Dualism

Dualism is the claims that the mind is distinct from the body, or any part of the body . How do we know that the mind is not identical to some physical object, like the brain, or the central nervous system? In general, how is it possible to show that a thing X is distinct from (that is, not identical to) a thing Y? How do we prove that X and Y are two distinct things, rather than one and the same thing?

1 Leibniz's principle of identity

Consider a very simple example (always the best examples!). Suppose you know that X is a piece of chalk, and that Y is a piece of cheese: that is, Y is a NOT piece of chalk. In that case X must clearly be a distinct object from Y—X and Y cannot be one and the same. (For one object cannot both be cheese and chalk.) Or suppose that both X and Y are pieces of chalk, but that X is longer than Y. In this case too we know there must be two pieces of chalk. (For if X were the very same object as Y then X and Y would have to have the same length.) Or suppose that X and Y are both pieces of chalk, and in addition they are the same length: but in this case, X occupies a different position from that occupied by Y. Again, X must be a distinct object from Y. (If X were the very same object as Y then X and Y would have to occupy the very same position.)

The principle we have been appealing to in these three simple cases is this: if X has a property which Y does not have then X is a distinct object from Y. The same principle can be stated in a slightly different form: if X is the same object as Y then X has exactly the same properties that Y has. We will call this Leibniz's principle for it was first clearly formulated by the seventeenth century German philosopher Leibniz.

If we could show that the mind has a property which no physical object has, or that all physical objects have a property which the mind clearly lacks, then (by Leibniz's identity principle) it will follow that the mind and the body are distinct items.

2 Some arguments for dualism

2.1 Two arguments from the survival of death

There are two arguments we could develop on the basis of the idea that the mind survives the death of the body. One is a simple argument, the other slightly more subtle.
Firstly, many people have reported near-death, out-of-body experiences. Such reports are typically associated with heart attacks, road accidents, and so on. In such cases the person's body ceases to function normally, and after bodily functions are restored the person reports having had extraordinary experiences during that period, experiences of the mind leaving the body, and of being conscious of the body from a position outside it; also, of being conscious of another 'world'; of coming into contact with people they knew to be deceased, and so on.
Suppose these reports are trustworthy (since we have no good reason to suspect that the people involved are deliberately trying to deceive us). Then there are two possibilities. Either what these people report really happens, or they are suffering from some kind of illusion or hallucination.
Consider the first possibility. If what they report really happens then the mind is distinct from the body. The mind could not leave the body and carry on functioning after the body had ceased to do so, unless the mind was a distinct item from the body. We would have an argument for dualism that looks like this:

Premise 1: The mind continues to function after the body (including all parts of the body) ceases to function.

Premise 2: The body does not continue to function after the body ceases to function.

therefore

Conclusion: The mind is not identical to the body.

Consider the second possibility. Suppose that all these people are deceived about their out-of-body experiences. (After all, you might argue, the brain would surely be under stress in such near-death situations, and so not at all unlikely to induce weird experiences.) Even if you take this line then the story is coherent, or logically possible. There is no obvious logical contradiction in the story. If this is right then it seems that we can construct another argument for dualism.

Premise 1: It is logically possible for the mind to continue functioning after the body ceases to function.

Premise 2: It is not logically possible for the body to continue functioning after the body ceases to function.

therefore

Conclusion: The mind is not identical to the body.

2.2 The argument from doubt

The seventeenth century French philosopher Descartes wanted to push doubt to the extreme to see what, if anything, would survive. He imagined that there is an omnipotent demon whose sole aim is produce in you as many false beliefs as possible. For example he makes you hallucinate the entire physical world. Thus it is at least possible to doubt the existence of the physical world, including that part of it which seems to be your physical body. Is there any proposition which even the omnipotent demon could not make your falsely believe? Descartes discovered that he could not sensibly doubt that he himself doubts. Putting all this in the first person, I cannot doubt that I doubt. For if I doubt anything then I thereby demonstrate that I doubt. So the proposition I doubt or I think is one the mere contemplation of which is sufficient to guarantee its own truth. This is Descartes' famous cogito: that is just Latin for 'I think'. Now since it is impossible for me to doubt or think without also existing, it is also indubitable (for me) that I exist. As Descartes put it: cogito ergo sum: that's Latin for 'I think, therefore I am'. Even if my sense experiences are wholly deceptive I can still be sure that I exist.
Thus we get the following argument

Premise 1: My body (or any part of it) is such that it is coherent for me to doubt its existence.
Premise 2: My mind is not such that it is coherent for me to doubt its existence.

Therefore

Conclusion: My mind is not identical to my body (or any part of it).

2.3 The spatial argument

The spatial argument, also due to Descartes can be developed in various ways, but the basic idea is that whereas it makes sense to attribute spatial properties to bodies, it does not make sense to attribute them to minds. With each body it makes sense to ask where it is, how big it is, what volume it takes up, and so on. But these questions applied to the mind seem to be nonsense (unless one is already convinced that the mind is identical to the brain, or something like that). So the argument looks like this:

Premise 1: Each physical body is extended in space.
Premise 2: My mind is not extended in space.

therefore

Conclusion: My mind is not a physical body.

The argument from divisibility is closely related to the argument above:

Premise 1: Every physical body is (in principle) spatially divisible.
Premise 2: My mind is not (even in principle) spatially divisible.

therefore

Conclusion: My mind is not a physical body.

The argument is valid (the premises entail the conclusion) so someone who denies the conclusion must deny one of the premises. Both these arguments seem compelling. It does not seem to make sense to ask for the physical or spatial dimensions of a mind, nor does it make sense to talk about spatially dividing a mind.

(We do talk about our minds being divided, being in two minds, and so on, but this need not be taken seriously as an objection to the divisibility argument. Firstly, it is not spatial division that is at issue; secondly, when a mind is divided in this sense it is one and the same mind which is aware of the attractions of two different alternatives.)

Questions: Does it make sense to say that the mind has physical dimensions, that a mind weighs a certain amount, takes up so much space etc? If not, is the definition of mind wrong? Can you think of an alternative definition?

3 The problem

The problem with dualism is basically the problem of interaction. How could a non-physical mind interact causally with a physical body. In particular, would not the causal efficacy of the mental violate well-confirmed physical principles, like the conservation laws. How (and where?) could the mind start pushing brute matter around?