Let us start by recalling what dualism is: in philosophy the idea traces back at least to Descartes (though some would consider Plato a dualist), and his contention that while everything else in humans and animals is “mechanical” (i.e., made of matter), the mind is an exception, since it’s made of some kind of distinct mind stuff (he was pretty vague about exactly what this mind stuff might be, and so is Chalmers, incidentally). Descartes immediately got into trouble because he couldn’t provide an account of how is it that the non-material mind seems to interact so well with the physical world and have tangible effects (that’s what happens every time, say, you make up your “mind” of wanting a martini, and your body responds by walking you to the bar and starting the shaking of spirits and the piling of olives).
Descartes then famously dug himself even deeper into an intellectual quagmire when he proposed that the place where (somehow) mind stuff and body stuff “meet” is the pineal gland, at the center of the brain. Turns out that we now know that the pineal gland is actually involved in the production of melatonin which, while important in keeping the day/night cycle straight (it helps with jet lag), and likely relevant to normal sexuality (the gland is larger in children, where it inhibits sexual development) is far from the seat of the soul. If it were, most of us would be in trouble because in many adults the gland calcifies, becoming essentially non functional.
Back to Chalmers. He claimed to have revived dualism -- despite the generally bad reputation the word has even among philosophers -- and developed the following argument in support of his startling conclusion (this is Hanrahan’s, I think fair, formalization of if):
premise 1: We can conceive of a world populated by some zombie twins, who act exactly as we do, have our same physiology and internal structure, our brain and our psychology. But they do not have any conscious experience whatsoever.
premise 2: Conceivability provides us with a guide to possibility.
premise 3: It is possible that there is a world populated by our zombie twins.
conclusion 1: If this is possible, then materialism [the idea that everything is made of matter/energy] is false.
conclusion 2: If materialism is false, then dualism is true.
QED (Quod Erat Demonstrandum, as we wished to demonstrate)
I refer you to Hanrahan’s short and eminently digestible article for the details, but you can easily see that even if one of the stated premises is not true, then none of the conclusions will follow. Now, let’s take a look at the three premises, beginning with the last one, p3.
I suppose that it is possible that there may be living beings that are made like us but do not have conscious experiences, although they would be really strange beings. I find that possibility to be extremely unlikely, but I don’t see that it contradicts any known physical or logical law. p1 is also true: we can indeed conceive of the kind of zombies that Chalmers imagines, though it must be noted that these are not the lively flesh-eating chaps we see in the movies.
The real problem, of course, is with p2: conceivability is not a reliable guide to possibility. I can conceive of impossible things, such as the idea of squaring the circle, or of a god that is omnipotent and yet can make a mountain so big that she couldn’t move it, and so on. Chalmers comes up with an (admittedly ingenious) little story, and we are supposed to deduce from it the momentous conclusion that there is more than matter/energy to the universe? When things appear to be too good to be true there is often good reason to think that they in fact are too good to be true.
Moreover, although this is an unstated implication of the above deduction, possibility in turn is not a particularly good guide to reality. There are plenty of things that are possible but that are not in fact realized in the actual world. Remember that the question that Chalmers wishes to answer is whether human mental experience is compatible with materialism, as is strongly suggested by the fact that nobody has ever seen a mental state occur independently of the presence of a physical brain (what some philosophers call the “no ectoplasm clause”). While I think it is very reasonable to assume that anything that is real must also be possible (either that or our logic is seriously faulty), it is just bizarre to suggest that one can go from possibility to claims about reality, the way Chalmers does. Has he not learned anything from the failure of the rationalist program in philosophy?
Incidentally, I would note that mine (or Hanrahan’s, from what I can tell) is not an argument against thought experiments in general. They can be useful to evaluate our intuitions, and -- contrary to popular belief -- they are not just something that “armchair philosophers” (a redundant phrase if there ever was one) engage in. Scientists from Galileo to Einstein have used thought experiments to sharpen their thinking about the world. But just like any tool, it needs to be used according to reasonable instructions and to solve the appropriate problems, just because all you’ve got is a hammer that doesn’t turn everything into a nail.
As readers of this blog know, I am a scientist with a background in philosophy, and am very sympathetic to the whole philosophical approach to things. But just in the same way I think a lot of scientists do a disservice to themselves and to the public by not taking philosophy seriously, I am also convinced that people like Chalmers don’t help philosophy even a bit, either among scientists or the public. Let’s relegate zombies to B-movies and try to be a little more serious about our philosophy, shall we?