Kirk - 'Sentience and Behaviour

7/30/2019 Kirk - 'Sentience and Behaviour'

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Mind Association

Sentience and BehaviourAuthor(s): Robert KirkReviewed work(s):Source: Mind, New Series, Vol. 83, No. 329 (Jan., 1974), pp. 43-60

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Sentience and BehaviourROBERT KIRK

The sceptic's suggestion that others, despite their anatomical andbehavioural resemblance to himself, might after all be insentient-without sensory experiences of any kind-is familiar enough. Butone of the assumptions on which his position rests is not, I believe,generally considered as seriously as it deserves to be. For the scepticmust assume that it is at least logically possible for there to be anorganism indistinguishable from a normal human being in allanatomical, behavioural and other observable respects, yetinsentient. And this assumption does not involve any obviouscommitment to scepticism: to adopt it seems to be compatiblewith claiming to know that in fact there are no such organisms.But the question of its validity has immediate relevance to themind-body problem, as well as to the philosophy of perception. Ifit can be shown to be valid, then certain widely held views aboutthe analysis of reports of experience must be abandoned, and it ishard to see how Materialism can survive. But if it can be shown tobe invalid, then it seems that Materialism is almost home anddry.Some philosophers have made use of the assumption I havedescribed1; and perhaps many people will regard it as obviouslyvalid. But there are objections to be met. Can it make sense to saythat something indistinguishable from a man might still bedifferent in the radical way imagined? Even if the description canbe adequately clarified, doesn't it involve contradictions or con-ceptual absurdities?My aim is to show that it is indeed logically possible for there tobe organisms answering to the description I have given (Zombies,for short). My starting point will be the story of Dan, who, itseems, turned into a Zombie by stages.I E.g. Thomas Nagel in 'Armstrong on the Mind', Philosophical Review, 79

(1970), 394-403; Moreland Perkins in 'Sentience', Journal of Philosophy,68 (1971), 329-337. Keith Campbell in Body and Mind, New York, I970,uses a rather similar assumption.43

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44 R. KIRK:

One day Dan (who had been perfectly normal until that time)accidentally cut his hand and started to behave very strangely.True, his behaviour included features indistinguishable from thetypical behaviour of a normal person suffering pain of the kind andintensity to be expected in the circumstances. He winced, ex-claimed 'Ouch!', nursed his hand, answered 'Yes' to questionsabout whether it hurt, and so on. But in addition to, and as itwere superimposed upon, these normal bits of behaviour was asecond kind of behaviour wildly different from the typical be-haviour of someone in pain. He expressed astonishment; and heuttered sentences and behaved in ways which, in more normalcontexts, would have been taken to be protestations to the effectthat he felt no pain whatever, that it was as if he were 'totallyanaesthetized'. He appeared to be astonished by two things: first,by the fact (as he seemed to regard it) that he really felt no pain atall in spite of a fairly serious injury; and second, by the fact that hewas nevertheless wincing, groaning, and uttering such sentences as'It hurts like hell'. Thus his behaviour as a whole was bafflinglyincoherent. (I am using the word 'behaviour' here and throughoutin the sense of mere bodily movement, without commitment as towhether or not it consists of actions. Similarly for other wordsused in specifying Dan's behaviour, e.g. 'say', 'express'.)After a time-about as long as it would have taken for a normalperson in that situation to have ceased feeling pain-Dan went onto describe his experiences (as it seemed) in the following terms.He continued to insist that he really had not felt any pain, and hesaid that so far as his groans, winces, exclamations, complaints andother 'pain-behaviour' were concerned, they had been a series ofhappenings over which he had had no control: he had felt 'like apuppet'; he had merely noticed their occurrence as one mightnotice one's finger involuntarily twitching. Indeed, he said that itseemed to him that the whole series of items constituting his'pain-behaviour' had been a series of extraordinarily complicatedinvoluntary twitches.Now obviously there can be no logicalL absurdity in thisdescription of Dan's behaviour. A good actor could match it. SoI Throughout I use 'logical' in a very liberal sense, so as to include whatsome might prefer to call 'conceptual'. I doubt if Quinean scepticism aboutthe legitimacy of such notions seriously affects my main points.



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SENTIENCE AND BEHAVIOUR 45let us imagine that we have actually found Dan behaving asdescribed; and that from that time on, whenever he is in a 'pain-situation' (i.e. a situation in which one would expect a normalperson to suffer pain) his behaviour corresponds, mutatis mutandis,with that described above (although there is less apparent surprise,and more resignation, as time passes). Could we make any sense ofthese phenomena?

IIDan's behaviour would be intelligible, I suggest, on the hypothesisthat he has simply ceased to feel pain in pain-situations, despitethe occurrence of behavioural features of types normally associatedwith pain. It is a natural enough suggestion, given the bizarre data.But I need to make very plain what a sound hypothesis it is, sincethis point will carry a lot of weight later in my argument.The hypothesis that Dan felt no pain would explain at leastthree puzzling features of his behaviour. Firstly, it would explain(what appeared to be) his expressions of astonishment at notfeeling pain in pain-situations. On this hypothesis they would begenuine expressions of astonishment, not mere behaviouralsimulacra. Secondly, it would explain his seeming expressions ofastonishment at his pain-behaviour. These too would be genuineexpressions of astonishment. Thirdly, it would explain his de-scription of the way in which his apparent pain-behaviour wasproduced as 'quite involuntary', 'something that happened' to him,'like a series of very complicated twitches'. For on the presenthypothesis, what appeared to be his pain-behaviour would benothing of the sort, so that its occurrence could be expected to fitsuch descriptions. Moreover, although on this hypothesis theseitems of behaviour were not produced in response to any feelingsof pain, they need not be totally inexplicable. For the workings ofDan's central nervous system might by themselves be sufficient tocause the physical movements in question: the contortions of thefacial muscles involved in wincing; the workings of the organs ofspeech, and so on.Contrast with this hypothesis any hypothesis on which Dan stillfeels pain in pain-situations. How would it be possible, on such ahypothesis, to account for (what seem to be) his expressions ofastonishment? Well, perhaps these items of behaviour are thebasic, inexplicable datum, corresponding to the basic assumption



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46 R. KIRK:of the first hypothesis, to the effect that Dan feels no pains. Butnotice how many questions this hypothesis raises, compared withthe first. On the first hypothesis it is certainly mysterious that Danfeels no pain in pain-situations. But an explanatory hypothesiscannot be expected to explain everything. And on this one, as Isaid, Dan's pain-behaviour need not be wholly uncaused: it mayreasonably be supposed to be caused by his neural states, which inturn are caused by damage to bodily tissues. On the rival type ofhypothesis, though, the expressions of astonishment come out ofthe blue; and apart from the mystery of their spontaneity there isthe additional mystery of why they are expressions of astonishment.For not only is there nothing to cause any kind of astonishment:the actual situation is, ex hypothesi, one (viz. Dan's genuinelybeing in pain) in which the expression of astonishment at notbeing in pain is hardly intelligible.A further difficulty for any hypothesis on which Dan really feelspain in pain-situations is his own subsequent descriptions (as theyseem to be) of his experiences. Not only does he continue to insistthat he has felt no pain: he also describes his pain-behaviour as'quite involuntary'. If he really felt pain, such remarks would beinexplicable. Of course, it would be possible to introduce furtherhypotheses to the effect that these utterances themselves are merelya set of complicated involuntary twitches. But if they are, how is itthat Dan fails to comment on this fact? He could hardly avoidnoticing them. And this difficulty also arises in the case of theexpressions of astonishment. For it is strange that Dan, who doesfeel pain on the hypothesis we are now considering, should notnotice the fact that such expressions are out of place, and evincesome surprise.I conclude that if we were faced by Dan or someone behavinglike Dan, it would be not merely intelligible, but very reasonable,indeed justifiable, to say that he had ceased to feel pain, despite hisapparent pain-behaviour.

IIIDan continues for six months in the abnormal state I havedescribed, and then suddenly starts to show a further set ofbehavioural peculiarities. This time they concern his sense ofsmell. Asked to smell some roses, he sniffs, utters some suchremark as 'They smell marvellous'-but then shows astonishment



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SENTIENCE AND BEHAVIOUR 47and distress. He appears to be astonished by two things. Firstly bythe fact that, although he has hitherto enjoyed a keen sense ofsmell and is not suffering from a cold, he cannot detect the scent ofthe roses at all. And secondly by the fact that, despite his nothaving been aware of any scent, his facial muscles have justformed an expression of enjoyment, and his vocal organs haveproduced a remark that would have been appropriate only if hehad really detected, and enjoyed, the scent of roses. The facialexpression and the movements of his organs of speech were all (sohe seems to think) involuntary, just as in the case of his productionof sentences 'expressing' or 'describing' pains in pain-situations.If all this were really happening we should expect Dan to showdistress, especially in view of his continuing abnormalities inpain-situations. And so he does, and with increased intensitywhen the same sort of thing is repeated in all subsequent situationswhere a normal person would be aware of some smell.

Clearly there is no logical absurdity in this description. Nor, forreasons similar to those alreadygiven in connection with the case ofDan's behaviour in pain-situations, is there any logical absurdityin the hypothesis that Dan has been unable to detect smells sincethe time in question. Indeed, for the same kind of reasons as weregiven for the earlier case, it would be a reasonable hypothesis: itwould explain more than any hypothesis according to which Danstill detected smells.The story continues. After each of the next few periods of sixmonths an additional peculiarity, analogous to those alreadydescribed, suddenly appears in Dan's behaviour, affecting, orseeming to affect, one sense after another. Similar arguments tothe foregoing make it a reasonable hypothesis that Dan is pro-gressively being deprived of sensory experiences of the variouskinds in question. The reasonableness of this hypothesis is rein-forced by the fact that Dan himself, after the first couple ofsensory losses (or apparent losses), begins gloomily to speculate

(as it seems) about which sense will be the next to go, and whetherhe will eventually lose all varieties of sensory experience while(macabre prospect) his body generates an exact simulacrum ofnormal conscious behaviour. For it fits in well with the hypothesisthat the hypothesis should itself be adopted by Dan; and the factthat Dan behaves, within the limits shortly to be described, as ifhe has adopted it is itself hard to explain unless the hypothesis istrue.



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48 R. KIRK:Let us suppose, then, that Dan has now reached the stage where,according to the hypothesis that I am assuming we have adopted,

he is left with the enjoyment of only two senses: sight and hearing.Most of his verbal and non-verbal behaviour (still, of course, inthe neutral sense) is appropriate to his circumstances and to theplans, purposes and intentions that a person in those circum-stances might reasonably be expected to have. But there are grossabnormalities.At times, and with what looks like growing desperation, Daninsists that he has lost all his senses but sight and hearing, anddreads the time when one or both of them will go too. He insists onthe involuntary nature of all or most of that behaviour the appro-priateness of which for normal people would depend on theirhaving full enjoyment of the senses which, on our hypothesis, henow lacks. For instance, he sometimes hears himself 'complaining'of giddiness (i.e. uttering appropriate sentences) even though, ashe insists, and as our hypothesis has it, he really has no bodilysensations at all. He expresses surprise when he hears such re-marks issuing from his mouth.Thus, by his account, quite lengthy sequences of his ownbehaviour have become inaccessible to him: they occur auto-matically, without his noticing or being able to attend to them.Moreover, even when he is able to attend to some parts of hisbehaviour by sight or hearing, he is powerless to inhibit them. Hehears himself 'describing' the flavour of an apple, say, even when(as he insists) he is most keenly aware of the absence of any corre-sponding experience. It is only in the gaps between such involun-tary pseudo-linguistic performances (which is what they are on ourhypothesis) and then only with great effort, that he is able tointerpose his own account-which, of course, contradicts what hisapparently automatic utterances would normally be taken to haveexpressed. The effort he apparently has to put into forcing out hisown descriptions of his situation has been increasing with eachstage of his deterioration.

Loss of sight, as it seems to be, comes after the next interval ofsix months. Dan (or the individual I am still calling Dan) wakesone morning and with a prodigious effort (as it seems) manages togroan: 'I've gone blind: can't see a thing.' Yet he prepares to go towork as usual, with only a few effortful complaints about havinggone blind. When he says 'Goodbye' to his wife, or at any rateproduces that utterance, as if it were a normal morning, he



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SENTIENCE AND BEHAVIOUR 49struggles to exclaim 'No! I can't see!' But he goes on to drive towork as usual. Once there, he makes a tremendous effort andshouts: 'But I can't see anything! I want to go home!' He istaken to a mental hospital. After six months, plus another week orso, he is pronounced cured, and returns home, where his wifegratefully accepts the tale the doctors tell her.

However, those of his friends who have been closely and intel-ligently following the course of Dan's troubles fear that it might bemore reasonable (though perhaps not more natural), in view of hishistory, to suppose that instead of having been cured at the mentalhospital, what happened was that his last sensory link with theworld-his sense of hearing-had, after the usual six months'interval, been cut. They fear that in his case it really is 'silent anddark within' -he has turned into a Zombie. But as time passes,their hypothesis gradually comes to seem absurd. After all, Dannow seems to be normal, and they can hardly avoid treating him asif he were. So they come to accept that he is normal.But even if it is psychologically very difficult, even impossible, inthe face of behaviour indistinguishable from normal human be-haviour, to believe that the individual who exhibits it is reallyinsentient, this could still in fact be true: what it is psychologicallyimpossible to believe need not therefore be false. I think thatDan's history so far makes it a tenable hypothesis that he hasbecome, or been superseded by,2 a Zombie. At least it seems tomake this suggestion intelligible. However, a different suggestionmight be thought more reasonable in the circumstances: that whatcame back from the mental hospital (let us call it D) was neither aZombie, nor Dan, but a differentperson, who had been taking overDan's senses one by one. I shall now take the story a little further,to a point where the hypothesis that D was in fact a Zombiebecomes not only tenable, but justifiable. (I shall need to use thisconclusion at a later stage.)In general, D behaves for six months in ways indistinguishablefrom the ways in which his friends would have expected Dan tobehave. For example he (or it) answers to the name Dan. However,I A phrase used by Iris Murdoch in rejecting analytical Behaviourism, TheSovereignty of Good (London, I970), p. 13.2 It might be argued that Dan, being a person, could not become or turn intoa Zombie, which no doubt is not a person. Cf. David Wiggins, Identity andSpatio-Temporal Continuity (Oxford, I967), passim. But the issue is ir-relevant to my argument, since my concern is only to show the logicalpossibility of Zombies, and that we could have good reasons for supposingthat we had been confronted by one.



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50 R. KIRK:when six monthshave elapsed,D occasionally roduces,apparentlywithgreateffort, narticulatenoises whichseemto expressdelight.After a time thereare sentencessuch as 'I can hearagain!PerhapsI'll get backmy othersenses.' After anothersix months: 'Marvel-lous! I can see againas well as hear! I wish the next couple ofyearswould passquicklyso that I could be myselfagain.I seem tohave been asleep,but I vaguelyremember he horribleexperienceof losing all my senses, one afterthe other.' I need not fill in muchmore detail,butone thing is important:at no time does D produceanything ike expressionsof distressor apprehension.

Now accordingto the hypothesis we are examining,D is apersonwho hastakenover from,or superseded,Dan: whenD hastakenover completely, Dan is, if not non-existent, then withoutany form of sensory experience,while D has all his senses abouthim. But then D's productionof the sentence 'I can see againaswell as hear'would markthe secondstageof his being takenoverby a different individual (Dan, on a reasonablehypothesis): itwould markthe lossof sight by the person D is supposedto be.But a normalperson, suddenly deprived of his sight, would beexpected to show some distress,and, as my storyhas it, D showsnothing of the sort. Thus there is a significantasymmetrybe-tween what happens at the onset of the successivestages of D'stake-overby (presumably)Dan, and whathappenedatthe onset ofeach of Dan's originalsensory osses: Dan showeddistress,but Dshows none. This fits in well with the hypothesisthat what cameback from the mental hospitalwas a Zombie, but not with thehypothesis hat it wasaperson.Fora Zombie,despiteappearances,hasno senses to lose,and thereforecannotbe distressedat the lossof a sense.It wouldbe possibleto fill in furtherdetails,but I thinkI havesaidenoughto make t clearthat the hypothesis hatDan hadbeensuperseded by anotherperson would be less plausiblethan thehypothesis that he had turned into, or been superseded by, aZombie; and that faced by the sorts of behaviourthat I haveascribedto Dan, we should be justified in supposing that, for atime, we had been dealingwith a Zombie.

IVNow so far I havesaidnothingabout the contents of Dan's skull.Let us suppose that, to anyone familiarwith the workingsof



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SENTIENCE AND BEHAVIOUR 5Inormal human brains, Dan's neural activity at any time appears tobe consistent with his physical activity at that time. This meansthat in the stages intermediate between normality and putativeZombiehood his neural activity as a whole, like his behaviour as awhole, is bafflingly incoherent. To a Materialist, indeed, it willeven appear to infringe the principle of Sufficient Reason. Suchneural activity as corresponds, for example, to Dan's protestingthat, despite appearances, he is not in pain, will appear not to havebeen caused by any preceding neural or other physical activity atall (although a Dualist could say that it was caused by somethingnon-physical, or not wholly physical). But when Dan has, puta-tively, reached the stage of complete Zombiehood, his neuralactivity will be indistinguishable from that of a normal humanbeing. And here my position comes into conflict with certainphilosophical views, according to which it would be impossible forsomething to be both indistinguishable from a man in all observ-able respects, and radically different in the way envisaged.One, which I shall discuss at a later stage (sec. VI), may beroughly characterized as verificationistic. The other takes the linethat sentience follows logically from the fact that what I callZombies are indistinguishable from living men. One version ofthis second approach would be based on analytical Behaviourism.But since the arguments I shall present against a rather differentview will (if they are good) incidentally refute analytical Be-haviourism, I shall not discuss the latter separately.The argument I want to examine goes as follows: (i) All humanbehaviour (in the neutral sense) is in principle fully explicable inphysico-chemical terms; (2) The performance of certain causalfunctions connected with behaviour is a logically sufficient con-dition for the sentience of any organism in which they are per-formed; therefore (3) Contrary to my description of them, what Iam calling Zombies, must, logically, be sentient.For the moment I shall ignore the possibility that (i) is false,and confine myself to a consideration of (2). (2) is entailed by thecausal analysis of the concepts involved in sentience. According toDavid Lewis:

'.. . the definitive characteristic of any experience as such isits causal role. The definitive causal role of an experience isexpressible by a finite set of conditions that specify its typicalcauses and its typical effects under various circumstances.



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52 R. KIRK:By analytic necessity these conditions are true of the experi-ence and jointly distinctive of it.'"

According to this analysis, then, if a process now going on in someindividual's brain has the causal role ascribed to, say, pain, itfollows logically that the individual is in pain. So if, as I amassuming, all human behaviour is explicable in physico-chemicalterms, there are physical (presumably neural) processes in Zombieswhich have the causal role ascribed by this analysis to pain, fromwhich it follows, if the analysis is correct, that Zombies feel pain;and similarly for other varieties of sentience.Now I think it is quite a good primafacie argument against thecausal analysis that each of us can imagine that what happened toDan should happen to himself. Of course we could hardly imagineourselves actually being in Dan's final stage; but we are familiarwith the idea of sleepwalking, and Dan's situation as a Zombieneed not be very much unlike that of a sleep-walker. But my aimin this paper is to give a stronger justification than this kind ofintuitive plausibility to my thesis that Zombies are logicallypossible, and I am therefore going to provide further arguments.V

We are assuming that all human bodily movements can in principlebe explained in physico-chemical terms. We may therefore regardthe brain as a device which causes the body to move in certainways, and modifies these movements in response to various kindsof stimulation of its sense organs. Thus the brain performs acertain (very complicated) causal role, or set of causal roles,connected with the movements of the body to which it belongs.Now there is clearly nothing logically objectionable in thesuggestion that a thing different in many ways from a humanbrain might nevertheless perform exactly the same causal roles,with respect to the movements of a certain human body, as wouldnormally be performed by a human brain. But, as I shall shortlyexplain, unobjectionable cases can be described in which it is veryobvious that the mere performance of those causal roles would notentail sentience. Nor, as I shall also argue, are the differencesbetween such logically possible cases and the actual human case

D. K. Lewis, 'An Argument for the Identity Theory', Journal of Philosophy,63 (I966), I7-25. See pp. i9 f. Cf. D. M. Armstrong, A Materialist Theoryof the Mind (London, I1968), passim.



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SENTIENCE AND BEHAVIOUR 53such that their removal would allow this logical gap to be filled. Ifthis is correct, it will follow that in the normal human case, too,contrary to what is implied by the causal analysis, the brain'sperformance of the relevant causal roles does not entail sentience.

There can be no logical objection to the following: A team ofscientists removes the brain from a living human being, makesalternative arrangements for the circulation of blood, etc., anditself takes over the causal functions that are in question. (One wayin which this might conceivably be done would be by arrangingforall the impulses from afferent nerves to be registered by char-acteristic radio transmissions to themselves, while they trans-mitted impulses to the efferent nerves, causing the body to moveabout, eat, and speak in ways indistinguishable from those of aman.) The body, thus controllable, would be a kind of super-puppet, and the team of scientists (whom I will call the BrainTeam) would jointly perform the role of puppet-master.Here, then, is a situation where the relevant causal roles arecertainly being performed, but where, very obviously, that factdoes not entail sentience: the behaviour of our super-puppet nomore entails sentience than does the behaviour of the wood andcloth variety.

Of course this case is very far removed indeed from the actualhuman situation, and it might be doubted whether the point Ihave just made has any bearing on our problem. I shall now try tomake it plain that the differences do not affect the logical situation:removing them will not produce an entailment of the sort required.Two glaring differences are in the size and location of the BrainTeam. But if (as is possible, though fantastic) the Brain Team weremicroscopically small, or indeed if the super-puppet were giganti-cally large, they might be ensconced within its skull, controllingthe body rather as the flight crew controls an aircraft. Yetobviously these modifications of relative size and location would notproduce the required entailment: it would not follow that thecomplex entity which consisted of the body with its controllingBrain Team was a sentient being. This consequence would nomore follow from their being inside the body than it follows that apantomime horse, for example, is a sentient being from the factthat i-tsanimatov% re nsideiit.Is it, perhaps, the consciousness and purposiveness of the mem-bers of the Brain Team which blocks the entailment in the case Ihave described? No: all the causal analysis requires is that the



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54 R. KIRK:relevant causal roles be performed. It imposes no conditions onexactly how they may be performed, e.g. it does not prohibit theirbeing performed by conscious, purposive individuals. Moreover,if the causal analysis were to be revised by the addition of somesuch condition, the modification would be unwarrantable. Forthere is no logical absurdity in the suggestion that conscious,purposive individuals might be found playing vital roles within thebodies or brains of other conscious, purposive individuals-evenwithin the reader's own head, despite his undoubted sentience.For similar reasons, the structure or chemical composition ofwhatever mechanism performs the causal roles in question cannotrelevantly affect the logical situation. The causal analysis imposesno conditions on the structure or composition of the mechanism,and even if it did, their imposition would be unwarrantable: wecannot rule out a priori what kinds of structures could be those ofa sentient being, so long as they performed the relevant functions.However, someone who objected to the claim that Zombies arepossible might rest his position on a simple assertion of the one-way entailment, rather than insist on the full causal analysis. Hemight assert that the functioning of a brain indistinguishable froma normal human brain was a logically sufficient condition forsentience. But it is clear that if (as we are assuming) other kinds ofstructures could perform the same causal roles as a human brain, itis merely arbitrary to insist on this entailment. It is only because healready knows that human beings are sentient that he is inclined tosay that having a normal human brain entails being sentient.Besides, the concepts of structure and chemical composition seemto have no relevant links with those of sentience.It appears, then, that neither the Brain Team's size, nor itslocation, nor its members' consciousness and purposiveness, norits structure, nor its chemical composition can account for the factthat its performance of all the relevant causal functions of a normalhuman brain does not entail the sentience of that entity which itcontrols. If this is correct, then even when all these differencesbetween the Brain Team and a normal human brain have beenremoved, the alleged entailment still cannot hold. But to remove allthese differences would simply be to replace the Brain Team bysomething not importantly different from a normal human brain.I conclude that the performance of the relevant causal functionsby a normal human brain does not entail the sentience of the personwhose brain it is,



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SENTIENCE AND BEHAVIOUR 55I have been assuming, of course, that all human bodily move-ments are in principle explicable in physico-chemical terms. If

they -are not, then presumably physical mechanisms could notperform the relevant causal functions in beings observably in-distinguishable from men. In that case Materialism would befalse, or in deep trouble, and the possibility of Zombies wouldlose some of its interest. However, there would still be no contra-diction in the description of Zombies: they would flout theprinciple of Sufficient Reason, no doubt, but that is presumablynot a logical principle, nor even one whose occasional infringe-ment would involve conceptual absurdity.That concludes my main argument against the causal analysis.If the argument is sound, then the causal analysis fails to provide abasis for rejecting the logical possibility of Zombies, because itfails to take account of some of the phenomena necessarily in-volved in sentience. Now among these, it is generally held, is thephenomenon of private detectability:the fact that the way in whichthe subject of a sensation or sensory experience is put into a posi-tion to describe it is quite different from the ways in which othersmight be put into a position to describe it. That the causal analysisfails to do justice to private detectability may also be seen, I think,by considering the following possibility. (This will be a secondargument against the causal analysis.)

Suppose that human beings never felt pain in pain situations,but that those sorts of stimuli which, as things are, typically causepains in humans, caused something different. Instead of painsthey cause, let us suppose, an uncontrollable trembling of all thelimbs, together with an automatic tendency of the hands to touchwhatever part of the body has been damaged or (in the case ofinternal damage) whatever part is closest to the site of the damage.And suppose that the degree of violence of the involuntary trem-bling varies with the degree of seriousness of the damage. Supposefurther that when in pain-situations a person starts trembling inthe way described, there is a suspension of the normal operation ofkinaesthesis, so that he can discover the occurrence of the trem-bling only by observation or inference. This means that there is noway for him to detect the trembling that is not open to others. Inother words, the peculiar trembling process typically caused bythe stimuli which actually cause pain in us is not, we are to sup-pose, privately detectable. All this is clearly a logical possibility.Now if all this were the case it is obvious that for a person to be



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56 R. KIRK:in the trembling state brought on by tissue damage would be:disturbing and disruptive of normal activity; likely to provokewinces, groans, or even screams, depending on the degree ofviolence of the trembling; and likely to cause the subject to wantto take all possible steps to remove the cause of the disturbance. Itis also clear that the phenomenon I have described would servevery much the same life-preserving functions as in fact the occur-rence of pains serves for us. It would tend to make us do whateverwas possible to repair damaged tissues, and to avoid situations inwhich such damage was likely to occur.

Thus the typical causes of the trembling-states would beidentical with those of our own pain states; and their typicaleffects on our behaviour and dispositions would be, if not iden-tical, then substantially similar to those of our pain-states. Yetdespite these very close similarities between the causal roles ofpain-states and these trembling-states, the trembling-stateswould not be privately detectable. I do not see how this could beso if the causal analysis had any validity.VI

I come now to the second, epistemological, objection to the sug-gestion that Zombies are logically possible. It goes as follows:'You are committed to allowing the logical possibility of twoindividuals, A and B, who cannot even in principle be distinguishedon the basis of observation of their current physical structures andstates, yet are radically different in a very important respect: A is aman; B is what you call a Zombie-insentient. But how can itmake sense to assert the existence of a difference that cannot bedetected by observation? Ex hypothesi, both A and B conform toall the criteria on the basis of which we ascribe sensations andother experiences to people. How can you avoid the conclusionthat they must both have sensations and other experiences? For ifon the basis of all possible tests one of them comes out as sentient(and A, since he is human, certainly will), then so will the other.You may perhaps have shown that no contradiction follows fromthe description of a Zombie if one attends only to the details ofthe description. But these more general considerations stronglysuggest that Zombies are not, after all, genuinely conceivable: thenotion turns out to be incoherent.'

The starting-point for my reply is the story of Dan, Notice



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SENTIENCE AND BEHAVIOUR 57first that, because he was observably distinguishable from anormal human being in every stage but the final stage of putativeZombiehood, and because the hypothesis of gradual sensory losswas shown to be a reasonable one, the objection can at most applyonly to the description of him as a Zombie: it has no force againstthe assertion that a man could fit Dan's description and pro-gressively lose his senses in the way hypothesized.-But at thispoint a supplementary objection might be raised. The descriptionof Dan's behaviour in the intermediate stages is so peculiar, itmight be said, that our ordinary concepts of pain, hearing, sight,experience and the rest have no natural footholds, and what we sayabout him is largely a matter for decision rather than for 'reason-able hypotheses'. But I think this misrepresents the situation.Dan's behaviour is certainly bizarre. But, as I argued in sectionsII and III, the hypothesis that he has ceased to feel pain, to be ableto smell things, etc., has real explanatory power. Once we have hitupon that hypothesis, it can be seen that our ordinary concepts areadequate to deal with Dan's case. Nor is there any reason to sup-pose that the sense in which Dan may be said to have 'ceased tofeel pain' is any different from that in which perfectly normalpeople cease to feel pain when anaesthetized. The situation isdifferent from those cases where, even after the most carefulconsideration of possible explanations, we are at a loss what to say;where no matter what hypothesis or description we adopt, we areleft with a sense of conceptual strain. Take the example oflobotomized patients who say they still feel pain although it doesnot bother them. If we say that they do still feel pain, we are leftuneasy by the oddity of a pain which does not bother the sufferer.But if, instead, we say they are not in pain, we are left with theoddity of a person who says he is in pain when he is not. But Dan'scase is not relevantly analogous. In his case, once the explanatoryhypothesis has been found, there is no residual sense of con-ceptual strain-although of course the hypothesized facts maystrain credulity, and are genuinely mysterious. The main reasonwhy there is no sense of conceptual strain, no suspicion of a changeor extension or attenuation of the meanings of such words as'pain', 'feeling', and so on, is that according to the hypothesiswhat appears to be pain-behaviour is no such thing: it is just aseries of complicated involuntary movements. Dan is not reallyboth avowing his sufferings, describing his headaches, and so on,and also denying that he feels any pains. All he is really doing is



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58 R. KIRK:the latter. So there is, on this hypothesis, no conceptually disturb-ing incongruity between his assumed experiences on the one hand,and what he says and does on the other. Hence Dan's case isindeed a matter for empirical hypothesis, not for arbitrary con-ceptual decision.But now, if this epistemological objection has no force againstthe proposition that someone behaving as Dan does really couldlose one sense after another, is it strong enough to resist the pro-position that he could become or be superseded by a Zombie? Ifhe could lose all but one of his senses, while his behaviour assumedthe patterns sketched in section III, why should he not also lose thelast, while there remained something which behaved in waysindistinguishable from the normal? It would be in accordance witha prima facie reasonable hypothesis that this should be so. For thesix-monthly alterations in Dan's patterns of behaviour, taken to-gether with the subsequent apparent reversal of the process, pointunambiguously in that direction. Indeed, Dan himself (on ourhypothesis) formed the expectation that he would lose all hissenses and become a Zombie. It would have been a comfort to himif someone could have proved that what he dreaded involved aconceptual absurdity. But does the objection we are consideringprovide the materials for such a proof? I shall argue that it does not.The objection lays great weight on the point that the differencebetween a Zombie and a man is not detectable by observation.Now it is true that my position is incompatible with the view thatall actual differences must be publicly observable. But the exampleof Dan shows, I contend, that we could nonetheless be justified, onthe basis of observations made at other times, in saying that agiven individual was, or had been, a Zombie. If I am right, theview just described is mistaken. So one of the things I claim to haveshown is that there are no good a priori arguments against thepossibility that two things should differ in an important respect andbe indistinguishable by means of observation of their currentstructures and states. Objections which merely assume the validityof some general epistemological view incompatible with myposition therefore need to be supplemented with detailed explana-tions of why I have failed to show that sentience provides acounter-example. To the extent that my imaginary objector relies onverificationistic dogma, then, his point has already been met.But the objector also makes some use of the notion of a criterion,and it might be thought that here his point is valid. This, again,



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SENTIENCE AND BEHAVIOUR 59seems to me to be a mistake. If the satisfaction of the criteria isheld actually to entail sentience, we have already seen that there isno such entailment (sec. V). But if, as would be more usual, thesatisfaction of the criteria is not held to exclude the logical pos-sibility of insentience, then there is logical room for Zombies. Tobe sure, the satisfaction of the criteria gives the best possiblejustification, ceterisparibus, for believing that whatever descriptionis in question does apply; but it leaves open the logical possibilitythat it does not apply. Naturally I agree that if a Zombie were tocomplete its life without any Dan-like episodes, then everyonewould be justified in saying that it was a normal human being. Butin such a case what we should all say (with every justification)would be false: unidentified Zombies could not truly be describedas sentient, no matter what people said. My position is that insome circumstances other factors than what was currently ob-servable could justify the hypothesis that what was indistinguish-able from a man might be a Zombie. (This sort of possibility is,indeed, allowed by one use of 'criterion'.) The objector has over-looked the possibility that there are other ways of being justifiedin saying that two indistinguishable things are radically differentthan what can be gleaned from observation of their structures andstates at the time in question. I conclude that the epistemologicalobjection fails.If my assertion of the logical possibility of Zombies necessarilycommitted me to sceptical doubts, I should now have to take on aphalanx of further objections, including all the valid points thathave been made against the intelligibility of the notion of aprivate language. But it is not as if the bare logical possibility ofbeing wrong about some statement made it impossible to knowthat the statement was true. I may consistently claim to know thatthere are no Zombies even though I think it is logically possiblethat I am wrong about this, and I therefore see no reason to sup-pose that anti-private language arguments have any force againstmy position.

VII[f Zombies are logically possible, then not only must analyticalBehaviourism be false, but also any view (such as the CentralState Materialism of Smart and Armstrong) according to whichstatements about sensory experience are analysable or trans-latable in 'topic-neutral' terms; and any view (such as those of



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6o R. KIRK: SENTIENCE AND BEHAVIOURFeyerabend and Rorty) according to which Materialism could bevalidated by appropriate, if inconvenient, changes of linguisticconvention or conceptual scheme. (For it is clear that Zombieswould be radically different from the rest of us no matter how wetalked or thought.) Indeed, it is hard to see how any intelligibleversion of Materialism could be reconciled with the logical pos-sibility of Zombies, given that we are sentient. It was, after all,because the 'nomological danglers' seemed to be incompatiblewith the rationale of Materialism that the possibility of topic-neutral analyses of sensation statements was originally mooted.But I shall not pursue that question here; nor shall I do more thanassert that a recognition of the logical possibility of Zombies canhelp to clarify central isues in the philosophy of perception andthe philosophy of action. Perhaps I have said enough to show thatif it is valid, the idea of Zombies, fantastic as it is, has useful workto do.UNIVERSITY OF NOTTINGHAM

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Kirk - 'Sentience and Behaviour