2. Substance dualism, part 1

Martín Abreu Zavaleta

May 28, 2014

1 Descartes’ arguments for dualism

First stab: second meditation

A bit of background will be useful: Descartes has started the project of doubting everything he knew until the time he wrote the Meditations and is now trying to find out if he can know anything with certainty. He says:

I will suppose then, that everything I see is spurious. I will believe that my memory tells me lies, and that none of the things that it reports ever happened. I have no senses. Body, shape, extension, movement and place are chimeras. So what remains true? (Meditation 2, p. 17)

He presents the following argument that at least he can convince himself with full certainty that he exists. He reasons as follows: if I can think of something, that means that I must exist, but right know I am thinking (doubting, etc.), so I must exist, and as long as I am thinking something, I can rest assured of my own existence.

Suppose that you are in Descartes’ position, and you have now convinced yourself that you can be certain of your own existence. Now you can inquire into the nature of the thing whose existence you have discovered (i.e. yourself). Since we have decided to doubt everything of which we are not certain, and we are not certain that we have bodies, we may be tempted to conclude that the substance we are after (the I whose existence we have argued for) is not a body! Questions: Is this a good argument? Can you find a place where Descartes addresses this question? If so, what does he think about this argument?

Descartes continues his investigation about the nature of the I he just discovered. He adds that just like he can’t doubt his own existence, he can’t doubt that he really is thinking when he thinks that he is thinking, or that he really is doubting when he thinks that he is doubting, and similarly for his other mental states, including the claim that he certainly seems to see and to have other sensory perceptions, even if in fact those sensory perceptions don’t really track anything.

He argues that “even bodies are not strictly perceived by the senses or the faculty of imagination but by the intellect alone". His argument for this claim is not particularly relevant for our purposes, but it would be a good exercise to attempt to reconstruct his argument.

A conceivability argument: sixth meditation

In his second meditation, Descartes presents a tentative argument that he is distinct from his body, but he ends up rejecting it. Now he presents a stronger argument, which he fully endorses. He begins with the premise that anything one can conceive without contradiction is possible:

There is no doubt that God is capable of creating everything that I am capable of perceiving in this manner; and I have never judged that something could not be made by him except on the grounds that there would be a contradiction in my perceiving it distinctly. (p. 50)

In p. 54, after presenting this assumption, he presents the rest of the argument:

I know that everything which 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 [...] 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. [...] 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.(p. 54)

We can reconstruct Descartes’ argument as follows:

Everything that I can conceive clearly, distinctly, and without contradiction, is possible.
Thus, if I can conceive that something exists without something else, then it must be possible for the former to exist without the latter.
I can conceive of myself existing without my body.
I can conceive of my body existing as a non-thinking thing.
So it must be possible for me and my body to exist without each other; therefore,
I am really distinct from my body, and can exist without it.

One way of understanding the argument is by appeal to the following premise:

Necessity of identity:
if A and B are the same thing, then necessarily they are the same thing.

Given premise 5 of the argument, 6 follows by modus tollens.

Another way to arrive to the conclusion is by means of Leibniz’s law, explained below.Question: Once you read the part about Leibniz’s law, how can we reconstruct Descartes’ argument so that it makes explicit use of it?

An argument for dualism using Leibniz’s law (sixth meditation)

Some pages after stating the previous argument, Descartes introduces some further reasons to think that the mind is different from the body, even though he takes his argument from conceivability to have established this. His new argument appeals to the following principle:

Leibniz’s law:
If A and B are the same thing, then they must have all the same properties. Equivalently, If A and B differ in at least one property, then they are not the same thing.

Leibniz was not the first person to use this principle in an argument, since Descartes used it before in the following argument:

there is a great difference between the mind and the body, inasmuch as the body is by its nature always divisible, while the mind is utterly indivisible [...] This one argument would be enough to show me that the mind is completely different from the body, even if I did not already know as much from other considerations. (p. 59)

Using Leibniz’s law, we can reconstruct the argument as follows:

If A and B differ in at least one property, then they are not the same thing (Leibniz¬ťs law).
The mind is always divisible.
The body is indivisible; therefore,
The mind is not the body.

Questions: What do you think about this argument? Can you think of other arguments for dualism that use Leibniz’s law? Is Leibniz’s law an implicit premise in Descartes’ argument from conceivability? Can we use any property to make an argument for distinctness using Leibniz¬ťs law?

2 Descartes’ dualism

Descartes’ view about the nature of the mind is usually called substance dualism. It is so called because it posits two kinds of substances: mental and material—or physical, as we would now call them.1

Descartes’ dualism attempts to recover the common sense intuition that our minds interact with our bodies: our mind can control our body at will, and our body can influence our minds by producing in it pains, hunger, and other sensations including sense perception. For this reason, Descartes’ view is also a version of interactionist dualism. The main tenet of interactionist dualism is simply that, though the mind and the body are different kinds of substances, they can interact with each other. The following remarks by Descartes show his commitment to this view:

There is nothing that my own nature teaches more vividly than that I have a body, and that when I fell pain there is something wrong with the body, and that when I am hungry or thirsty the body needs food and drink, and so on. So I should not doubt that there is some truth in this (p. 56).

I am not merely present in my body as a sailor is present in a ship, but ... I am very closely joined and, as it were, intermingled with it, so that I and the body form a unit. If this were not so, I, who am nothing but a thinking thing, would not feel pain when the body was hurt, but would perceive the damage purely by the intellect, just as a sailor perceives by sight if anything in his ship is broken. (p. 56)

Descartes’ view was forcefully attacked since the very beginning, which is why we will consider some objections raised by Princess Elisabeth of Bohemia.

3 Elisabeth’s challenges

Recall Descartes’ working definition of a body:

By a body I understand whatever has a determinable shape and a definable location and can occupy a spas: in such a way as to exclude any other body; it can be perceived by touch, sight, hearing, taste or smell, and can be moved in various ways, not by itself but by whatever else comes into contact with it. (p. 17)

Moreover, Descartes has argued that he (his soul, his mind) can exist without a body, and without having any extension. This is what puzzles Elisabeth:

Given that the soul of a human being is only a thinking substance, how can it affect the bodily spirits, in order to bring about voluntary actions? The question arises because it seems that how a thing moves depends solely on (i) how much it is pushed, (ii) the manner in which it is pushed, or (iii) the surface-texture and shape of the thing that pushes it. The first two of those require contact between the two things, and the third requires that the causally active thing be extended. Your notion of the soul entirely excludes extension, and it appears to me that an immaterial thing can’t possibly touch anything else. (Elisabeth to Descartes, 6.V.1643)

Descartes has claimed that the mind and the body causally interact with each other, and Elisabeth is asking for an explanation of how the interaction is supposed to obtain. Elisabeth challenges is as follows: causing a material thing to move requires being extended, but Descartes claims that minds are not extended. So how can minds cause bodies to move?

In another letter, Elisabeth presses the challenge:

I’ve never been able to conceive of ‘what is immaterial’ in any way except as the bare negative ‘what is not material’, and that can’t enter into causal relations with matter!

I have to say that I would find it easier to concede matter and extension to the soul than to concede that an immaterial thing could move and be moved by a body. On the one side, if the soul moves the body through information, the spirits would have to think, and you say that nothing of a bodily thing thinks. On the other side, you show in your Meditations that the body could move the soul, and yet it is hard to understand that [...] a soul that can exist without the body, and that has nothing in common with the body, is so governed by it. (Elisabeth to Descartes 10.VI.1643)

1Remember our working definition of substance: a substance is something that can exist independently of other things.