I The Zombies Advance

We're back! Actually, we've never been away, so what I really mean is that my fellow zombies and I are, I hope, about to announce to the world that we're here! It's true I haven't yet asked anyone else to agree to expose themselves as I am now doing, but I'm hoping that my own public outing will encourage, or perhaps even shame others to come forward and join me.

Admittedly there's a bit of a problem about all this, as I've no idea how many other zombies there are. For the sort of zombie I am, a ‘zombie’ in the philosophers' use of the term, is a human being who doesn't differ in any physical ways whatsoever from the usual run-of the-mill, admittedly pretty varied sort of human, and who furthermore behaves in all the normal – again pretty varied – sorts of ways, but who is, as it is sometimes put, ‘dark inside’, meaning that we have absolutely no mental occurrences such as visual and auditory sensations, aches, pains, itches, feelings of love and hate, anger and pity, etc. So we aren't the common or garden type of zombie, to be found in Haiti and elsewhere, who no doubt has brain states altered by drugs or spells and who therefore behaves in altered, unusual, ‘zombie-ish’ ways, sometimes making such humans quasi-mechanical slaves under the control of zombie-masters.Footnote ¹ Philosophers' ‘zombies’ are different sorts of creatures; third parties cannot in any way distinguish the philosophers' type of zombie from any other human being who isn't a zombie. Perhaps the zombie informs them, as I am now publicly doing, but on what possible grounds could such a confession or deical be believed? Only a zombie knows ‘for sure’ that he's a zombie. So embarrassingly enough, when I wrote that we're back, it may turn out that I'm using the royal ‘we’: that is, for all I know, there may be only one of us, at least here on earth. As I shall later explain, that's only one of the reasons it's embarrassing. I may be in a minority of one, or there may be just a few of us, or we may even be the majority. Or perhaps everybody is a (philosophers' type of) zombie, but they're too embarrassed to say so. How can I tell, unless I ask? So please, please, please, if you are such a zombie do let me know.

From now on, where I use the term ‘zombie’, I shall mean a philosophers' zombie. As it has been so difficult for me to admit that I totally lack what just about everyone else seems so confidently to claim that they have, I'll let you know what led me to do so. One morning in April when I woke up I noticed that the light seemed completely different from normal, and looking outside I was completely surprised to see snow gently falling onto the London gardens, already softly carpeted with a thin white powdery layer. It made me feel sad, and yet somehow quietly content. What was it doing at this time of the year, with all that worry about global warming? It somehow made me think of times long gone, and looking so peaceful, it presumably must have evoked in me those mixed emotions of surprise, sadness and contentedness. It felt good.

I'm sure these conflicting emotions must somehow have led me to feel that I should confess what I've kept secret for so long. I can understand why people might take it amiss; but then I think ‘Why should I keep it secret? All these years of doing so must have taken their toll. And nobody would ever have known, if I hadn't “confessed” at some time or other. How could they have?’

Now I can imagine you asking: ‘How on earth can you say that you feel and experience nothing, when just now you wrote about what the snow looked like, and just how it made you feel?’ Well, that's just the problem. Almost every non-philosopher, and probably most (but certainly not all) philosophers, confidently affirm that there's more to seeing, smelling, hearing, feeling sick or elated, emotionally overcome, in love, upset, having a headache etc. than merely behaving in the variety of ways that different people do behave in those situations, together with having the relevant goings-on in their brains and other parts of their nervous systems, internal organs, joints etc. Philosophers speak of ‘raw feels’, and ‘what it is like to experience things “from the inside”’, and the experiencing subject's ‘qualia’, and other academic peculiarities, but in any event, such philosophers are completely confident that there is more to experiences than mere bodily ‘matter in motion’ with physical causal input and output, and many even believe that this ‘more’, this ‘extra’, is all there is that makes life worthwhile, or hell on earth, or something in between. So you'll appreciate why it's embarrassing to admit that I don't have any of this extra ‘stuff’, and even haven't too much clue what they're talking about. But I do believe them, or anyway half-believe them. For why would the majority of the human species go around lying in this way?

Well, one can sort of see why they might. We probably all know someone or other who is tone deaf, others who actively dislike or are bored by a whole range of music that we find profoundly moving, or exciting, others who apparently appreciate what sounds to us merely a hideous din, and I even know someone who, incredibly as I first thought, tells me that he doesn't like any music at all. None. If he hears some on the radio, whatever it may be, he simply turns it off. So we can quite easily understand people having a certain sort of experience and pretending they don't, or not having it and pretending they do. So perhaps a good many of those people – and could it possibly be all? – don't have any experiences at all, and just pretend they do, out of sheer embarrassment. It must be incredibly difficult to keep this up. But perhaps they do. Or perhaps, like me, they have never had to pretend, because nobody has ever asked me. Think about it: has anyone ever asked you?

In any event, I'm not pretending to have experiences, these so-called ‘extras’, that in fact I don't have. I just find myself talking and reacting in all ways as though I'm having them, but I'm not. So when I said that I find certain music moving, or hideous, I just meant that I find myself reacting in various complicated sorts of ways to those sorts of music. I'm not even sure what kind of thing it is that (just about) everyone else claims they have, but which I'm confident I lack. If you've never seen, or had described to you, or seen a picture of, say, a pterodactyl, then how can you know what you're allegedly missing? But in fact I have a very good reason to think that I'm missing something. You see not only am I a zombie, but I'm an alien. I come from the planet Zog, and good King Zack of Zog had me sent to Earth when I was a baby to replace someone of the same name, physical and genetic make-up, and the same parents, and who did have experiences. As King Zack has since explained to me, by telecommunication, on Zog the laws of nature are different from earthly laws. We Zog-humans have the same genetic and other physical make-up as Earthly humans and behave in just the same sorts of way as Earth-humans, but we don't have experiences: our physical make-up suffices for all that we do. But when Earth-humans have the equivalent physical goings-on these are all (inevitably, on Earth) correlated with various experiences, and these experiences are required for many of their responses. They have so-called autonomic responses, where the nervous impulses suffice for their responses, but also non-autonomous, where their corresponding experiences kick in with causal input.

Earthly philosophers apparently find it difficult, if not impossible, to explain how experiences, if they are not physical things, can have this causal input. On Zog this problem never occurred to anyone, because we don't have experiences.

It seems as though it was pretty risky of King Zack to send me to Earth – well, not risky for him of course – because there would be no guarantee that Zogians would go on functioning satisfactorily on Earth, where there are different laws of nature from those on Zog. But Zack is a kind man – first he sent several animals that Earthlings usually claim have experiences, such as cats and dogs, but which lack them on Zog, and they all seemed to function perfectly. Later he sent some Zog-humans: he has told me I'm not the first one, but I'm not at all sure where the others might be, for the reason I have explained. So I hope he won't be unhappy that I've decided to come out. (Those of us lacking experiences have as much free will as the next man.) But in truth I haven't asked him, or anyone else. First of all I'm worried that my girlfriend will be terribly upset if she reads this article. I can just see what will happen if she does. I'll need to point out to her that it hasn't, doesn't, and I trust won't make a blind bit of difference to our relationship; things can carry on just as before. But I can see that they won't. ‘You absolute sod’ she'll say (or actually something quite a bit worse). ‘You never told me. All these years you've been deceiving me: you've felt nothing. You've been pretending that you have had all sorts of feelings for me when you haven't. How could you possibly have done this to me?’ It won't be any good my protesting that I haven't been pretending: I have simply been behaving like more or less every other sort of man.

There'll be floods and floods of tears. I've been abusing her, unbelievably abusing her. It won't be any good my saying that if she'd ever asked me I'd have admitted – well, I hope I would – that I never experience anything at all. I've sighed, groaned, sent flowers, protested my love, as adamantly as the next man. But I know it, I know it: it will be of no avail. So you see that's a further reason why I have been so reluctant to come out. She really is a wonderful woman. So I wouldn't have made these sacrifices of both her and me if I hadn't thought it for the greater good of humanity. I need to do it because some philosophers have claimed that (philosophers') zombies are impossible.

Of course I wouldn't have cared at all about this if the ‘impossibility’ claims had remained in the obscurity of seldom read academic journals. But now they have reached the wider public by being published in Think. If these claims are right, then on one interpretation of them we have a solution to the philosophical problem of explaining how the alleged ‘extras’ – private subjective experiences, which Earth-humans allegedly have and zombies lack – how these alleged mental ‘extras’ can have a causal input into purely physical aspects of humans. For if zombies are impossible then there can be no such things as entities physically, behaviourally and structurally identical with humans except that they lack experiences. But we should note that this could be for either of two reasons. The first would be that anything physically identical with a zombie would inevitably have the mental extras: it is impossible to have the one without the other. This would still leave open the question how the extras, if not physical entities, can have a causal input into humans and other animals. The second reason would be the subtler and extremely surprising idea that to be physically identical with a zombie just is the same thing as to have experiences: there is no further ‘extra’ to have. The claim would be that there is nothing more to experiences than – to put it roughly – what goes on physically in humans; so the answer to the problem of explaining how the extras can have a causal input is that there simply are and can be no extras to require any sort of explaining. This is a typically reductionist theory, reductionists being philosophers who claim that seemingly awkward entities of some type A are actually no more than non-awkward entities of another type, B. On this account, zombies are impossible not because anything (physically) identical with a zombie would inevitably have something extra, but because anything (physically) identical with a zombie just is something with experiences: to be the one is the same thing as to be the ‘other’, so there can be no such thing as being the one without the alleged ‘other’. It would be like suggesting that something might be a vixen without ‘also’ being a female fox. There is no ‘also’ to it. To be the one just is to be the ‘other’; or, one might say, there is no ‘other’ to be the same as the one. So, very subtly, if to be a zombie is to be a human being with suitable behaviour and internal physical goings-on but without experiences, then such a creature is impossible, because to have suitable behaviour and physical internal goings-on just is the same thing as to have experiences.

This view about humans and other animals is very surprising and counter-intuitive, which is why it requires extremely long and complicated books to be written about it explaining why the common sense view is both primitive and a terrible obstacle to the advance of science. One version claims that experiences are the same things as brain processes. To the objection that this seems obviously ridiculous, because experiences seem clearly to be very different sorts of things from brain processes, or because I can be fully aware of my experiences without having any clue about my brain processes, or that I can look at your brain processes but cannot look at your experiences, the answer usually given is that there are many situations where one thinks one is looking at or thinking about two different things, but it turns out that they are one and the same: one can simply be ignorant about the matter. But I think there is a simple knock-down argument against this particular version of mind-matter reductionist theory, one that I haven't seen published anywhere. Consider someone hearing a simple, unchanging musical note, or seeing an undifferentiated and unchanging colour on a wall. For these unchanging experiences to continue through time surely requires processes to be changing in the brain. But one and the same thing cannot be both changing and unchanging. So experiences cannot be the same thing as brain processes. Thus so far as I can see at least this particular ‘identity’ theory must be false. This fact leaves untouched my personal ability, as a zombie, to have these things going on in my brain and this sufficing for everything I do: it's just that I don't have experiences of unchanging musical notes or undifferentiated colours, or indeed of anything else. Other people apparently do have, and I'm beginning to feel quite jealous. Although I don't know: their experiences of pain are apparently quite horrible.

Clever, up-to-the minute philosophers often make derogatory remarks about the common sense view of the nature of the alleged mental ‘extras’; Beenfeldt refers to them in this journal as popularly believed to be ‘metaphysically supernumerary, causally inefficacious, mystical ingredients … forever undetectable by science’.Footnote ² And he claims that the famous philosopher Daniel Dennett has actually shown that zombies are impossible, zombies being entities with no qualitative awareness of any kind; so presumably he thinks that humans do have qualitative awareness but they don't have these mental ‘extras’ – perhaps because being in the relevant physical states is all there is to qualitative awareness.

Robert Kirk has also claimed in this journal that zombies are impossible.Footnote ³ Now it so happens that one summer when we were young lecturers Robert and I both attended a philosophical conference at which we shared a bedroom. (Don't wait with baited breath for amazing revelations, or anyway not ones of interest to the gutter press). A charming chap, Robert, but informing someone that you're a zombie isn't the sort of thing one simply blurts out to someone one has only just met, even if one is sharing a bedroom, so I certainly didn't tell him. Many years later I find he is arguing that zombies are impossible. This makes me suspicious. Could it be that actually he too is a zombie and is trying to pretend that he isn't one on the spurious grounds that they are impossible, whereas if that evening I had confessed, he might also have done so? An opportunity missed, like so many in life? And now Robert is trying to keep it even more secret by arguing that zombies are impossible? So his arguments, and those by other ‘impossibilists’, have now, perhaps unfairly, led me to me suspect that perhaps Dennett, Kirk and the others are zombies themselves, like me. Could they really be pretending that they believe such things are impossible, because they are too embarrassed to admit they are zombies themselves? Could they be ‘self-hating zombies’?

If they are, then I now ask them to come out and publicly ‘admit’ it. So come along now Bob: please join me. Nobody is going to criticize anyone for a public recantation. But just in case he and others are unwilling to do so, in the second part of this article I shall criticize one of the major arguments he uses for the ‘impossibility conclusion’, and very briefly indicate how I think the causal problem can be solved for the common sense view of the nature of experiences as subjective entities over and above mere physical goings-on.

II The Zombies Retreat

OK, OK, I admit it. I was joking. I am not a zombie.

Of course I didn't exist on the planet Zog, where laws of nature are different, as I pretended in Part I of this article. But it is only in these sorts of circumstances, I think, that (philosophers') zombies would be possible, a philosophers' zombie being unlike an ordinary earthly zombie in that it would be indistinguishable from an earthly human in all physical and behavioural ways, but would lack experiences. I also cheated shamelessly when, in order to persuade you that I might be a zombie and you wouldn't know unless I told you, I pretended that such a zombie might go on functioning on Earth. It (contingently) couldn't, any more than we could function if we went to a planet where gravity affected only inorganic elements, unless we had some amazing machine which overcame this ‘defect’. So a zombie sent to Earth, even by King Zack, would need some sort of physical ‘driver’ to overcome the changed laws of nature, and thus wouldn't be physically indistinguishable from us, people whom I would argue have on the contrary ‘mental drivers’. But zombies would be possible in other worlds.

We need to distinguish two different sorts of possibility, and of impossibility. It is physically impossible, or in fact impossible, for humans to jump unaided higher than 30 feet on earth. Conditions on earth, and the laws of nature, make it in this way impossible. We might therefore, rather unnaturally, call this a ‘natural’ impossibility, to cover the suggestion that perhaps there is more to nature than just the physical – perhaps the mental, or the abstract. We can on the other hand easily imagine a human making an unaided jump of more than 30 feet on earth; it could happen if the gravitational pull on earth were much weaker. There are conceivable circumstances in which this might occur. Another way of describing this is to say that there are possible worlds in which humans can jump more than 30 feet on earth. We owe the idea of possible worlds to the great philosopher Leibniz, so in his honour we can say that it is ‘L-possible’ to jump higher than 30 feet … etc., but not physically (naturally?) possible. I consider it perfectly satisfactory to read ‘L-possible’ as ‘logically possible’, but I should warn that many modern philosophers object to this (for what I consider spurious reasons).

My claim has been that it is in fact impossible to create zombies on earth, but that this is a ‘natural’ impossibility, i.e. the laws of nature prevent us from so doing, whereas it isn't an L-impossibility: there are possible worlds, with different laws of nature, where zombies might be created, and I have been pretending that our universe actually contains at least one such ‘possible world’, namely on the planet Zog. Perhaps it does and perhaps it doesn't. (Of course we anyway live in a possible world: if it's actual, then it's possible. But most possible worlds aren't actual, or real – they remain merely possible, or imaginary.) My claim is that on earth, humans and other creatures with reasonably complex central nervous systems have so evolved that correlated with various occurrences in their nervous systems they have experiences which are something different from, or ‘extra’ to, the physical occurrences. This anti-reductionist account needs to explain how such experiences, together with the physical occurrences, jointly cause various outputs of internal physical changes, outward behaviour, and further experiences. In circumstances with different laws of nature – in other possible worlds, as we say – the same nervous systems would have evolved without the concomitant experiences. Either such creatures would be able to react in the same ways as do the corresponding earthly creatures, or they wouldn't. My claim is that in some possible worlds, although lacking experiences, they would be able to react in the same way as humans and other animals do in our world; but in such worlds the laws of nature would have to be different to allow this to occur. So philosophers' zombies are possible, but presumably not on Earth, and quite likely nowhere in the entire universe – only in different possible worlds. They are L-possible. So my belief is that zombies are L-possible, that there aren't any on earth (or probably anywhere else in the universe), and that if there were such creatures on earth we might know about it because they might say the same sorts of thing as I have written in Part I, for example, and with some embarrassment: ‘What on earth are you talking about, so-called subjective experiences as entities different from the physical goings-on inside me?’ But the funny thing is that there are such creatures on earth, namely certain philosophers. So are they in fact themselves zombies after all, and refusing to admit it, so their existence doesn't contravene our laws of nature, or are they intellectually confused, or am I?

Of course all the above is philosophically contentious. If zombies really are L-impossible then there must be something wrong with what I have claimed, so I shall look in a minute at an argument claiming that they're not possible. But the reason why I want to insist that they are L-possible is that if they are, then we need to understand how there can be something more – and contingently more – to earthly animals that have experiences than merely their physical goings-on, and there is a real philosophical problem to be solved about how such ‘extras’ can have causal input. It is a problem going back at least to Descartes, who wondered how the mind, if not a physical object, can affect the body. If, to put it crudely, minds aren't anything different from bodies, then there is no such problem to solve.

But are there, as we normally believe, such things as experiences which are different from the goings-on in peoples' bodies, things which embody the ‘how-it-seems’ to the individual alone, ‘from the inside’ (to use a very misleading spatial metaphor)? I am very fond of spinach, and a friend of mine hates it. He wonders just how I can possibly like that disgusting stuff, as he would put it. Of course it isn't actually the spinach that he hates and I love: it's the taste of spinach. And he has told me he sometimes wonders: does it taste the same to both of us? If it tastes differently to him from how it tastes to me, then it is easily understandable how he can hate it and I can love it, because what we are hating and loving are two different ‘its’. If it tastes differently to me then what I am liking is something different from what he is hating. What he finds difficult to apprehend is how anyone can like just that particular taste that he finds so disgusting, the thing that he experiences. So there surely is a question: does it taste the same to both of us? If it tastes differently to each of us, then our subjective experiences of ‘the’ taste of spinach are different: if it tastes the same then our experiences are qualitatively the same. How could we tell which of these two alternatives is correct? Before at least waving towards a way of answering this, we should note that ‘how it tastes to him’ just is what we call his subjective experience of the taste, and ‘how it tastes to me’ is my subjective experience of the taste. These two experiences are necessarily distinct things, even if (as we guess?) often they are qualitatively identical, i.e. it tastes to him the same as it tastes to me. And one aspect of the so-called ‘mind-body’ problem is the problem of explaining how such subjective somethings, the experiences themselves, can have causal input – if indeed they can have.

Turn now to the subject of colour blindness, the phenomenon of being unable to distinguish between light of two or more different wave-lengths that most people can distinguish between. One of the most common is red-green colour blindness. There is an extremely clever and also extremely simple, objective way, of testing for this. If red dots are made into a pattern on a green background, or vice versa, a normally sighted person will be able to see the pattern but a red-green colour-blind person will not. So this is a completely objective test of how things subjectively look to the individual being tested. It certainly is a scientific test. Now consider someone who is colour-blind in this way. He cannot distinguish between red and green, and we might now ask ourselves: is this because red things and green things both look red to him, or because they both look green to him? In asking this question I take it that I am asking whether both red things and green things look to him in the same way as red things look to you and to me, or as green things look to you and to me (assuming that you are not colour blind, and, as I point out below, that they look the same to you as they do to me, and we don't both see as red what everyone else sees as green, and vice versa!). How things subjectively look to Frank is an example of ‘what it is like to be Frank’ vis-à-vis his experiences, that is, what they seem like to Frank. So there is such a thing as what it is like to be Frank in this respect which is not the same as what is going on in his body.

Scientific testing can ascertain whether Frank is colour-blind in this respect. But if he is, then can science ascertain whether everything red and everything green looks red to him, or whether they all look green to him? And when I made the distinction above, I assumed that you and I, both passing the colour-blind test, see red things in the same way, and see green things in the same (but of course different!) way. But is there any way of telling whether we do see all red things in the same way? Could science answer this question, and if it can't, then is it just nonsense? It certainly seems to make sense, and if it does, then there is such a thing as how things seem to you in the world and how things seem to me: our experiences seem to be mental contents that may or may not be qualitatively identical.

Now we might think that science could ascertain whether colour blind Frank sees red and green things as red, or red and green things as green, by ascertaining what goes on at his retina and in his visual cortex or perhaps other parts of his brain, and comparing it with the occurrences in normally sighted people. We normally take it that this would provide excellent evidence concerning whether he sees both these colours as red or as green. But we can see that it isn't an absolutely knock down proof. Perhaps the parts of his visual cortex that are active when he sees either colour are the same as the parts when others see red, so it is reasonable to conclude that he see both colours as red. But isn't it possible that when those parts of his cortex are stimulated then he is having experiences as of what we see as green (cf. the spinach question in the previous paragraph)? This simply pushes us into the field of general scepticism, about which I don't want to say much. What I mainly want to show is that to find out what is going on in the cortex etc. is a different matter from how things seem to Frank, what his experiences are like. And what we would like to know is how, if at all, the actual experiences, the ‘how it subjectively seems to him’ can have a causal input to his actions, or whether the total causal input comes solely from what is going on in his central nervous system etc. My argument about the zombies has been to suggest that there could be – and hence are – possible worlds where the total causal input comes solely from the physical states, but that our world is one in which there also is causal input from experiences correlated with those physical states. It would be very strange, it seems to me, if experiences and other subjective conscious states which arise from evolution have absolutely no function, no causal input whatsoever into what goes on in animals' behaviour. But if they do have, we need some philosophical explanation of how this is possible – for it often seems to people that only the movements of physical bodies can have causal influence.

I wrote in the last paragraph that I don't want to say much about scepticism. But I shall say this much: despite the fact that philosophers still carry on discussing and apparently trying to ‘solve’ the ‘problem of scepticism’, I believe that it was certainly solved by John Wisdom in his series of articles ‘Other Minds’, later published in book form.Footnote ⁴ What Wisdom showed is that sceptics can always push their opponents into admitting that no possible evidence would prove absolutely beyond a shadow of doubt that they know what they think they know, or even that what they think they have good evidence merely for believing is indeed good evidence for those beliefs. This is usually because the move from evidence to that for which it is evidence is neither deductive nor even straightforwardly inductive: so, sceptics often conclude, we have no evidence whatsoever for what we often confidently affirm, or affirmed before philosophical probing. The fact – the L-fact – always is, Wisdom affirms, that in claiming that our evidence is not ‘knock-down’, logically conclusive, the sceptic is just about always right, and if he – perhaps confusedly – marks this truth by saying that in those circumstances we cannot ‘know’ that our conclusions are true, then this too is correct, but put in a very misleading form of words. It then might look as though there is something we are missing, some other evidence that might give us the knowledge that we so painfully lack. But there is and can be no such thing. To move to the example we have been discussing, it is surely right that we cannot know how things look to Frank in precisely the way that Frank knows how things look to Frank. Suppose I get wired up to his brain, and all these things then look green to me, via what is going on in his brain. Does it follow that they look green to Frank? Well, maybe they do and maybe they don't, for Frank is seeing things through the use of his brain, whereas in this thought-experiment it is I who am ‘seeing things through Frank's brain’, (and actually, also through my brain as well, which Frank is not). And these are two different things. So it seems as though there is something I lack, namely, seeing things through the use of Frank's brain in the way that Frank sees things through the use of Frank's brain. But, surprisingly, there is no such thing. So there is nothing that I lack.

Why is there no such possible thing, and thus nothing that I lack? Suppose you say that you would like to own my house. Being a generous chap I say ‘Help yourself’. I get out the legal papers so that it becomes yours, perhaps charge you the minimum required to make it a legal sale, say £1, and hand you the keys. You move in. Are you happy? ‘Oh dear,’ you say, ‘this is no longer your house: it's mine. So I haven't got your house at all: all I have now is my house that merely used to be yours.’ Perhaps it will be clear that if that is what you want then there isn't anything that would count for you as owning my house: either it's mine or it's yours, but not both. What follows from this is that there isn't anything relevantly in the way of houses that you might have but in fact lack. This, by the way, is why the truth that ‘God cannot perform the logically impossible’ is no constraint on God's omnipotence. The reason why God ‘cannot’ cut out an area of paper that is both square and circular is that there is no conceivable area that is both square and circular; so making an area that is both square and circular isn't a ‘something’ that God, despite His omnipotence, unfortunately cannot do. It isn't anything at all, actual or possible. Similarly, my knowing how things look, smell, feel etc. to Frank in the way that Frank knows how they look, smell, feel etc. is no possible state of affairs, so nothing that I lack. If this is correct, then Wisdom solved the philosophical problem of scepticism, and it is a great shame that many contemporary philosophers continue to come up with amazing (and implausible) attempted ‘solutions’ to it, none of which can possibly work.

Let us turn now to the arguments for the impossibility of zombies.

In his article ‘Zapping the Zombies’ Robert Kirk introduces the idea of (philosophers') zombies as ‘beings physically exactly like ourselves … fully functioning human bodies … [but with] … no conscious experiences’ (p. 47). That is exactly the sort of creature that I pretended in Part I that I am: Kirk argues that they are impossible. They might be thought impossible either because for some reason there is no possible world in which a creature physically exactly like us except lacking experiences would be able to function like us, or because any creatures physically exactly like ourselves would inevitably, either in our world or in all possible worlds, have conscious experiences that are something ‘extra’, over and above their purely physical make-up, or because, utterly surprisingly, to be a fully functioning human body exactly like ourselves just is the same thing as to have conscious experiences. If the latter were the case, then there couldn't be ‘the one thing without the other’, for there simply wouldn't be two things, one of which might (or might not) be able to exist without the other. But this latter view is the philosophical ‘reductionist’ view which the ordinary man in the street would find so hard to swallow, and which I too find hard to swallow. That, alas, isn't enough to show that it is wrong.

However, my claim is that although it is factually impossible for such zombies to be formed on earth, with our laws of nature, nevertheless they are L-possible, that is, there is nothing self-contradictory in the idea of such a creature, which therefore could exist in some possible world, and thus even might, for all we know, exist in our world, namely in a part of it having different laws of nature (or perhaps initial conditions) from those existing on Earth. My claim is that humans (and other conscious animals) do have something more, experiences and other mental states that have causal input into their behaviour and bodily internal physical states, as well as into their other mental states, and thus there is a philosophical problem of spelling out the (non-reductionist) nature of such mental states, and how they can have causal input. So it is important, from my point of view, to claim that zombies are possible, not merely because I think it's true, but because it underlines the fact, if I am right, that there is something further to be explained about human and other animal behaviour.

The structure of Kirk's argument, as he tells us on p. 51, is that if zombies are possible, then (it follows that) something else is possible, but that ‘something else’ is impossible, hence zombies too are impossible. Kirk makes thankfully plain that he is talking of L-possibility and impossibility: he claims on the same page that his argument shows that ‘the zombie idea is … incoherent’, (ie. self-contradictory). The ‘something else’ that he claims is impossible, and yet the possibility of which follows from the possibility of zombies, is a creature physically identical with a zombie (and hence with a human being) but having what he calls ‘non-physical’ experiences that are caused by physical occurrences in the creature – presumably in its brain – but which have no effects on the physical world, the view known as ‘epi-phenomenalism’.

I shan't discuss Kirk's complex and subtle argument attempting to show that their possibility does indeed follow, an argument which I thoroughly commend to the reader, or his claim that such epi-phenomenal creatures are in truth impossible. Instead I want merely to raise some questions about his use of the concept of the physical in the course of his argument.

Kirk says that if zombies were possible then there would be ‘more to human consciousness than can be accounted for in purely physical terms … [and that we would therefore have] … non-physical components whose presence explains the fact that there is something that it is like to be one of us’ (p. 47, my italics), and he argues that this view is wrong. At other places he refers disparagingly to belief in non-physical ‘components … items … and properties’, at least with respect to human beings. But we need to know what ‘being accountable for in purely physical terms’ amounts to; could it be that humans do have something ‘extra’, something non-physical, but this ‘extra’ can be accounted for ‘in purely physical terms’? We also need to understand what it is to be physical, and hence what to be non-physical.

Kirk states (p. 48) that ‘In the nineteenth century scientists began to think there were good reasons to suppose that every physical effect has a physical cause’. He then uses this supposition as a premise in his later arguments, so presumably he thinks there are other good reasons for accepting it, since scientists have believed all sorts of strange things, and still do. As Kirk expresses it, the supposition is anyway compatible with some physical effects also having some non-physical causes, since there are almost always very many causes of any effect. But from what he goes on to say, it is pretty clear he means that physical effects have only physical causes.

Now we normally, and usefully, contrast a person's physical properties such as height with mental properties such as intelligence: we also distinguish between physical and mental strength. But intelligence and mental strength clearly have effects on people's behaviour, so any arguments to conclusions that they don't must either have gone awry, or the conclusions need somehow reinterpreting. Such conclusions might be misleading ways of claiming either that it's only the physical (e.g. neural) correlates of these properties that have causal influence, or that mental properties or other entities really are physical properties or entities. Either of these claims obviously needs a good deal of support.

Properties are not physical objects, like pebbles, pushcarts or even people, all of which have spatial location. Even if some properties have spatial location, which I doubt, it's clear they don't all have. The property of being president of the United States passes from person to person: the current president is currently somewhere, but the property of being the president is not also there, hovering over him like a ghostly … ah … mind? Suppose we just decide to classify and thus call all (and only) properties of physical objects ‘physical properties’, in contrast to properties of abstract objects, such as that of being a prime number. We would then, simply by fiat, classify intelligence as a physical property. But even with this terminology, it doesn't seem true that every physical effect must have (only) physical causes, if physical causes and effects are changes (or continuations) in physical objects or properties of physical objects. According to Relativity Theory the bending of space has physical effects; but the bending of space isn't a property of a physical object, because space isn't a physical object. Empty space also has the properties of transmitting light and other electro-magnetic waves, but not sound waves, and therefore affects the transmission of these waves.

If these doubts about a version of ‘physicalism’ are right, it leaves open the possibility of the mind being as ‘mystical’ or mysterious as you like, and still having a causal input. My own belief, however, is that one needs no such mysteries to understand the nature of mind. My view, somewhat alluded to above, is that minds, including even experiences, are properties of organisms, which are themselves of course physical bodies. If something's being a property of a physical body is enough for it to be a ‘physical property’, then minds are physical properties. Life too is a property of organisms, and so isn't a physical object, but most non-religious scientists and philosophers think it can be ‘accounted for’ in purely physical terms simply in the sense that when and only when various objects (organisms) have certain physical set-ups, those objects have the (further) property of being alive. I believe the same holds for minds, and for experiences, which means that they are not identical with, or the same thing as, either the physical objects, or the processes going on spatially within the objects having those properties.

One reason why minds often seem so mysterious is that, being properties, they are nowhere: in some circumstances and about some mental entities, such as thoughts, we feel very inclined to believe this, and about others, such as sensations, we do not. If right this still leaves open the question just how they can have causal input, which requires a view on the nature of causation. I hope to write about these matters elsewhere.