Alan Saunders: Hi, Alan Saunders with you for today's brain-dead edition of The Philosopher's Zone.

[film trailer audio]

Alan Saunders: Zombies have been the source of many enjoyable, and even more not so enjoyable, films over the years. Though perhaps 'enjoy', which implies inner consciousness, is not the word we want here. Hollywood has given us countless depictions of the revenge-seeking corpse with a fervent hunger for human flesh; while the Haitians will have you believe that zombies are a kind of robotic slave -- the living victim of sorcery. But what do philosophers make of the zombie, and what does it have to teach us about human consciousness?

Today we're stepping into the world of the philosophical zombie, whose physical existence is surprisingly similar to our own. Here to convince us that God could have created a world of mindless zombies is Professor David Chalmers, a philosopher at the Australian National University. David is also the director of the Centre for Consciousness at the same University. David, thanks so much for coming on to the show.

David Chalmers: Thanks Alan, it's a pleasure to be here.

Alan Saunders: Now, as I said at the beginning, there are three kinds of zombies, aren't there? Hollywood zombies, Haitian, or voodoo, zombies, and philosophical zombies.

David Chalmers: That's right. They're creatures that are a lot like us, but they're missing something vital that we have; and then you have the different zombie traditions, if you like. So there's the Hollywood movie zombie tradition, where zombies are a lot like us, except they're dead! They're missing life, they're risen from the grave, and they go around eating brains. A lot of people think the Hollywood zombie tradition is a bit of a rip-off of the Haitian or voodoo zombie traditions, which goes back for, you know, centuries in Haiti. There, the thought is that upon getting a certain kind of voodoo poison people's free will is somehow sapped away from them -- they become zombie slaves. I don't think they're supposed to actually die in this tradition. But somehow they lose all their free will and volition, and they can therefore be slaves. So these zombies are a lot like us, but they're missing free will.

Now the philosophical zombie is the more recent entrant into the zombie stakes. Here the idea of the philosophical zombie is a creature which is physically identical to a normal human being but lacking consciousness, lacking a mind. There's nothing it's like to be this kind of zombie from the inside.

Alan Saunders: In your 1996 book The Conscious Mind, you introduced the philosophical zombie as a thought experiment. Tell us about that.

David Chalmers: That's right, so... One question you can ask is whether zombies really exist. And I guess no-one thinks that the Hollywood zombies exist, and even the Haitian zombies are... I guess people think there's more of an element of folklore than anything else. But these philosophical zombies in particular, which are the ones I want to concentrate on -- nobody thinks those really exist. So the idea is, the question is, does the idea even make sense?

So here's the idea: someone physically identical to you or to me. Looks like you from the outside, even has a brain like yours on the inside. Walks like you, talks like you, but there's nothing going on on the inside. It feels like nothing at all; when you stab them, they don't feel any pain; when they open their eyes they don't get a big swirl of colours and shapes the way that we get; they don't have a stream of conscious thought. It's all blank on the inside.

The important thing here is that we're not like that. We're not these automata. We're conscious. We have rich inner, subjective experience. But to reflect on the relationship between that consciousness and our underlying physical state is just really interesting to ask, 'Well, could it be? Does it even make sense to suppose that someone could be physically just like us, but without any consciousness at all?' And here I and various other people think that, although there's not much reason to believe that there are zombies in the actual world, the idea at least makes sense, or, as philosophers say, it's logically possible.

Alan Saunders: You've said that it's conceivable that there are zombies, even though you don't think there are zombies. It's important to know exactly what you mean by 'conceivability'.

David Chalmers: That's right. So the thought is that there are some things which are just a contradiction in terms, like maybe a 'married bachelor' or two plus two being five. Those are things that we can rule out a priori, without even any empirical evidence, just by reasoning. There are other things that seem not to exist in the actual world, but still seem to make sense. Maybe an anti-gravity machine; you know, you drop a ball and it falls upwards. There's no logical contradiction in the idea of an anti-gravity machine. It seems to make total sense. It just seems that the character of our world rules that out. So we'll say an anti-gravity machine is conceivable, even though it's not possible in our world. That's the kind of status, I want to suggest, that zombies have. They're conceivable, even though they couldn't really exist in our world.

Alan Saunders: I wonder, though, I mean just to compare zombies with the anti-gravity machine... Yes, I suppose I can conceive of an anti-gravity machine; I can conceive a machine that enables me to drop something and it falls upwards. But that is actually not consistent with an awful lot of other laws about the universe that I accept. So there is a sense, isn't there, in which it is logically not conceivable?

David Chalmers: I think you're right that an anti-gravity machine would contradict certain laws of nature in our world. I'll say the same thing about zombies. I think the existence of zombies would contradict certain laws of nature in our world. It seems to be a law of nature, in our world, that when you get a brain of a certain character you get consciousness going along with it. But actually this gets to the background, the motivation for this whole zombie thought experiment, really comes, or at least arose for me, from thinking 'what is the nature of the relationship between the brain and consciousness?' Is consciousness somehow just a totally automatic, derivative consequence of the brain that you get for free, or is it something distinct, that may be connected to the brain by laws of nature, but it's not just a consequence that you get 'for free', as it were?

I mean, people think that... In the case of certain properties of human beings, like their height, for example, or maybe even someone's weight, or somebody's... maybe somebody's being alive. All that is totally fixed by your physical properties. You couldn't even imagine someone who had exactly the same physical structure as you but that didn't have a height. Or had exactly the same physical structure as you but that wasn't alive. That, somehow, doesn't make sense.

But the idea of a zombie that has the same physical structure as you but lacks consciousness -- has nothing it's like on the inside -- that seems to make sense in a different way. So I think it would indeed contradict certain laws of nature connecting the brain and consciousness, but it still suggests, at least to me, that consciousness has a special status here different from that of, say, life or height or something; and that you actually need these laws of nature, special laws of nature, to connect the brain to consciousness in the first place.

Alan Saunders: So are we saying that -- let's put it in theological terms -- that God created the human race; he created their bodies, he created all the inner bits as we know them; the internal organs, the brain, the synapses firing across the brain, and so on. Having done that, he had one further job, and that was to add consciousness?

David Chalmers: Something like that. I mean, in our world there is consciousness. But the question here, I suppose, is could God, to use the metaphor, if God had so chosen, have created a purely physical world; physically just like ours, but with no consciousness at all? That is, a zombie world. I've used that, and other people have used this idea to illustrate the idea that somehow there's more to consciousness than the physical processes in the world. For God merely to have created the physical processes in the world would have been consistent with the absence of consciousness. We do have consciousness, therefore there's something more in our world than the physical processes alone.

Alan Saunders: Well, we'll come on later perhaps to the question of whether I am conscious. But there's a common phrase, and it's a phrase that you've already used, which is that there is nothing that it is like to be a zombie. Does this mean that when a zombie is in a position in which the rest of us would, say, smell freshly baked bread, the zombie feels nothing? That it, as I think you've referred to this: it lacks qualia.

David Chalmers: That's right. I mean the key to consciousness, the way I think about it, is the way it feels from the inside. You know -- someone is conscious if there's something it feels like to be them. Or if there's something it's like to be them. I think there's probably something it's like to be a dog. If so, dogs are conscious. Maybe, who knows, is there something it's like to be an earthworm? If there is, then they're conscious; it feels like something from the inside. But for a zombie, by definition, they're not conscious, if you like, they no first person perspective on the world at all.

Alan Saunders: But it's... let's say it can drive, it stops at red lights.

David Chalmers: That's right. People right now are devising self-driving cars with sort of robot-like systems which have these capacities to do things, like to walk and to drive, and so on. I take it no one thinks those things are actually conscious, though; there's nothing it's like on the inside to be them. They're just machines. So, the zombie, I guess, is an extension of that.

Alan Saunders: Well, talking about robots, the great philosopher Descartes in the 17th century held that non-human animals could be wholly explained in terms of their physical mechanisms; he actually didn't think there was anybody at home in the case of a dog or a cat. Now today, of course, technology has allowed us to imagine what Descartes thought impossible, a machine which functions and behaves just like a human being but is not conscious. What do you think Descartes would have had to say about the philosophical zombie?

David Chalmers: It's an interesting question. Descartes basically thought that dogs and cats are zombies. You know, mechanical systems without consciousness. He thought humans are not zombies, and for this reason he thought that, the body and the mind are ... well it's connected to the fact that he thought the body and the mind were totally different things. Interesting question: would he have allowed that there could be a body just like ours without a mind at all? Descartes talked a lot about the reverse possibility: he talked about the idea that you could have a mind just like mine without a body at all. And Descartes certainly thought that was conceivable, and maybe even possible.

Zombies in a way are the reverse of that classic Cartesian thought experiment. Instead of keeping the mind without the body, we keep the body functioning just as it is without the mind. It's complicated because Descartes thought the mind played a central role in pushing around the processes in the brain and the body; he thought they communicated in two directions through the pineal gland. So you might think that with Descartes if you got rid of the mind, you'd just have a gibbering wreck of a body that couldn't do anything, but would he allow that it was at least logically possible? I think maybe he might have.

Alan Saunders: Talking about automata and machines, what about artificial intelligence? Are computers zombies?

David Chalmers: I think this is the way that most people think of the computers we have right now. I mean, they're very unsophisticated zombies right now; we don't have artificial intelligence that's got remotely the capacities of a human being; maybe we're up to earwig level, or earthworm level, or something, in artificial intelligence. And I take it even the more die-hard proponents of artificial intelligence don't claim that we yet have artificial consciousness. Nobody thinks that the chess-playing computer, or you know the one that beat the champion at Jeopardy! earlier this year... I take it that nobody thinks that Watson, that computer, is actually having feelings of triumph as he beat his human opponent.

So for now, I guess, probably it looks like these artificially intelligent creatures really are zombies. I suppose the question is, as they get more and more sophisticated, if we ever have artificially intelligent creatures which are as sophisticated in their behaviour as humans, will they still be zombies, or will there be something going on on the inside; will they be conscious. You might think that will make all the difference to how we treat these beings. Because if they're conscious, then they're basically people, and they deserve the status of people.

Alan Saunders: What does the zombie scenario make of physicalism, the thesis that everything including our biology, and our moral lives are essentially physical? Must it be wholly rejected in order to take on the zombie argument?

David Chalmers: I think it's sort of... it goes roughly the other way 'round, the way I see it. Because the idea of zombies seems to make sense, and seems to, in a certain sense, be possible, I think one can use that to argue against the thesis that everything is purely physical. Now many people, I think, agree that the idea of zombies are conceivable, including people who want to be physicalists. I actually started out myself as a physicalist, trying to explain consciousness wholly in terms of the brain. It was partly through thinking about zombies that this started to me to seem untenable. After all, any physical description of a brain seems at least to be logically consistent with the absence of consciousness. So somehow you need to put an extra ingredient in there. Now, that said, of course I've argued this at some length... physicalists have had all kinds of responses. They roughly fall into two classes: some people think the whole idea of zombies is just silly, or inconceivable; it doesn't make sense, it's contradictory, it's a contradiction in terms. Other people say 'well, sure it's conceivable, but not conceivable in any sense that makes a difference to what's actually metaphysically possible', and they try to argue that physicalism is consistent with zombies at least being conceivable. So we've had both kinds of responses from physicalists.

Alan Saunders: On ABC Radio National, you're with The Philosopher's Zone and I'm talking zombies with David Chalmers from the Australian National University. David, does the apparent possibility of zombies pose a problem for evolutionary theory? I mean we can ask, can't we, why creatures with qualia, creatures with these inner states, survived rather than their zombie counterparts? So how could consciousness possibly have an evolutionary function?

David Chalmers: The zombie hypothesis here does raise a really difficult question about the function of consciousness: what does consciousness actually do? It seems to be this most central feature of our existence, but what difference does it make to our brains and our behaviour? And people have put forward a thousand hypotheses here, but every time someone puts forward such a hypothesis... someone points out: 'look, that function you're talking about; consciousness is used for language, or consciousness is used for planning, or for decision making; in principle we could have an explanation of those things without bringing in consciousness.'

So the zombie thought experiment, I suppose, is just extending that point by saying look, it at least makes sense that we could have lacked consciousness altogether, and have still gotten by evolutionarily just as well. Now, one reaction to this is to say that consciousness is just a big, complicated by-product of all of that physical processing; maybe there are fundamental principles out there that say 'when you've got the right kind of information processing in the brain, as a matter of fact, you then get consciousness.' Maybe there are laws of nature -- to use your word from earlier -- laws of nature that connect processing in the brain to consciousness, and then consciousness would come along kind of as a bonus.

Another view would be that consciousness is actually playing a role inside the system all along; actually driving our behaviour, and evolution selected it because without it we couldn't have done such smart and sophisticated things. Then I suppose the question would be, well why didn't evolution just produce zombies; and then this, I think would have to get back to the idea that there's something about the natural structure of our world that rules out zombies, even if they're logically possible. Something about the laws of nature in our world rule out zombies. You get the structure, you get consciousness, and evolution has somehow found a way to exploit that fact.

Alan Saunders: What about the possibility of the 'zombie within', which some psychologists have been playing around with? The idea that human activities, learning and memory, or I suppose touch-typing on a computer, or even driving to work, can be done unconsciously. What do you make of the 'inner zombie'?

David Chalmers: This is really interesting, because this zombie idea started as a thought experiment from philosophers, and it wasn't really meant to be taken seriously as science, but in the last decade or so a lot of psychologists and neuroscientists have been taking seriously an idea which is at least a lot like the zombie idea, the idea that we have zombie systems; we all have zombie systems within us. Roughly, unconscious systems that control a whole lot of our behaviour. So it's become a more and more popular idea that there are these, maybe, two systems; the vast unconscious system and the slow conscious system. And we feel that we're in control and, you know, I'm picking out something to eat for dinner because that's what I'm choosing to do consciously; but it turns out more and more there's evidence that our choices and many aspects of our behaviour are actually dictated by these unconscious systems within us. So when you get up and go to the fridge and get out some food, maybe that's not so much a product of conscious reflection that hey, I want the food, but some... it's just your unconscious zombie systems taking you there, and you have this illusion that you're in charge, but it does raise some questions about just how much we're in charge.

Alan Saunders: The idea of zombies is being criticised, strangely enough, for lacking imagination. The philosopher Daniel Dennett says that the zombie argument underestimates the complexity of the human body. What's your take on that?

David Chalmers: Well, Dennett has got fairly strong commitments in the other direction here. He's certainly of the camp that thinks there's really not that much about consciousness to be explained, and we can explain it in terms of the right functional goings-on in the brain. So he thinks that: explain all the things that brains do, the walking, the talking, the deciding, the planning, and you've basically explained everything there is to explain about consciousness.

I think what the zombie thought experiment brings out is something which Dennett denies, which is there's something further to be explained: the way it feels from the inside; and to explain all that complicated functioning is not to explain the way it feels from the inside. Now around this point Dennett basically says 'Ah, but the functioning is really complicated; there's a lot going on and so on in the brain', and I think the question is what does the appeal to complexity explain here? I think it explains the things that the brain does; again, the language, the learning, the planning, the discrimination. But the key conceptual point I think is that to explain those things is not to explain consciousness: the way it feels from the inside.

Alan Saunders: You actually created a profile of your own zombie twin, didn't you?

David Chalmers: A profile?

Alan Saunders: Yes.

David Chalmers: I'm trying to recall...

Alan Saunders: Well, you did talk about Zombie Dave.

David Chalmers: Yeah... So I wrote a book, The Conscious Mind, a few years ago, where I talked about my zombie twin, who looks rather a lot like me from the outside; he's got the same long hair and dodgy accent, and thinks about philosophy a lot, but there's nothing it's like to be him on the inside. Now, he probably doesn't exist in this world, but in some other possible world, as philosophers put it. Maybe there's a possible world where my zombie twin is conducting this radio interview with you right now, having just this conversation, but there's nothing going on on the inside.

Alan Saunders: Okay. Finally, David Chalmers, I can reveal the truth now: I think that I am a zombie. Can you prove to me that I'm not?

David Chalmers: I can't prove to anybody else that you're not. I can't prove to anybody else that I'm conscious. All your behavioural evidence is consistent with my being conscious. So I think from my own point of view, the one case where I can be certain that someone is conscious is my own case. And I suspect the same is true of you. At this point we may need to go beyond the thought experiment and conduct a real experiment. Maybe we'll get a nice sharp needle out, and we'll prick you in the arm and see if you feel anything. Now if, then, you feel something, then by definition you're not a zombie, because zombies don't feel a thing.

Alan Saunders: Oh, well now I'm not sure what I feel. I've been talking to David Chalmers, the director of the Centre for Consciousness at the Australian National University. David, it has been fascinating speaking with you today.

David Chalmers: Thanks Alan, it's been terrific.

Alan Saunders: And David not only writes and thinks about zombies, but, as you'll soon hear, he sings about them as well. Get ready for a singing philosopher! This week's show has been prepared by Joel Tozer, Kyla Slaven is the producer, and Charlie McCune did the mixing. Thanks for joining me, Alan Saunders, hope you can tune in again next week; bye for now!

[song]

David Chalmers:

I act like you! I act like you!

I do what you do

But I don't know

what it's like to be you

what consciousness is

I ain't got a clue

I got the zombie blues