Descartes - Lecture 9
9. Descartes’ Arguments for a Real Distinction between Mind and Body
The Argument from Doubt
Descartes is sometimes tempted to believe that he can show that there is a real distinction between mind and body by appealing to the so-called Argument from Doubt. Thus it would, for example, be quite easy to read the `Second Meditation' as attempting to put the Argument from Doubt to such a use. After Descartes has come to the conclusion that it is certain that the statement ‘I exist’ is true whenever it is formulated by him, he attempts to clarify the nature of the entity picked out by the word ‘I’ by refusing to affirm of it anything that can be called into question through the method of doubt. Unfortunately this leads him to express himself in the following terms ‘I am, then, in the strict sense only a thing that thinks, that is, I am a mind, or intelligence, or intellect, or reason – words whose meaning I have been ignorant of until now.’ Although Descartes subsequently attempted to distance himself from this argument by protesting that he merely intended it to show what he could KNOW about his nature, there are compelling grounds for holding that Descartes had once been of the opinion that this argument was a cogent one. In a letter written to Jean de Silhon on the eve of the publication of the Discourse on the Method, Descartes gives the following statement of his position: `a man who doubts everything material cannot for all that doubt his own existence. From this it follows that he, that is his soul, is a being or substance which is not at all corporeal...'. And in Part IV of the Discourse itself, Descartes writes as follows: `I saw that while I could pretend that I had no body ... I could not for all that pretend that I did not exist.... From this I KNEW I was a substance whose whole essence or nature is simply to think, and which does not require any place, or depend on any material thing, in order to exist.'
This argument appears to depend on the version of Leibniz’s law known as the principle of the indiscernibility of identicals. This principle states that if something is true of a at a particular time but not true of b at the same time, then a is not identical with b. In many contexts this principle seems relatively unproblematic. If it is true of a that a has blue eyes and it is true of b that b has brown eyes, then a is not the same person as b. However the principle seems far less convincing in modal contexts (those involving the concepts of necessity and possibility) and intentional contexts (those involving concepts such as belief and desire).
We would normally accept, for example, that four is the number of the gospels. Now four is necessarily twice two, but it is not necessarily true that the number of the Gospels is twice two - if the appropriate decisions had been taken by the Church Fathers there might have been three or five Gospels. Thus there is something true of the number four that is not true of the number of the Gospels. However this point is not likely to persuade us that our original claim that four is the number of the Gospels is mistaken.
Similarly, Venus may be believed to be a planet without the Morning Star being believed to be a planet. Nevertheless this point does not prevent it from being true that Venus and the Morning Star are one and the same thing.
Unfortunately for Descartes the predicate `... can be pretended by me not to exist' provides a context that is both modal and intentional as it concerns the POSSIBILITY of a particular PRETENSE. Thus we have reason to be acutely suspicious of any attempt to invoke the principle of the indiscernibility of identicals in this case. Indeed the following example seems to establish conclusively that this principle will only lead us astray here. The criminal underworld of Gotham City is obviously extremely interested in discovering the real identity of Batman. Let us suppose then that the Joker finds himself locked in single combat with Batman and takes the opportunity to indulge in the following piece of reasoning, `I can doubt that Bruce Wayne is alive (after all, I haven't seen him recently and that bomb I posted to his house might well have killed him); but I cannot doubt that Batman exists; therefore Bruce Wayne is not Batman.' At this point the Joker is knocked unconscious, but when he recovers he hastens to tell his criminal confederates that his punishing encounter with Batman has at least enable him to discover that the rumour suggesting that Bruce Wayne is Batman must be mistaken. Plainly the Joker’s reasoning, although formally analogous to the reasoning in Descartes' letter, is hopelessly flawed?
A more classically erudite example. Marcus Tullius Cicero (106 – 43 BC), a Roman statesman, lawyer and philosopher, is generally known today as Cicero. However it was once more common for him to be referred to as Tully. Thus someone A studying philosophy today might well be aware, on the basis of good evidence, that Cicero was a philosopher. However if A were to encounter the name Tully in an old-fashioned library catalogue, A might fail to realize that Tully and Cicero are one and the same person. Let us suppose, then that A attempts to argue that as he cannot reasonably believe that Cicero was not a philosopher while he can reasonably believe that Tully was not a philosopher, it follows by Leibniz’s law that Tully and Cicero are not one and the same person. This argument would again be formally analogous to the Argument from Doubt, but it would plainly be taking A to a false conclusion.
The Argument from Clear and Distinct Perception
When Descartes first deploys the `Argument from Clear and Distinct Perception', he argues that the fact that he can clearly and distinctly conceive of mind apart from body shows that the two are really distinct because he knows that everything which he clearly and distinctly understands is capable of being created by God so as to correspond exactly with his understanding of it (Meditations, `Sixth Meditation', AT VII 78). However Arnauld's example of the right-angled triangle appears to show that this argument is untrustworthy. I can have a clear and distinct idea of a right-angled triangle without having a clear and distinct idea of a triangle that has sides such that the square of the hypotenuse equals the sum of the squares of the remaining two sides. Nevertheless it is necessarily true that any right-angled triangle is also a triangle whose sides bear the latter relationship to one another. Indeed one can exploit Arnauld's example still further. It doesn't seem impossible that someone might have a clear and distinct idea of a right angled triangle and a clear and distinct idea of a triangle that conforms to Pythagoras' theorem without having the slightest notion that the two ideas are necessarily co-extensive. Thus it doesn't seem impossible that someone might have a clear and distinct idea of mind and an equally clear and distinct idea of body, and yet fail to be aware of some necessary relationship that holds between the two. If Descartes wants to argue from his clear and distinct perceptions to the separability of his mind and body, then it seems as though he would need to maintain that he clearly and distinctly perceives that his mind is separable from his body. And although Descartes seems to stumble into making such a claim in the middle of the paragraph cited above, I think that most of us would find ourselves unable to locate anything akin to such a clear and distinct perception within our own experience. Moreover it is extremely noticeable that Descartes himself doesn't seem to be aware that this highly ambitious claim differs in any important respect from other claims about what he can clearly and distinctly perceive that patently fail to license the ontological conclusion he is seeking.
Descartes on Clear and Distinct Perception
In the `Replies to Objections' Descartes apparently attempts to respond to Arnauld’s objection by maintaining that one does not genuinely have a distinct idea of x unless one is aware of all those things that are necessarily true if x exists. However this response fails to be faithful to his own explicit definition of the term `distinct'. Descartes defines a clear perception as one which is `present and accessible to the attentive mind - just as we say we see something clearly when it is present to the eye's gaze and stimulates it with a sufficient degree of strength and accessibility' (Principles of Philosophy, AT VIII 22). And a perception is `distinct' if `as well as being clear, it is so sharply separated from all other perceptions that it contains within itself only what is clear'. Nothing is said here about a distinct perception of x being a perception that brings with it knowledge of everything that is entailed by the supposition that x exists.
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