Three Arguments for Dualism

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This is the 47th post in my Theories of Knowledge and Reality series. The last post finished up the section devoted to issues related to freedom and moral responsibility. This post begins the next topic, the human mind.

Materialism - the physical or material world is all there is (or, more particularly about the human being, we are merely physical beings). This view is also called physicalism.

Dualism - there are two fundamentally different kinds of things in the universe - physical and mental things. In the case of the human mind, that means our mind (or soul, as some would call it) is a non-physical thing. (This view is technically called substance dualism. Another version of dualism comes up later.)

Leibniz's Law:
If A = B, then A and B share all and exactly the same properties
(In plainer English, if A and B really are just the same thing, then anything true of one is true of the other, since it's not another after all but the same thing.)

It's pretty common in introductory philosophy classes to present three dualist arguments roughly tracing back to Rene Descartes that rely on this principle. If A just is B, then A and B will have all the same properties. If Clark Kent really just is the same guy as Superman, they'll have all their features in common, even if Lois Lane doesn't know it. Many people think these arguments for dualism are unconvincing, but that wouldn't show Leibniz's Law to be false. Leibniz's Law is one of the most sure principles we can get. The arguments have to be questioned some other way.

Argument from Disembodied Existence

1. My mind can exist separate from anything physical.
2. No physical part of me can exist separate from anything physical.
3. Therefore, by Leibniz's Law, my mind isn't a physical part of me.

We seem to be able to imagine ourselves existing apart from anything physical. That's why the first premise seems right. We can't just float off outside our bodies like astral projection, but Descartes didn't think things had to be that way. If it had been different, we wouldn't have had physical bodies. This seems possible. If so, there's a property my mind has that it doesn't share with anything physical - it could have existed without any connection to a physical world.

But maybe this is like Lois Lane's thinking Superman and Clark Kent are different. She's wrong, and maybe we're wrong when we think this disembodied existence could have been possible. How do we know we can imagine a truly possible disembodied state without assuming dualism? If materialism is true, the first premise is wrong, so it won't convince a materialist, who denies premise 1.

Another way to resist this argument is to say the mind could exist apart from any physical part of me (maybe by being converted into a computer program or something). That doesn't mean dualism is true. Some people don't think computers could ever be able to think, but others find this sort of view attractive.

Argument from divisibility:

1. My physical parts are divisible.
2. My mind is not divisible.
3. So my mind is distinct from any of my physical parts (by Leibniz's Law).

This second argument is much simpler but has exactly the same problem. If the premises are true, the conclusion will follow by Leibniz's Law. The problem is that we can't be sure the premises are both true. The first premise is hard to question, but the second involves the same sort of imagination as the first argument's claim that we can imagine ourselves to exist disembodied. Descartes was sure you couldn't divide his mind, because it seemed as simple as anything could get. How could you divide someone's thoughts or choices? But is he right? How do we know that it's impossible to divide the mind without assuming that dualism is true? If materialism is true, maybe our minds are divisible. Maybe our minds are just our brains, and then they're divisible in the same way brains are divisible - with a knife and scalpel. Some people mention actual cases when people seem to have divided minds (multiple personality disorder or split-brain cases where the two hemispheres of the brain are severed).

There's another way around the divisibility argument. A materialist can think the mind isn't just a physical part of us. It's more than that, but it's not an immaterial thing. It would be more like the software, and the brain would be like the hardware. That doesn't require going beyond materialism. This has the same weakness as above, with John Searle's argument to deal with, but if you have a response to that, you could go with this.

Argument from introspection:

1. I can come to know about my mind (mental states) by introspection.
2. I cannot come to know about my brain (or any physical states) by introspection.
3. Therefore, my mind and my physical parts are distinct (by Leibniz's Law).

I can know about my mind through looking into myself. I can know about my brain through external investigation. A materialist has no problem with those two claims. But will a materialist admit the second premise? What if mental states just are brain states and we understand them in two different ways? It's like the same guy that Lois Lane knows under two ways of thinking about him - under one way, she thinks some things about him (i.e., that he's a powerful hero with incredibly good vision), and under the other way she thinks different things about him (i.e., that he's a glasses-wearing reporter). But he's the same guy. She just doesn't know it. So it would be if our brain is our mind. We can think of it in terms of beliefs, memories, and desires - from within. We can also think of it in terms of neurons, electrical signals, and gray matter - as if of from an outside point of view. But maybe it's the same thing we're thinking about, just in two different ways.

As before, a materialist might say our mind isn't just our brain, admitting that the conclusion is true, but still say the mind isn't non-physical. If this is so, then the conclusion is true, but materialism is also true. Some materialists prefer to think of the mind as just the brain, and this move would be unattractive to them, which would require a more complex response.

One thing to notice about all these arguments is that they fail only if materialism is true. That is, the arguments won't convince materialists that materialism is false, but the responses are only any good if materialism is true. If materialism is false, all of the objections to the arguments fail. That would mean that the arguments are sound arguments if dualism is true but unsound if it is false. The arguments are unconvincing to the materialist, but it's easy enough to see why a dualist would think they are sound arguments. It doesn't resolve the question, but I think these arguments are often treated as worse than they are in that respect.

The next post will look at a different kind of argument for dualism.


In the intersection of the worlds of Stand-Up Comedy and Metaphysics, no one comes close to Steven Wright. He said, someone broke into my house last night and replaced everything with exact duplicates.

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