I'm not convinced by the fact that showing 'possibility' of zombies is enough to eliminate physicalism. The argument they use is too slippery for me to grasp, but it basically sounds to me something like this:

Skeptics think that the laws of physics are sufficient to describe the universe. It suffices to show that magic is possible, and then skepticism is false. This is why opponents of skepticism do not have to point to actual cases of magic being performed; it is enough if such things are possible.

Second, along the lines of conceivable -> possible, here's an interesting thought experiment.

I can conceive of an immovable object. An object such that when it is inserted into our universe, it can never be moved with respect to some frame of reference. Such an object is much easier to conceive of than a zombie.

I can conceive of an irresistable object. This object is such that it moves with constant velocity relative to some frame of reference, and its velocity can never be changed.

I find it hard to accept that the possibility of such concepts can be used to show anything useful about the universe.

Further on, I found that the best counterexample included was that of Daniel Dennett, who compares the idea of consciousness against the idea of health - to quote the quotation:

Supposing that by an act of stipulative imagination you can remove consciousness while leaving all cognitive systems intact — a quite standard but entirely bogus feat of imagination — is like supposing that by an act of stipulative imagination, you can remove health while leaving all bodily functions and powers intact. … Health isn’t that sort of thing, and neither is consciousness (1995, 325). <end quotation>

Finally, it's nice that they went out and mentioned all the issues with semantics around the concepts of 'conceivable' and 'possible', because that was the part that feels like it wants to give me a migraine.

Last thought: One of the ideas was that whatever it is that distinguishes us from zombies does not exist in a physical sense. But we have devices that can read our thoughts - or at least show that our physical state is changing while we think.

My MRI shows brain activity, therefore I am.

I think this argument is rubbish. Claim 2 seems like obvious bullshit, unless "identical" is given some weird technical meaning that makes claim 1 bullshit instead. Further, with the sense of "possible" that's relevant here it's very far from clear that anyone's shown that zombies are possible. (The SEP page treats this point quite well.)

I have used the following parallels before: I can imagine electric current flowing without any charged particles being present, "therefore" electric current is not the same thing as the movement of charged particles; I can imagine my computer continuing to do what it does without its circuitry and what happens therein, "therefore" the processing my computer does is not a mere activity of its circuitry. What twaddle. (See http://www.mccaughan.org.uk/g/log/2007/inconceivable.html for a longer rant on the subject, complete with a couple of quotations to show that some philosophers really do think such silly things in the case of consciousness.)

If A == B, then B just is A, we're saying A == A. A possible world where B != A is a world where A != A. You seem to believe that A != B even in our world.

(1) is the essence of the physicalist claim. You can certainly declare it bullshit if you want...

The leap in in (3). While brains and mental states as we know them must be identical, according to physicalists, other worlds may do brain-like and mental-state-like things not as we know them.

For instance, we may have misunderstood our own world.

Oddly, (4) gets back on track. If zombies can be proven to be possible (which conceivability does not do) then physicalism is false. (Although, this generally implies epiphenomenon, which also implies causality is false, which puts consciousness outside of logic {P -> Q} let alone physics.)

I only think 1 is bullshit if you use something like the "synonymous" definition. (Can we agree that it's crazy to say that physicalism requires believing that "consciousness" just means some particular set of brain processes?)

I think your comment about 3 basically amounts to agreeing with me that physicalism doesn't say that mental states and brain states are "identical" in any sense that makes the rest of the zombie argument work.

I agree, of course, that if 1-3 are correct then "zombies are possible" implies "physicalism are wrong".

Epiphenomenalism doesn't imply that causality is false. And if consciousness is "outside of logic" then I say: so much the worse for the notion of consciousness. (But I don't see any reason why we should think consciousness is outside of logic, whether zombies are possible or not.)

It's a good accessible read, mentions a few of the interesting questions zombies can shed light on, and has a fair number of reader comments and responses.

http://wiki.lesswrong.com/wiki/Sequences

This is the strongest argument that mind is matter: mental things like qualia clearly do cause physical events (e.g. discussions about qualia), which seems to suggest that qualia etc are themselves physical, since we expect physical events to have physical causes.

(And yet I can't stop wondering how being a collection of atoms feels like anything.)

To me, there just seems to be an undecidable knot there that can never go away, even if you say it doesn't exist like Dennett and others.

Another thing that never gets said is that questions of consciouness are really questions about solipsism in disguise. The real question isn't whether what it's like to be my neighbour is different than what it's like to be someone who died 100 years ago. The only real "obvservable" difference is that it's like something else to be me.

It doesn't, but someone has to have them in order for the concepts to get introduced in the first place.

Imagine a world that has always been populated entirely by p-zombies. By definition, that world should be indistinguishable from this world, so how on earth did they get started talking about consciousness and qualia?

In your example of the "ouch" printing program, someone had to have pain and say "ouch" before such a program was made.

The answer to the question "how did they get started talking about qualia" is: by definition, the hypothetical Z-world is indistinguishable from ours. In our world, people talk about qualia. Therefore, in the Z-world, zombies talk about qualia. What's more, the beginning of "zombies talking about qualia" would be the same as the beginning of "people talking about qualia".

I have a feeling I'm misunderstanding you.

In your example of the "ouch" printing program, someone had to have pain and say "ouch" before such a program was made.

Why? What if the program is the output of a random number generator? Does that mean the random number generator experiences pain?

Right. Which implies that the reason that we talk about qualia is not that we have them (otherwise zombies wouldn't), but for some completely different reason that just happens to exactly line up with the qualia that we in fact do have. That would be an astounding coincidence and is a major strike against the theory per Occam's Razor.

Huh? People talk about dragons and other things that don't exist. With qualia et al, the only difference is that folks are claiming to have it.

Maybe telekinesis or other psychic powers are a better example.

Y's are exactly like human in every physical respect but they don't have Z. Therefore Z is not physical and since we're vacuously assuming Z, human must have something besides physical qualities.

I'm sure you could use something like the Flying Spaghetti Monster in the discussion (http://en.wikipedia.org/wiki/Flying_Spaghetti_Monster).

I had (what I feel is) a similar question when I first encountered people writing about Lisp symbols.

That made people very uncomfortable about a lot of things, and really demonstrated the group feeling that consciousness has to be some other substance. A lot of great conversations about verifiability, positivism, philosophy of mind, and naturalism/materialism were started with Zombies.

The arguments always assume that we have qualia as a prerequisite and that a zombie doesn't. However, the very people who believe in qualia will themselves say that animals don't possess qualia as they're not conscious. Yet we as conscious thinking humans do. So how did we acquire this from a logical standpoint? Because surely a biologically identical zombie, built atom for atom the same way as a human would itself possess qualia. If the zombie doesn't posses qualia, how did we acquire it? I'm genetically related to a rat and at one point in history we were the same species, so how did I acquire qualia and it not?

It eventually just strikes me that it's an asinine argument, and only unthinking zombies would waste their time with it, thus we have proof of p-zombies in the form of the qualophiles and qualophobes, because no one else gave a big enough shit for long enough to think past an initial idea and the two camps have been left arguing as futilely as if they were arguing over the existence of god.