Descartes' arguments for dualism
Thomas (Janice L.)
Source: Thomas (Janice L.) - Mind and Person in the Philosophy of Religion
Paper - Abstract

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  1. Essential reading – 21
  2. Further reading – 21
  3. Introduction – 21
  4. The argument from the possibility of doubting the body – 22
  5. Criticisms of Descartes' first argument – 23
  6. The argument involving clear and distinct ideas – 27
  7. The argument involving the indivisibility of the mind – 27
  8. Learning outcomes – 28
  9. Sample examination questions – 29
  10. Tips on answering the sample questions – 29
Full Text
  1. Essential Reading
  2. Further reading
  3. Introduction
    • Anyone who has heard of the French seventeenth-century philosopher, Rene Descartes, probably knows that he searched for a proposition which was immune from even the most determined doubt and found that proposition in the little sentence ‘I think therefore I am' (better known in the Latin version of the formula: 'Cogito ergo sum'). Descartes could not doubt his own existence at any time when he was thinking: in The Meditations and The Discourse on Method he used the ‘impossibility of self-doubt when thinking' as the leading thought in his main argument that his mind and his body were distinct, non-identical substances.
    • Descartes himself thought that the reasonings and succession of thoughts in The Meditations formed a seamless whole and that no single argument could be lifted out and studied independently of its place in the total enquiry. Many thinkers have gone against Descartes' wishes, ignoring, for example, the cosmological argument for the existence of God in Meditation III in order to consider Descartes' use of hyperbolic doubt early in the work or his brief treatment of the ontological argument in Meditation V. The same fate (of being singled out and considered apart from the work as a whole) has befallen his arguments for substance dualism in Meditations II and VI. And it does seem that these arguments are comparatively free-standing and can be studied independently. However, in order to have some sense of the place of Descartes' discussion of dualism in his thought as a whole (which will help you understand the arguments better), you should begin by reading Meditations I, II, and VI straight through without taking notes, but trying to form a picture of the drift of his thought.
    • Meditation II brings Descartes to his first certainty (‘I think therefore I am'), but leaves him wondering ‘What is this I of whose existence I am so certain?' When contemporary critics found fault with his arguments for the distinctness (independent existence) of his mind and his body in Meditation II, Descartes claimed that he had never offered arguments for his duality until Meditation VI. You might ask yourself as you read whether this claim is true: is Descartes trying to establish that he consists of two substances in Meditation II? Or does he only do so in Meditation VI (pp. 54 and 59 of the Cottingham translation)? Certainly there is a marked similarity between some points he makes in Meditation II and the argument for the distinctness of his mind from his body in The Discourse on Method (127). You might want to look up this passage in The Discourse as well.
    • After this preliminary reading of Descartes, you are ready to look at his arguments for his dualism (sometimes referred to as ‘Cartesian dualism') in detail. As has already been indicated, there is controversy among Descartes scholars about where and what exactly Descartes' arguments for substance dualism are (see also Smith and Jones 37). But philosophers of mind seem agreed on one, which is often simply cited as ‘Descartes' first (or main) argument for dualism' (see Graham 133); this is an argument which turns on the possibility of doubting the existence of body.
    • The second argument which will be summarised and discussed here is one which appears in Meditation VI and relies on Descartes' doctrine of clear and distinct ideas and his views on the creative power of God.
    • Finally, also in Meditation VI, is an argument which, comparatively speaking, has been ignored by the commentators. This is something of a pity since, whereas the first two arguments are afflicted with fatal flaws, this third argument seems quite strong and — up to a point — successful. This third argument relies on distinguishing body from mind/soul on the basis of the physical divisibility of the body contrasted with the impossibility of giving a sense to the notion of physical divisibility when applied to the mind. You should find this third argument familiar from the Phaedo — it bears a striking resemblance to certain elements of the Affinity Argument for the immortality of the soul.
  4. The argument from the possibility of doubting the body
    • Reading
      "Priest (Stephen) - Theories of the Mind", 1991, pp. 24-25
      "Smith (Peter) & Jones (O.R.) - The Philosophy of Mind - An Introduction", 1986, pp. 36-39.
      "Van Inwagen (Peter) - Metaphysics", pp. 156-57.
    • Descartes reaches a point in Meditation II where he is convinced that it is an unshakeable certainty that he exists. Of course he does not mean that he exists of necessity — he did not have to exist. History could have got along without him. Rather, he means that whatever he is thinking — if it is ‘I wonder if I exist', ‘I doubt that I exist' or even ‘I am convinced that I do not exist' — as long as there is something wondering, doubting or being convinced (however mistakenly), then it is true that that thinking something exists. So as long as he is thinking his existence is indubitable by him.
    • But the same thing cannot be said of his body. He finds it altogether possible to doubt that his body or indeed any material substance exists. Surely, it is conceivable or imaginable that he might be a disembodied1 mind being systematically tricked into believing he has a body?
    • It is worth pointing out that Descartes is not really afraid that his body does not exist. He is considering the grounds for his beliefs. For the belief that his mind exists he has the best possible grounds: it is in the very nature of the belief that he cannot suspend judgement about it. It would be irrational to think ‘I think there is some thinking going on (or a thinker here) but I could be wrong.' How could anyone wrongly think there was some thinking going on when there was not?
    • This basis for belief in the existence of himself as a thinker is quite different from his basis for his belief in the existence of his material body. It might seem exaggerated caution or scepticism to suspend belief in the existence of your body but it is not absolutely against logic. Contrariwise, it defies logic to try to doubt or suspend belief about the existence of the very thing (a thinker) which is required if there is to be any doubt or suspending of belief. To doubt or suspend belief about anything is something only an existent thinker can do.
    • So Descartes' first argument for dualism is this:
      1. I can doubt (or suspend judgement about) the existence of my body.
      2. I cannot doubt the existence of my mind.
      … Therefore,
      3. My body and my mind are different substances.
    • Behind this argument is a principle of reasoning which we all accept in many areas of life including, for example, when we are reading a detective story. Imagine a story in which the hero tries to prove that the heroine, who stands accused of the crime, is innocent. One way to prove this is to establish that she was not at the scene at the time the crime was committed. If he can prove her alibi she will not be convicted despite any damning circumstantial evidence against her. This is because if the heroine has a property the criminal lacks, she cannot be identical with (cannot be the same substance as) the criminal. And we may suppose she does have such a property — perhaps she was a hundred miles away when the crime happened and several independent and highly respectable witnesses can be found to testify to the fact.
    • In general, if X has a property Y lacks, or Y has a property X lacks, then X and Y are not the same single thing — they are genuinely two. This principle is called (or is a version of) Leibniz' Law2 (after the philosopher who wrote and thought extensively about its implications).
    • Although Leibniz came some years later than Descartes, it is usually said (for brevity) that the principle behind Descartes' main argument for dualism is this version of Leibniz' Law3. Descartes is saying that his mind has a property (indubitable existence while he is thinking) which his body lacks. Also his body has a property (dubitable or doubtful existence) which his mind lacks. Since each has a property the other lacks they are definitely, Descartes believes, distinct substances.
  5. Criticisms of Descartes' first argument
    • There are two main objections which have been levelled at this argument by many critics:
    • First, there is the criticism that says that logicians and metaphysicians are well aware that there are exceptions to Leibniz' Law4. Sadly for Descartes, his argument exemplifies two different sorts of exception to Leibniz' Law5 and thus is doubly flawed.
    • Second, there is the criticism which points out that the form of argument Descartes employs here must be invalid. This is because we can construct many analogous arguments of exactly the same logical form or structure whose premises are (as in this case) true but whose conclusion is obviously or self-evidently false. If an argument form leads from true premises to a conclusion known to be false then it must be an invalid argument form.
  6. The first criticism: exceptions to Leibniz' Law6
    Reading: Look at:
  7. First exception
    • Logicians have long been aware that two sorts of situation generate exceptions to Leibniz' Law7. Remember the form of Leibniz' Law8 in question. It says ‘If X has a property Y lacks or vice versa, X is not Y.' Ordinarily (as in the alibi case), this is a very reliable rule. However, suppose that the property in question is one involving a psychological verb, such as ‘wants', ‘believes', ‘hopes for'.
    • To take an example: perhaps as a matter of fact I have such a weak heart that a win on the lottery is just the thing the surprise of which would give me a fatal heart attack. The win might well be wanted by me while anyone would agree that the fatal surprise would not be wanted by me. I might set up an argument:
      … The win is wanted by me (has the property of being wanted by me).
      … The fatal surprise is not wanted by me (has the property of being not wanted by me)
      … If X has a property Y lacks (or vice versa), X is not (identical with) Y.
      … The win is not the fatal surprise.
    • But of course it would be a mistake to argue that a win is not, and could not be, a fatal surprise just because the win is wanted by me and the surprise is not.
    • Being ‘wanted by me' is not a property that can be used to distinguish one item from another. It is always possible that one and the same thing might rouse certain wants, beliefs and feelings in my mind in one situation and quite different or opposite psychological reactions in my mind in different circumstances, depending on the aspects it presents to me. I may see it in a certain light and characterise it one way while, unknown to me, it could also equally well be categorised another and opposite way.
    • When Descartes uses the property ‘dubitable by me', he is using the psychological notion of doubting. But it is not difficult to see that I might find it easy to doubt something when it presents a certain aspect and yet find it impossible to doubt that very same thing when it presents a different aspect. I might find it impossible to doubt a claim I read which was written by a respected scientific authority and yet find it very easy to doubt the words of a pompous-seeming, ineffectual and rambling speaker at a meeting - even though, when I am introduced to the speaker, I find that he is in fact the scientific expert I have always (without being acquainted with him) respected. It was easy to doubt his words when I didn't know who he was.
    • So too for Descartes. Ask yourself whether Descartes' capacity to doubt the existence of his own body could (or should) survive the discovery that thinking requires a physical organ (for example, a brain). Surely, if his existence as a thinker is indubitable, then so too is the existence of whatever items, physical or otherwise, turn out to be necessary conditions of thinking.
    • Of course, Descartes does not believe that thinking requires the existence of any (part of a) physical body, but he cannot legitimately argue that because he can doubt the existence of his body his body could not (unknown to him) be a necessary condition of his thinking.
  8. Second exception to Leibniz' Law9
    • Since Descartes' first argument for dualism tries to use Leibniz' Law10 together with a property having a psychological aspect to show mind and body are distinct substances, it is already irreparably flawed. For completeness, however, it is worth adding that the notions of possibility and probability (and, indeed, necessity) operate in a very similar way to the way psychological notions work to create exceptions to Leibniz' Law11. And Descartes' first argument involves possibility as well as the psychological notion of doubting.
    • The easiest way to explain this is by means of an example. Suppose that you accept — what all biologists and human physiologists would tell us — that it is not naturally possible for a five-year-old child to be more than six feet tall. Now, wherever you happen to be right now, ask yourself whether it is possible that the next person who comes down the road may be over six feet tall. You are likely to say ‘Yes, it is possible; nothing in logic or natural law rules out that the next passer-by may be over six feet tall.' But it is easy to see that the following argument would not be right:
      … The next person is possibly over six feet tall.
      … A five-year-old is not possibly over six feet tall.
      … If X has a property Y lacks or vice versa, X is not (identical with) Y.
      … … Therefore,
      … The next person will not be (identical with) a five year old.
    • This argument cannot be right. We know that the next person to pass by could be a five-year-old: again, nothing in logic or natural law rules it out.
    • The problem is that different descriptions can easily apply to a single individual (in this case, two applicable descriptions could be ‘next person' and ‘a five-year-old'). But different descriptions envisage different possibilities. Whether both or either or neither of those possibilities are genuinely open is a matter of how things are in the actual world, outside the head of anyone thinking about them. The person who actually comes along could be either a five-year-old or a six-foot-three adult or someone else altogether.
    • Just because Descartes thinks it possible that his physical body does not exist while he cannot think it possible that his thinking self (his real self) does not exist, that does not prove that the two descriptions ('thinking thing' and ‘physical thing') could not, in actuality, outside Descartes' mind, apply to one and the same substance. Perhaps ‘physical things' are the only sorts of things there are so his thinking self, if it exists at all, must be one. Descartes' doubly flawed argument fails to establish otherwise.
  9. The second criticism: Descartes' argument is of an invalid form
    • Reading: Look at:
      "Priest (Stephen) - Theories of the Mind", 1991, pp. 133-34.
      "Smith (Peter) & Jones (O.R.) - The Philosophy of Mind - An Introduction", pp. 40-41.
      "Van Inwagen (Peter) - Metaphysics", pp. 156-57.
    • The second line of criticism which has been directed at Descartes' main argument for dualism is the criticism that the logical form or structure of the argument must be defective since exactly analogous arguments (ones with exactly the same logical form or structure) whose premises, like Descartes', are true, nonetheless have conclusions which are obviously or self-evidently false.
    • In the readings listed above you will find a number of examples of arguments which have exactly the same form as Descartes' argument, whose premises are true and yet their conclusions are known to be false.
  10. Activity
    • See if you can invent an example of your own. The ingredients of such an example are as follows:
    • You need a statement of identity which you know or accept to be true. A statement of identity has the form ‘A is the very same thing as B' or ‘A is identical with B' or just ‘A is B' (or ‘A = B'). A good example that Priest uses in a related context is ‘Dr Jekyll is (the very same person as) Mr Hyde'. (Or you could use ‘Clark Kent is Superman.')
    • You need a description which explains the possibility of doubt about the person or thing mentioned in the identity statement. With the same example we could say, ‘No one is in doubt that (in the story) Mr Hyde committed many horrible murders' but such is the good doctor's excellent reputation that ‘Everyone doubts that Dr Jekyll is a villain.' ('Everyone doubts that Clark Kent is brave and strong.'; ‘No one doubts that Superman is brave and strong.')
    • Now you need to construct an argument with two true premises about the entity referred to by each of the terms in your identity statement:
      … No one doubts Mr Hyde's guilt.
      … Everyone doubts Dr Jekyll's guilt.
      … If X has a property Y lacks, or vice versa, X is not identical with Y.
      … Mr Hyde has undoubted guilt (which Dr Jekyll does not have).
      …… Therefore
      … Dr Jekyll is not Mr Hyde.
    • Alternatively:
      … No one doubts Superman is strong and brave.
      … Everyone doubts Clark Kent is strong and brave.
      … Superman has undoubted strength and bravery.
      …… Therefore,
      … Clark Kent is not Superman.
    • The argument seems to disprove the identity statement you began by accepting. So - given that its premises are true but its conclusion (of non-identity) is false - it would be a mistake to regard the form of the argument as sound or valid.
  11. The argument involving clear and distinct ideas
    • Reading: "Priest (Stephen) - Theories of the Mind", 1991, pp. 28-31.
    • This argument, which appears in Meditation VI, has as a first premise:
    • ‘…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…. capable of being
      separated at least by God' (Cottingham 54).
    • That is, if I can form a clear and distinct idea of something, God could arrange for that thing to exist independently of other things and indeed that thing could exist even if everything else had been annihilated.
    • The second premise is that I can form a clear and distinct conception of myself as a thinking, non-extended thing and of my body as an extended, non-thinking thing.
    • The conclusion is that I — the thinking, non-extended thing — am distinct from my body. Mind and body are distinct substances, each capable of existing in the complete absence of the other.
    • The main criticism which this argument attracts is that the first premise simply does not convince us of its truth or at least it is hard to see that Descartes has established it. The fact that I have a fairly detailed and precise notion of what my mind is like and even of how different mind is from body, mental states from physical states, does not seem to provide any evidence that mind and mental states do not depend in any way on matter or body for their existence. As Priest points out, even if Descartes is right that thinking is one of his essential properties, he has given no evidence that it is his only essential property.
  12. Activity
    • Descartes relies heavily on conceivability and imaginability. Ask yourself whether you can conceive of or imagine your mind existing (or continuing) in the absence of your body (or indeed any body). Sometimes it is possible to picture things (imagine them in that sense) which, on reflection, one might want to deny are, strictly speaking, conceivable. For example, in animated cartoons characters are sometimes pictured falling from a great height, spreading like a puddle on hitting the ground and then springing up whole and apparently uninjured. Is imagining continuing to exist as a thinker despite the destruction of one's body like imagining that a living organism could be squashed (as in the cartoon) and yet continue living and functioning? Is it something we can picture which is nonetheless not genuinely possible?
  13. The argument involving the indivisibility of the mind
    • Finally, Descartes gives an argument for dualism in Meditation VI which uses Leibniz' Law12 to prove non-identity, but does not fall into any of the traps or exhibit any of the flaws of earlier attempts to do so.
    • Descartes argues:
      … My body (like all matter) is divisible into parts which can be spatially separated from each other.
      … My mind is indivisible (it is not divisible into spatially separable parts).
      … The body has a property the mind lacks.
      … … Therefore,
      … Mind and body are different substances. (Dualism is correct.)
    • This argument does not use a property defined in terms of a psychological verb (nor does it try to use one which involves possibilities) to distinguish one entity from another. It uses a perfectly unproblematic property - physical divisibility. Something which is physically divisible cannot be identical with (be the very same thing as) something which is literally indivisible in its nature. And many thinkers have believed the mind is an indivisible unity13. So, Descartes would seem at last to have established the non-identity of mind and body.
  14. Activity
    • Do you think a sense could be attached to the notion of physically dividing, or dividing and spatially separating the parts of, the mind? We sometimes talk about mental conditions like schizophrenia as a kind of ‘fracturing of the self', but this way of talking is metaphorical (non-literal). The divisibility which Descartes thinks the body has and the mind lacks is literal.
    • Suppose (as a kind of science fiction thought experiment)14 that a part of your brain were taken out of your skull and transplanted15 into the skull of another living person. Might not that transplanted16 part literally take a part of your mind to another place?
  15. Criticism of the argument for the mind's indivisibility
    • The third argument does in my opinion succeed in proving that mind and body are not identical and do have different natures, neither of which can simply be reduced to the other. Where Descartes' argument falls down is that it provides no proof that the mind as well as the body is a substance.
    • Think of it in terms of this analogy. My voice and I have some sharply distinct features which can be used to prove that I am not identical with (not the very same thing as) my voice. I am 5'2" and have brown eyes: my voice is not 5'2" or brown eyed. My voice is a light alto, but if I am an alto this is only by extension (a person is an alto if he or she sings most comfortably in this register). If I am rightly said to be ‘light', it is in a different sense (my hair or skin colour could be ‘light' in a sense opposed to ‘dark' or my weight might be considered ‘light' as opposed to ‘heavy'). But I as a person am not light as opposed to deep or strong (in the sense in which voices are deep or strong). These properties, true of my voice but not of me as a person, show that I am not to be identified with my voice, or it with me, but that does not incline me to think that my voice is a separate substance utterly, metaphysically, distinct from the person I am.
    • Descartes still needs to prove that his mind as well as his body is a substance before he will have proved mind/body dualism in the strong version of this view to which he subscribes.
  16. Learning outcomes
    • When you have finished working through the material in this section and the associated readings, you should be able to:
    • Outline the three arguments for Cartesian Dualism which appear in Descartes' Meditations and Discourse.
    • Explain, discuss and evaluate the success of both those arguments, and the criticisms which philosophers have addressed to those arguments.
  17. Sample examination questions
    • Descartes claims that he cannot doubt his own existence when thinking whereas he can doubt the existence of his body. How does he try to prove substance dualism from this fact? Assess his success.
    • Discuss the use Plato (in the Phaedo) and Descartes (in the Mediations) make of the idea that the soul or mind is indivisible.
  18. Tips on answering the sample questions
    • In answering either of these questions, you should provide a brief definition of ‘substance dualism'.
    • The first question asks for some mention of Leibniz' Law17 and how one form of it can be used to prove non-identity. The second question needs a discussion of what kind of divisibility is at issue.


Part 1 (The Metaphysics of Mind and Body); Section 1 (Dualism); Chapter 2. Hard copy filed in "Various - Papers on Religion Boxes (Heythrop)".

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