Via Leiter’s blog I happened to be looking at the list of recent SEP entries. I read this entry on impossible worlds that got me thinking.

The response to the zombie argument that I have been developing over the last couple of year appeals to the distinction between prima facie and ideal conceivability. Something is prima facie conceivable, roughly, if there is no obvious contradiction in the imagined scenario. Something is ideally conceivable if, roughly, there is no contradiction in the imagined scenario even upon ideal reflection. I have tried to argue that zombies are merely prima facie conceivable and may not turn out to be ideally conceivable (another way of putting it that is roughly equivalent is that zombies are epistemically possible but not metaphysically possible) since there are equally plausible parity arguments (zoombies and shombies). As a corollary of this line of defense I have argued that what people like Dave actually succeed in imagining when they *think* they imagine the zombie world is really just a world that is very similar to the actual world. Just as a point of clarification I have always meant this to be a different claim than the Russellian response that the zombie world may have different ‘inscrutable’ fundamental physical properties. What I mean is that since we do not yet know all of the facts about the brain, physics, or theories of consciousness, we may be inadvertently failing to include some crucial physical law, property, or theory of consciousness. So it is very easy, I claim, to imagine a world that is physical in roughly the same sense that ours is but where there is no consciousness. For instance, if the higher-order thought theory of consciousness is right then the ‘zombie’ world is really just a world like ours that lacks higher-order thoughts.

Now people like Dave often claim that they can conceive that we add this feature and yet still it is intuitive that those creature could lack consciousness. If this is really the case and the higher-order theory is true then Dave has imagined an impossible world. But it seems to me that we can at this point admit that the traditional zombie world is conceivable and go on to argue for a restriction on the second premise of zombie argument, which to remind us, is the claim that if zombie are conceivable then they are possible. This premise becomes possibly false since it may be the case that zombies are conceivable but not metaphysically possible, where this means that they inhabit an impossible world.

One response to this line of thought might be that the use of ‘conceivability’ here isn’t the same as that employed by the zombie argument. As used by Dave ‘conceivable’ means roughly imaginable without contradiction but in these impossible worlds we conceive of a world with a contradiction (by stipulation it contains a contradiction). But, of course, the point here is that one may not notice or be in a position to spot the contradiction, which is exactly one of the reasons for postulating impossible worlds (or in this case impossible scenarios in Dave’s sense). If one takes this line, as I am inclined to do myself, then the issue reduces to the original one of the difference between prima facie conceivability and ideal conceivability. But if one has a more generic version of conceivability one can argue that zombies are conceivable and impossible in way that seems different from the usual type-b line…