Derek asked why I think zombies are nomologically impossible. Basically, it's because if they weren't, then I'd have no reason to think that anybody else is conscious. Perhaps everyone I meet is a zombie. Sure, they say they're conscious: but non-conscious physical duplicates would have all the same causal properties, and so say all of the same things, in that exact same sincere-sounding tone of voice, but all without any lights on "inside". Now, the reason I'm justified in thinking that other people are conscious too, is that I know that I'm conscious, and that other people have a relevantly similar constitution to myself, so I guess they're conscious too.
The universe is such that entities composed like I am somehow end up being conscious. If the previous statement is true, then there can't be any zombies, given the natural laws that actually govern our universe. Being physically identical to me is nomologically sufficient for consciousness. That's my assumption. Without it, it becomes utterly mysterious as to why I'm conscious. What special features do I have, beyond my physical ones, that make me conscious? It sounds like we'd be getting into substance dualism, ghosts, and fairytales. So that's that.
The zombie identification issue, described in the first paragragh, raises a more general problem. To illustrate it, imagine a universe physically identical to ours, but lacking the psycho-physical laws (whatever they are) that bestow consciousness upon us. In particular, consider my zombie counterpart in that world, and call him 'Zichard'. Now, Zichard is physically identical to me, and so he interacts with the physical world in all the same ways. He goes where I go, says what I say, and types what I type. To the outside observer, we are entirely indistinguishable. But I'm conscious, and he isn't. Right now, he's typing up a blog post identical to this one. He writes about how the 'publish post' button looks red to him. But it doesn't, not really, because he isn't actually conscious, so nothing looks like anything to him. He's just a robot made of flesh and blood -- and he doesn't even realise it.
Now let's think about me. I'm physically identical to Zichard. When my fingers hit the keyboard, they have the same physical causes behind them as do Zichard's fingers in his zombie world. So consider the following statement of mine: The 'publish post' button looks red to me. Zichard typed that exact same sentence, mind. There's nothing non-physical in the zombie world, so Zichard's physical situation must have been sufficient to cause him to type the sentence. But we're physically identical beings, living in physically identical worlds. So my physical situation must also have been sufficient for my typing that statement. The fact of my consciousness doesn't seem to have entered into it at all. The fact that the button really does look red to me, doesn't seem to be any part of the causal explanation of why I typed that sentence.
That seems a real worry. Indeed, it strikes me as a powerful motivation for some form of reductionism about consciousness. Whatever those physical facts were that led me to type that sentence, my consciousness must consist in those facts, in order for it to be true that my description of the button's redness was caused by my experience of that redness. In other words, the following claims form an inconsistent triad:
(1) Consciousness is non-physical.
(2) Physical actions have physical causes.
(3) My description of the button's redness was caused by my conscious experience of that redness.
I'm inclined to reject (1). So I should say that zombies are metaphysically impossible after all. I guess dualists are going to reject (2) instead. But then, in light of my 'Zichard' scenario, to retain (3) they're also going to have to deny the following:
(2') If physical causes C lead zombie Z to perform act A in the zombie world, then the corresponding physical causes C' lead the physically identical non-zombie Z' to perform the corresponding act A' in the physically identical non-zombie world.
And how can you deny that?