A ``zombie'' is a creature whose intentional mental states are those of a typical human being, but who is not phenomenally conscious. When I set out to write this paper, my original goal was to discuss the best argument that zombies are possible and the best argument that they are not. Instead, I have discovered that the best argument for the possibility of zombies is seriously challenged, perhaps fatally so, almost from the start. This discovery is much to my dismay since I wholeheartedly believe in the possibility of zombies. At the moment, however, it appears that the opposition holds a distinct advantage in the quality of their argument.
Because the argument for the possibility of zombies is challenged so early on, I will not spend much time discussing this argument. Instead I will spend most of my time presenting the argument that zombies are impossible, since I think it is this argument that most needs to be addressed. I will end this paper by giving my opinion about how the argument for the impossibility of zombies might be countered. Unfortunately, at the moment my ideas on this front are just vague intuitions, not solid arguments.
I have chosen an argument by David Chalmers as the best argument for
the possibility of zombies and an argument by Gilbert Harman as the
best argument for their impossibility.
Actually, the
argument I will present as Harman's is not quite what Harman has
written. In my previous paper, ``The Knowledge Argument'', I pointed
out some serious flaws with Harman's argument. In this paper I will
``fix up'' Harman's argument, and discuss the new and improved
version.
Before continuing, I need to provide a bit of background. When philosophers argue about the possibility of zombies, they are not usually arguing about whether zombies are possible in our world (``nomological possibility''). Usually they are arguing about whether zombies are possible in any possible world (``metaphysical possibility''). That is, zombies are considered to be possible if there exists at least one possible world in which there is at least one zombie.
Often when philosophers talk about zombies, they talk about a zombie
``duplicate'' of a person. A zombie duplicate of me would be a
duplicate of me that is not phenomenally conscious. There are several
different kinds of ``duplication'' that are relevant. A physical
duplicate of me would be an exact copy of me down to the last
subatomic particle. Alternatively, a ``functional duplicate'' would
duplicate my functional states, yet might be completely unlike me
physically. An example of a functional duplicate of me might be a
carefully programmed robot with a brain made out of silicon chips.
Another example might be a fancy contraption made out of tiny tinker
toys and rubber bands.
A problem with the notion of functional duplicates is that there are many ways we can carve up a complex system functionally. If we choose a very fine grain of causal structure, we might insist that a functional duplicate of me must duplicate the richness of my causal structure all the way down to subatomic particles. Alternatively, if we chose an extremely coarse grain, I might be duplicated by a toggle switch. The causal structure we are usually interested in when we talk about a functional duplicate is the finest level of causal structure that must be preserved when we duplicate a person to not lose something important about his or her essential nature. Precisely where this level should fall is not clear.
Another important kind of duplicate is an ``intentional duplicate''. An intentional duplicate would duplicate my intentional states, which are states that have ``meaning''. There is no consensus among philosophers about exactly how intentional states come to be, but one popular theory is that a functional duplicate of the right sort would necessarily be an intentional duplicate. This position is known as ``internalism'' and is in contrast to ``externalism'', which maintains that a functional duplicate---even one of the right sort---is not necessarily an intentional duplicate because, according to the externalist, intentional state is not determined by functional state alone. Rather, it is determined by a combination of a person's functional state and the state of their external environment. That is to say, a person's intentional state is not determined ``locally'', it is determined ``globally''.
I am going to assume without arguing for it here that internalists are correct in saying that intentional state is determined locally. That is, I believe and will assume that a functional duplicate of the right sort would necessarily be an intentional duplicate. From now on, when I use the term ``functional duplicate'' I will use it to mean a functional duplicate of the right sort as to also make an intentional duplicate.
Before continuing, I should probably point out that Chalmers does not argue specifically for the possibility of intentional zombie duplicates. He argues instead for the possibility of physical zombie duplicates. If we assume that internalism is true and that there is no non-physical ectoplasm affecting intentional state, then a physical duplicate must also be an intentional duplicate. Therefore, if Chalmers' argument is sound, he has also shown that intentional zombie duplicates are possible.