- [Here] is the position taken by Descartes and most other ancient thinkers: The soul is of a different kind, but it interacts with the body. Descartes's position is technically known as substance dualism - there is a thought-stuff, a mind-stuff, and it is not like atoms; but it is causally potent, interactive, and leaves a visible mark on our universe.
Zombie-ists are property dualists - they don't believe in a separate soul; they believe that matter in our universe has additional properties beyond the physical.
"Beyond the physical"? What does that mean? It means the extra properties are there, but they don't influence the motion of the atoms, like the properties of electrical charge or mass. The extra properties are not experimentally detectable by third parties; you know you are conscious, from the inside of your extra properties, but no scientist can ever directly detect this from outside.
So the additional properties are there, but not causally active. The extra properties do not move atoms around, which is why they can't be detected by third parties.
And that's why we can (allegedly) imagine a universe just like this one, with all the atoms in the same places, but the extra properties missing, such that every atom moves the same as before, but no one is conscious.
The Zombie World might not be physically possible, say the zombie-ists - because it is a fact that all the matter in our universe has the extra properties, or obeys the bridging laws that evoke consciousness - but the Zombie World is logically possible: the bridging laws could have been different.
But why, oh why, say that the extra properties are epiphenomenal and undetectable?
We can put this dilemma very sharply: Chalmers believes that there is something called consciousness, and this consciousness embodies the true and indescribable substance of the mysterious redness of red. It may be a property beyond mass and charge, but it's there, and it is consciousness. Now, having said the above, Chalmers furthermore specifies that this true stuff of consciousness is epiphenomenal, without causal potency - but why say that?
Why say that you could subtract this true stuff of consciousness, and leave all the atoms in the same place doing the same things? If that's true, we need some separate physical explanation for why Chalmers talks about "the mysterious redness of red". That is, there exists both a mysterious redness of red, which is extra-physical, and an entirely separate reason, within physics, why Chalmers talks about the "mysterious redness of red".
Chalmers does confess that these two things seem like they ought to be related, but why do you need to assert two separate phenomena? Why not just assert one or the other?
Once you've postulated that there is a mysterious redness of red, why not just say that it interacts with your internal narrative and makes you talk about the "mysterious redness of red"?
Isn't Descartes taking the simpler approach, here? The strictly simpler approach?
Why postulate an extramaterial soul, and then postulate that the soul has no effect on the physical world, and then postulate a mysterious unknown material process that causes your internal narrative to talk about conscious experience?
Why not postulate the true stuff of consciousness which no amount of mere mechanical atoms can add up to, and then, having gone that far already, let this true stuff of consciousness have causal effects like making philosophers talk about consciousness?
I am not endorsing Descartes's view. But at least I can understand where Descartes is coming from. Consciousness seems mysterious, so you postulate a mysterious stuff of consciousness. Fine.
But now the zombie-ists postulate that this mysterious stuff doesn't do anything, so you need a whole new explanation for why you say you're conscious.
That isn't vitalism. That's something so bizarre that vitalists would spit out their coffee. "When fires burn, they release phlogiston. But phlogiston doesn't have any experimentally detectable impact on our universe, so you'll have to go looking for a separate explanation of why a fire can melt snow." What?