I don’t know who first introduced them into philosophy, but zombies are frequently quoted in discussions of consciousness. Perhaps the obsession with brains which Hollywood zombies share with cognitive scientists has something to do with it.
Philosophical zombies are indistinguishable from normal people: their appearance and behaviour is perfectly normal, but they have no inner life: no phenomenal experience, no qualia, just colourless information processing. There isn’t, to use Nagel’s phrase, ‘something it is like’ to be a zombie. They are, to use a word which is no longer so unambiguous as it once was, robotic. Philosophical zombies are therefore, in fact, quite different from Haitian zombies or the Hollywood variety.
Philosophical zombies are used in various ways, but the main point about them is that they provide one of the chief arguments for the existence of qualia. The key question is: could zombies exist? According to one school of thought, it is intuitively obvious that they could, and that fact establishes that qualia are an important part of the riddle of consciousness – perhaps the most important part. The other main view is that zombies could not exist. You might take this view if you believed zombies would necessarily have to have qualia if they were to behave in exactly the way we do; or you might think that there are just no such things as qualia, anyway – so that in the relevant sense we are all effectively zombies anyway.
Zombies are often conceived of as being identical to their qualia-having equivalents, on an atom-by atom basis. Dennett ‘s zombies, by contrast, only resemble normal people externally (Dennett finds the whole zombie idea ridiculous, so the atom-by-atom version is probably just too extreme for him, even as a debating position). Inside, any kind of jiggery-pokery could be going on, so long as it does the (presumably computational) job required. He goes on, however, to propose a super-zombie or ‘zimbo’, which besides its routine processing of inputs and outputs, is capable of addressing and ‘considering’ its own internal states. He asks the interesting question, what would such a zimbo ‘think’ about its own experiences?
It would presumably think it had qualia. In fact, if all of the external behaviour of a zombie (or zimbo) exactly matches that of a normal human being, then a zombie would somehow have to talk as if it had qualia even though it didn’t. It might well write a phenomenological dissertation on the subject, which would be indistinguishable from that of a real human writer. This is one of the main problems for a proponent of zombies. If they really are indistinguishable from normal human beings apart from qualia, that seems to make qualia a kind of ghostly irrelevance of a kind we’d be better off without.
It can’t be denied, though, that the idea still seems plausible. Suppose we were cataloguing the badly-organised stock of a sweet-shop. While we note down the figures, our colleague calls out the quantities of red or black licorice found on various shelves. Even if we visualize red licorice the first time, after writing down the twenty or thirty figures, we are surely not having any red qualia in association with the red licorice figures: but we are certainly acquiring information about the redness of various items. It seems perfectly reasonable to think that a zombie could function quite well with this kind of information, without ever having the kind we get when we actually see the licorice.
A second difficulty is over the issue of possibility. It seems unlikely on the face of it that zombies are possible in practice (Though how would we know? Everyone but you and I could be zombies – and I’m not completely sure about you), but it’s generally felt that that isn’t really necessary. They only have to be possible in theory – but what exactly does that mean?
Chalmers surely sets the bar too low when he claims that the mere intelligibility of the notion of zombies is enough. The idea of light without electromagnetic radiation is intelligible, but it does not establish that electromagnetic radiation is something over and above ordinary light. A more reasonable demand is that they should be logically possible – that is, they don’t have to be compatible with the laws of physics, but they must not involve contradictory suppositions. On these terms, zombies seem acceptable, but it could be argued that that only establishes that qualia exist, or could exist, in some other world with different laws of physics, whereas we are really concerned with the world we actually live in.
Perhaps, then zombies have to be possible in this world – and why not? Well, it is a basic assumption we generally make that under the same conditions, the same events occur. This is a hard assumption to give up, because if different events could occur in identical circumstances, the world would become much harder to understand and predict, perhaps even entirely incoherent. But if philosophical zombies existed, we would have two identical physical sets of circumstances (one with me, one with my zombie twin) in which wholly different qualic events occurred.
In the final analysis, I think even the most committed zombists would accept that the argument is an appeal to our intuitions rather than a knock-down logical one. But the continuing interest in the issue of qualia shows how strong those intuitions are.