated by a certain figure; that can be comprised in a certain place, and so fill a certain space as therefrom to exclude every other body.” (2.5)

One of the defining aspects of bodies is that they are extended in space: that they have certain dimensions. Should we conclude from this that Descartes thinks that bodies do not exist in space — that they have no dimensions? Does this make sense? Does it follow that they are not located anywhere?

5 Descartes’ view of the relationship between mind and body

So far, we’ve examined Descartes’ argument that the mind is not identical to any body. But this tells us what the relationship of mind to body is not; it does not tell us what it is. In one place, Descartes gives his view of the relationship of mind and body by an analogy:

“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.” (6.13)

What can we take from the idea that the relationship between mind and body is akin to the relationship between pilot and vessel? One thing a pilot does is control the vessel; by steering, pilots cause vessels to do things. So we would expect Descartes to think that minds sometimes cause bodies to do things. And this is what he thinks (see among other places, 6.12-13). Why this fits well with common sense.

It seems, then, that we can sum up the main points of Descartes’ dualist view of the relationship between mind and body as follows:

  1. The mind is not identical to any body. Nor are particular mental events (particular episodes of thinking, feeling, etc.) identical to any bodies.
  2. Bodies are defined by Descartes as things which have extension. Since minds are not identical to any bodies, minds do not have extension. So minds do not exist in space.
  3. Bodies sometimes cause effects in minds, and minds sometimes cause effects in bodies.

6 Varieties of dualism

We can separate out two parts of Descartes view, via the distinction between objects or substances on the one hand, and properties on the other. A way to get a handle on this distinction via the distinction between names and predicates.

Corresponding to the distinction between substances and predicates is a distinction between two kinds of dualism. The property dualist says that mental properties — like feeling a pain or thinking about food — are not identical to any physical property. The substance dualist says that there are mental substances — minds — which are not identical to any physical things. Descartes was both a substance dualist and a property dualist. In this first part of the course, where we focus on the mind-body problem, the most important part of his view is his property dualism. (Though Descartes himself spends most of his time talking about substance dualism.) In the second part of the course, when we discuss the nature of persons, we’ll return to substance dualism.

One good question at this point is: what is the relationship between substance dualism and property dualism? If substance dualism is true, does it follow logically that property dualism must be true as well? How about the other way around?

Another distinction between kinds of dualism is worth making here. We noted above that Descartes thought that minds could cause effects in bodies, and vice versa. So, despite thinking that minds and bodies are different sorts of things, Descartes thought that minds and bodies could interact. For this reason, his view is sometimes called interactionist dualism.

But not all dualists think this. Some dualists are epiphenomenalists: they think that mental events are caused by physical events, but that mental events never have any physical effects. So the line of causation always goes from physical to mental, and never in the reverse direction. Can you think of any reason why someone would find this view attractive? Why might it be preferable to interactionism? Does the view have any disadvantages? Can the epiphenomenalist, for example, give any explanation of how mental features could have evolved, if they never have any effects in the physical world?

A third variety of dualism is parallelism, which is the view that, although mental and physical events run ‘in parallel’, there are no causal connections between them. Why might one be attracted to this view? How could the correlations between mental and physical events be explained by a parallelist, if at all?