Dear Derek,

The Philpapers organisers' aim is for a professional philosophical forum with emphasis on discussion of current literature. My transitional status aside I hope we are in line with that and I take your basic point - that the 'P without Q' zombie argument against physicalism has problems with premises - as an important philosophical issue worth pursuing. (It would be good to get input from David Chalmers himself.) Nevertheless, I am worried that several arguments in above posts go off at a tangent from David's reasoning and slide back into folk psychological issues irrelevant to the zombie posit. My understanding is that the point of both philosophy and science is to identify inconsistencies, contradictions, or other inadequacies in folk accounts of the world and replace them with workable reasoned alternatives. I sometimes worry that current professional philosophy seems the opposite - a last bastion of intuition jealously guarded against reason! But let me get to the point.

The zombie posit is of an entity based on exactly the same instantiated laws of physics as a human, that behaves the way that we do in all respects but has no conscious experience. By definition we are considering something that will never fail a Turing test. My point about building robots that can pass Turing tests is not a point about building zombies but one of several arguments against the folk idea that human behaviour needs some magic ingredient called 'intention' that requires consciousness in some way. I will return to that but it is probably best first to address your very first query about what it means to have no consciousness.

David uses consciousness to mean that X is conscious if 'it is like something to be X' in Nagel's sense. I agree that although most people see what is intended, this phrase is pretty oblique. One could say that it is no help unless we can say what it is like. 'Dark inside' is also misconstruable. Both phrases are awkward metaphors. But this may be unavoidable because natural language has not yet moved on from a folk description format to a rigorous ontological one. We do not have the language structures we need. However, I think there may be an alternative to the Nagel phrase that is more robust:

X is conscious if (at least a limited internal part of) the world is like something to X.

This implies that the world in some way provides data for X and that these data have distinguishing qualities of appearance to X. X is acquainted with data in an overt way. Seeing red and blue is all we need to posit. For me, this meaning of consciousness is fully adequate to start out on the zombie argument. Moreover, most people get the idea. If you do not you might be a zombie. You may be a philosopher with your own precepts about consciousness in to which David Chalmers's conception does not fit (very likely). You might also be conscious but you may not be able to access the role your consciousness plays in the way you construct arguments. This is an interesting possibility partly because it is the opposite of your intuition and partly because it is probably true for all of us in one sense. One of the most difficult problems about consciousness in David's sense is how we can possibly talk about our acquaintance with data rather than just the data themselves. My own view would take too long to explain here but I conclude that although there is a mechanism we can never take the accounts of others about their consciousness as evidence of their consciousness. The same behaviour can always be simulated without. (And I think there is a real possibility that people differ in their access to the link between experience and behaviour but again the detail is too long.)

This comes back to your point that we cannot indeed ever know that a zombie is a zombie. But that is not the point of the zombie posit, which is a conjecture about ontology that ventures beyond positivist constraints. In passing, I think the issue about how long a robot (not a zombie) could keep passing the Turing test is a red herring. Suspension of disbelief in cartoon films indicates that we never make a final rational decision on whether or not something is conscious. We might have doubts about the robot but we would constantly ask 'is it really conscious after all?'. The reason is that our decision about what is conscious can never be rational anyway because we have no conception of how 'overt acquaintance with data' in the way we are familiar with, as opposed to just 'physical signal passing' could constrain behaviour (and as I say it probably does not). Moreover, passing Turing tests at a complex level relating to jokes and unrequited love etc. does not need to be invoked here. All we need talk about is the consciousness of the black on white of print. Computers decipher print but do not have our 'awareness of black'.

Having established that David's definition of a zombie is clear, my own problems relate to the meanings of 'conceivable' and 'physical'. Conceivability is a dangerous plaything. We are questioning our most cherished beliefs about ourselves so we need to question the reliability of our conceivings and there are plenty of reasons to be sceptical. The zombie is conceivable in the very weak and loose sense that if we do not know the nature of something any possible nature is conceivable, (we do not know the final rules that link known physical laws and experience within anybody). But my earlier post made the point that if we ask what we really mean by physical laws we find a clear contradiction because we have posited, by default, that the links are the same as in our conscious selves. (I do not think that inconceivability on the basis of an intuitive sense of a need for 'intentions' comes in to it.)

The real problem as I see it is the idea that physics is 'closed' or 'complete'. Dynamical physical laws form a closed chain of intermediary links within any causal description but are so far always incomplete when it comes to the final cashing out in experience - as Newton pointed out. There are extra laws of correspondence between dynamics and experience implicit in physics that we have yet to get a grip on. The solution to Jackson's Mary story is obvious. There is more to know about what determines experiences. Maybe I actually agree with David Chalmers in that physicalism is false but the physicalism falsified is a partial materialist physicalism that within physics itself is more or less a straw man. We know we need laws beyond current physical laws and maybe we could call these some other sort of law. However, since 'physical law' only means a law instances of operation of which determines experience, I am not sure why we should want to make the distinction. I suspect that people usually want to make the distinction because of a residual folk sense of 'physical' as 'stuff'.

Best wishes

Jo E