the inconceivability of zombies

The Inconceivability of ZombiesAuthor(s): Robert KirkSource: Philosophical Studies: An International Journal for Philosophy in the AnalyticTradition, Vol. 139, No. 1 (May, 2008), pp. 73-89Published by: SpringerStable URL: http://www.jstor.org/stable/40208892 .

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Philos Stud (2008) 139:73-89 DOI 10.1007/sl 1098-007-9103-2

The inconceivability of zombies

Robert Kirk

Received: 12 August 2006 /Accepted: 5 March 2007 / Published online: 15 May 2007 © Springer Science+Business Media B.V. 2007

Abstract If zombies were conceivable in the sense relevant to the 'conceivability argument' against physicalism, a certain epiphenomenalistic conception of consciousness - the 'e-qualia story' - would also be conceivable. But (it is argued) the e-qualia story is not conceivable because it involves a contradiction. The non-physical 'e-qualia' supposedly involved could not perform cognitive processing, which would therefore have to be performed by physical processes; and these could not put anyone into 'epistemic contact' with e-qualia, contrary to the e-qualia story. Interactionism does not enable zombists to escape these conclusions.

Keywords Zombies • Consciousness • Epiphenomenalism • Physicalism •

Conceivability • Conceivability argument • Qualia • Mental causation •

Privacy • Chalmers

1 Introduction

Zombies would be like us in all physical respects, but without phenomenal consciousness. It is widely agreed that if zombies are possible, physicalism is false. A much debated

argument for the possibility of zombies starts from the claim that they are conceivable, then urges that whatever is conceivable is possible. Many physicalists agree that zombies are conceivable - even in a strong sense - but disagree that conceivability entails possibility. Whatever may be the correct view on that last point, I think all are wrong about the

R. Kirk (El) Department of Philosophy, University of Nottingham, 97 Westhorpe, Southwell, Notts NG25 ONE, UK e-mail: [email protected]

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74 R. Kirk

conceivability of zombies. I will argue that zombies are not conceivable in any sense

strong enough for the conceivability argument.1 There are plenty of objections in the literature to the conceivability of zombies.2 But the

idea is so alluring that those who think zombies are conceivable tend to feel there must be

something wrong with the objections; the zombie idea may be problematic (they say) but

surely it is not actually incoherent. I will argue that, on the contrary, it is indeed incoherent, involving a grossly distorted conception of phenomenal consciousness. To counter its

appeal there needs to be a way to bring out the wrongness of that conception - an approach which has strong intuitive impact, but does not just pit one lot of intuitions against another. That is what I will try to provide.

My approach centers on a particular conception of consciousness, the 'e-qualia story', to be explained shortly. This is a variety of epiphenomenalism: the view that although all

physical events are physically caused, consciousness depends on non-physical 'qualia' which are physically caused but have no physical effects. The argument of this paper has two stages. The first is to argue that the e-qualia story is not conceivable (in the relevant sense) because it is contradictory. The second is to argue that if zombies are conceivable, so is the e-qualia story. In outline:

(A) The e-qualia story is not conceivable. (B) If zombies were conceivable, the e-qualia story would be conceivable.

Therefore zombies are not conceivable.3 (A) is defended in Sects. 3-9; (B) in Sects. 10 and 11.

2 'Zombies' and 'conceivable'

The idea of zombies suggests itself as soon as one accepts the causal closure of the

physical. If every physical effect has a physical cause, all human behavior is explicable in

physical terms. But then how does consciousness - phenomenal consciousness, the sort involved in there being 'something it is like' to have experiences - fit into the story? Apparently it can only be a causally inert by-product, and epiphenomenalism or paral- lelism must hold. In that case, as G. F. Stout argued,

it ought to be quite credible that the constitution and course of nature would be otherwise just the same as it is if there were not and never had been any experiencing individuals.4

What Stout envisaged is a 'zombie twin' of our world: a physical duplicate of the actual world on the assumption that the physical world is closed under causation - so that

everything physical goes on just the same - but without phenomenal consciousness. Zombies must be understood to be complete physical systems in the sense that all effects in

1 For the view that the zombie possibility entails the falsity of physicalism see e.g. Chalmers (1996) and Kirk (1974, 2005, pp. 7-23). For the conceivability argument see e.g. Kripke (1980), Chalmers (2002) and other essays in Gendler and Hawthorne (2002). 2 Indirect objections: Ryle (1949) and Wittgenstein (1953); direct ones: e.g. Dennett (1991, 1995) and Kirk (2005, pp. 37-57); Shoemaker (1981, 1999); Tye (2006). 3 This outline mirrors that of Kirk (2005, pp. 39-55). However, the arguments here are significantly different (and I think clearer and more cogent) and take account of objections not considered in the book. 4 Stout (1931, 138f).

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The inconceivability of zombies 75

them are produced physically. They can be defined as follows: creatures without phenomenal consciousness but physically just like us on the assumption that the actual

physical world is causally closed.5 Once the zombie idea has been explained it is almost impossible to resist the thought

that such things are at least conceivable. Most philosophers would agree they are conceivable in some sense, but that sense is often too broad for the purposes of the con-

ceivability argument. For our purposes it is enough to say that a proposition or situation counts as inconceivable if it can be known a priori to be false; otherwise it is conceivable.

3 The e-qualia story

Does the possibility of zombies entail epiphenomenalism? Some have argued that it does; but that seems to be a mistake.6 Why shouldn't zombies be possible even if the actual world is interactionistic? Later I shall defend a different claim: that the conceivability of zombies entails the conceivability of the e-qualia story, a particular version of epiphenomenalism. This story consists of theses (E1)-(E5) below. Although close to epiphenomenalism as usually understood, it does not aim to be a fair reflection of

epiphenomenalists' views, nor of current views about qualia.

(El) The world is partly physical and its physical component is closed under causation:

every physical effect has a physical cause.7

(E2) Human beings are physical systems related to a special kind of non-physical properties, 'e-qualia'. E-qualia make human beings phenomenally conscious.

(E3) E-qualia are wholly caused by physical processes but inert: they have no effects either on the physical world or among themselves.

(E4) Human beings consist of nothing but functioning bodies and their related e-qualia. (E5) Human beings are able to do such things as notice, attend to, think about, compare,

and (on occasions) remember their e-qualia.

I take it that (El) and (E4) are clear enough for our purposes, given (E2), and will explain (E2) now, and say more about (E3) and (E5) in the next two sections.

The notion of e-qualia is significantly different from at least one current notion of

qualia. David Chalmers defines qualia, or 'phenomenal qualities', as "those properties of mental states that type those states by what it is like to have them".8 It seems the existence of qualia in that sense could be accepted even by physicalists, since the definition does not entail that these properties must be non-physical. Nor, unlike the e-qualia conception, does it entail that they could be stripped off without affecting the physical world. To say qualia in that sense exist amounts to no more than saying we are phenomenally conscious. Clause

5 Some authors use "zombie" for merely behavioral duplicates, or for systems resembling us only in input- output functions; but those senses too are irrelevant here. Physicalists can consistently concede that behavioral and dispositional similarity is insufficient for mental similarity (pace Dennett, 1991, pp. 438-

440): Kirk (2005, pp. 97-118). The definition of zombie twin worlds is refined in Sect. 10 below. 6 See for example Perry (2001). Interactionist zombists must deny that causal closure holds in our world, hence cannot define zombies as physically like us. Chalmers says the conclusion of his anti-materialist

argument is not epiphenomenalism, but "the disjunction of panprotopsychism, epiphenomenalism, and interactionism" (1999, p. 493; see also Chalmers, 1996, pp. 150-160), and Sects. 10 and 11 below. 7 Even those epiphenomenalists who maintain that God intervenes in the world can accept it is conceivable that the physical world should have been causally closed. 8 Chalmers (1996, p. 359, n.2).

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76 R. Kirk

(E2) of the e-qualia story is crucially different: it emphasizes that what makes us conscious is our relation to these special non-physical properties.

Although perhaps not many philosophers would endorse the position outlined by (El)- (E5), many would assume it is at least conceivable. The argument in the following sections aims to show that the e-qualia story is not conceivable in the relevant sense because (El )-(E4) are incompatible with (E5). If there were e-qualia satisfying (E1)-(E4), the epistemic relations envisaged by (E5) would be ruled out. It is notorious that epiphenomenalism gets into trouble over our epistemic relations with qualia; I aim to show that this difficulty is terminal.

4 E-qualia, epistemic contact, and cognitive processing

By (E3), e-qualia are caused by physical processes but inert. E-qualists will acknowledge that the qualities of our conscious experiences (which is what e-qualia are supposed to be) seem to have effects on our behavior, but insist it is only apparent causation, not real. When a thorn in my finger hurts and makes me wince, most of us would say the harm to my finger caused pain, which in turn caused the wince. E-qualists, in contrast, will say that one

physical event (stimulation of pain-receptors) caused both a pain e-quale and another

physical event (wincing). It seems that the pain caused the wincing (e-qualists would say) but in fact what caused it was the physical event, which also caused the pain. To see that this account of conscious experience is not just strange but inconceivable, we must first note a consequence of the inertness of e-qualia.

Like all varieties of epiphenomenalism, the e-qualia story is an attempt to characterize the metaphysics of a world whose inhabitants are phenomenally conscious just as we are: that is, in the sense that there is something it is like for them to perceive the world around them, and to have sensations and other experiences. We are able to do such things as notice, attend to, think about, compare, and on occasions remember the qualities of our

experiences. So the e-qualia story must ensure that the inhabitants of a world satisfying its conditions - an E-world - engage in those activities too. And since in an E-world the qualities of its inhabitants' experiences are provided by e-qualia (by (E2)), its inhabitants must be able to do such things as notice, attend to, think about, compare, and remember their e-qualia: for short, they must be in epistemic contact with their e-qualia.9 Hence (E5).

Epistemic contact in that sense involves cognitive processes such as conceptualization and the storing and retrieving of information, which in turn involve the causation of changes and the persistence of unconscious items. The case of remembering illustrates both points. If information is stored about an event, that must leave traces which can have effects later, when the subject is recalling or otherwise using the stored information; but these traces are not normally conscious. Other forms of epistemic contact, such as thinking about, attending to, or comparing items, in turn depend on information being stored and retrieved. Epistemic contact also involves causation and unconscious persistence, because it depends on conceptualization, which requires more or less persisting cognitive structures contributing causally to the subject's ability to group things together.10

9 Chalmers uses 'epistemic contact' for what he calls 'acquaintance' with qualia (1996, p. 197). I will consider this notion later (Sect. 9, Objection 3); but I am using 'epistemic contact' in the broader sense indicated. 10 It might be suggested that causation is not necessary: mere counterfactual dependence would be enough. But the need for unconsciously persisting traces and structures would still prevent e-qualia from being capable of the necessary cognitive processing. See also Sect. 7.

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But e-qualia are inert, and so cannot themselves engage in the cognitive activities which would be necessary to put E-worlders into epistemic contact with their e-qualia. Also, they do not persist through time (at least not unconsciously) and for that reason too cannot

perform the cognitive functions necessary for epistemic contact. The point is crucial. By (E4), human beings consist of nothing but bodies and their associated e-qualia. It follows that in an E-world, the cognitive functions in question must be performed by physical processes. You might suggest that non-physical items other than e-qualia could perform those functions; but by (E4) there are no such items.

The fact that the cognitive work in an E-world must be done by physical processes presents a fatal difficulty for the e-qualia story.

5 How is epistemic contact possible?

According to the e-qualia story, physical processes cause a temporally extended complex of e-qualia constituting an individual's stream of consciousness. At any given time the individual is supposed to be in epistemic contact with whichever e-qualia are occurring at that time. But what ensures there is such contact? To get an idea of the difficulty we can start by imagining an E-world w that becomes 'zombified': at a certain time it loses all its

e-qualia. Since by (El) the physical component of an E-world is closed under causation, the absence of e-qualia leaves the physical component of w functioning as before; the difference is that its inhabitants are zombies. Suppose, then, that a single temporally extended complex of e-qualia, ^, is introduced into this zombie world. \j/ is the kind of e-

qualia complex that every E-worlder is supposed to be in epistemic contact with, but in this

example it is not caused by physical processes but has come into existence spontaneously. Might some E-worlder nevertheless be in epistemic contact with \j/: think about, attend to,

compare, or remember some of the e-qualia in it?11 No. ij/ is an e-qualia complex suitable for one individual; so at most one E-worlder could

be in epistemic contact with it. However, by definition there is nothing to connect ij/ with one particular E-worlder rather than with any other. Therefore nothing could put any particular E-worlder into epistemic contact with xj/. Even if it somehow constituted a stream of consciousness all by itself, it would be one whose constitutive experiences no one could attend to, think about, or remember. This would be so even if w had only one inhabitant. Nor would it matter how many such e-qualia complexes were introduced into the zombie world w\ nor how numerous its population was. The mere co-existence of e-

qualia complexes and zombies would not result in anyone's being in epistemic contact with those complexes.

However, the e-qualia story does not envisage mere co-existence. Each complex of e-

qualia is supposed to be caused by some of the physical (presumably neural) processes in a human-like body; and that might at first appear to ensure that those processes were in

epistemic contact with it. Causation alone would not be enough, however. Assuming physical processes can cause non-physical items in the first place (as we must for argument's sake) there is no reason a priori why the laws of nature in w should not be such that, for any arbitrary type of e-qualia complex, the neural processes in a given E-worlder' s

body caused an e-qualia complex of just that type. Suppose for example that I had a

counterpart K in w. There is no a priori reason why processes in K's body should not cause

1 ' E-qualists cannot deny that such a situation is possible. Since e-qualia are non-physical, neither their

existence nor their non-existence can entail or be entailed by anything physical (but see n. 18).

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78 R. Kirk

an e-qualia complex of a type that might have been associated with an Inuit whose life included dangerous encounters with polar bears. I am not an Inuit, have comparatively slight experience of snow and ice, and have never been close to a polar bear. So even if the

e-qualia story were true of me, very few, if any, of my neural processes could have put me into epistemic contact with the e-qualia in such a complex. Since the neural processes in

my counterpart's body mirror my own, they would not put anyone into epistemic contact with that particular e-qualia complex.

An obvious reply would be that the laws of nature in an E- world ensure that the relevant neural processes in its inhabitants' bodies cause only such sequences of e-qualia as stand in an appropriate isomorphic relation to (features of) those processes.12 If I am attending to a

sequence of clock chimes, for example, then my cognitive processing of the chimes must be temporally and otherwise correlated with the e-qualia they supposedly cause. E-qualists might now seem to have a satisfactory response to the original question. Shortly I will

argue that that response does not work; but we need to be as clear as possible about the

difficulty for e-qualists, and the resources they have for dealing with it.

6 The problem

It is agreed on all sides that we can refer to our experiences. But if referring requires the referent to have an effect - however indirect - on the referrer, then because e-qualia would be inert, no one could refer to them. For those who accept a causal account of reference, the

e-qualia story is mistaken for that reason alone. However, the same thought also leads

epiphenomenalists to reject the causal account of reference, so presupposing it when

arguing against the e-qualia story would be question-begging. My argument does not

depend on a causal account; nor is the problem I am focusing on the same as the problem epiphenomenalists have over referring to experiences.

The following question helps to clarify the problem:

(Q) How, in an E-world, could physical processes in an individual body contribute to anyone's being in the relevant sort of epistemic contact with e-qualia: what could hook them up, epistemically?

I will argue that e-qualists cannot give a satisfactory answer to this question. The structure of an E-world would prevent its inhabitants from being in epistemic contact with 'their' e-

qualia because the cognitive processing essential for the relevant sort of epistemic contact would have to be performed by physical processes epistemically insulated from all e- qualia. That general point is based on the following counter-example to the suggestion that if e-qualia are caused by and isomorphic to the relevant physical processes, then there is epistemic contact.

7 My cranial currents

Suppose that ours is an E-world, and that by some quirk in the prevailing laws of nature, those of our brain processes which allegedly cause e-qualia also induce minute patterns of electrical activity which are in relevant respects isomorphic to them - but have no effects on them. For each sensory modality these cranial currents mirror the relevant brain activity

12 Epiphenomenalists typically assume isomorphism. See e.g. Chalmers (1996, p. 243).

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just as it is supposed to be mirrored by e-qualia. The patterns formed by currents induced

by activity in my visual cortex, for example, would serve as a continuous record of my visual experiences - hence (as the e-qualia story implies) of my visual e-qualia.

As it happens, no one actually observes these induced currents or knows anything about them. It follows that their being caused by and isomorphic to some of my brain processes is not enough to put anyone into epistemic contact with them. From the point of view of my epistemic history they might as well not be there; they could cease or start up again, or

change character, without affecting the epistemic situation in the slightest. So they are a

counter-example to the suggestion that for such processes to cause or be isomorphic to

something would be enough to put someone into epistemic contact with it. That result applies directly to e-qualia. Even if my brain processes cause or are iso-

morphic to my e-qualia, that cannot put me into epistemic contact with them. My e-qualia, like my cranial currents, could cease or start up again, or change, without affecting the

epistemic situation. They are so thoroughly insulated epistemically that they might as well not be there at all. So far, then, e-qualists cannot answer question (Q).

But if causation and isomorphism will not do the trick, what else can e-qualists appeal to? It must be something additional or different. But by definition there is nothing in an E- world but its physical component and its e-qualia, so the only available factors are:

(a) The intrinsic properties of the physical component; (b) The intrinsic properties of e-qualia; (c) The ways in which natural necessity might relate those properties so as to constitute

subjects in epistemic contact with their e-qualia.

None of these can help e-qualists.

(a) If the intrinsic properties of my physical cognitive processes could put me into

epistemic contact with anything, they could do it for my cranial currents; which they don't. Nor could any changes to them make a relevant difference, since the argument depends only on the broad features of an E-world without reference to any physical details.

(b) Keep in mind that the cranial currents case is a counter-example to the general suggestion that if certain suitable physical cognitive processes cause or are

isomorphic to something - anything- which has no effects on them, then they are

enough to put someone into epistemic contact with it. It is a counter-example regardless of the intrinsic properties of the item supposedly caused by and isomorphic to the physical processes in question. So it remains a counter-example if that item

happens to consist of e-qualia, and regardless of what the intrinsic properties of those

e-qualia may be. Their intrinsic properties could be whatever you please (provided they remained inert); I should still not be able to notice, think about, attend to, remember, or compare them.

If e-qualists are going to be able to escape the cranial currents argument, therefore, it must be on the basis of (c) the ways in which the physical and non-physical components of an E-world are related (presumably by natural necessity). But if causation by, and

isomorphism to, physical cognitive processes are not enough for epistemic contact, what else might e-qualists appeal to? The only other suggestion I know of is that the counterfactual dependence of particular e-qualia on particular preceding physical events

might be enough. But that will not work because the cranial currents case remains a

counter-example. If ours were an E-world where the physical processes which caused

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80 R. Kirk

e-qualia also caused isomorphic patterns of cranial currents, there would be the same counterfactual dependence between those currents and subsequent physical events as the

present suggestion envisages between e-qualia and those physical events. Since I would remain out of epistemic touch with my cranial currents, the same must go for my e-qualia.

The cranial currents case is more than a counter-example to the two suggestions just discussed. It highlights features of an E- world which ensure that e-qualia are epistemically insulated from all cognitive processes. Both e-qualia and cranial currents are (we imag- ined) caused by the same brain processes, while neither has any effects on them. Those two cases have the same essential structure; the only differences are between the intrinsic

properties of e-qualia and those of cranial currents. But we saw two paragraphs back that the details of those intrinsic properties cannot affect the argument; so the differences between e-qualia and cranial currents are not relevant. It follows that if any feature of an E- world could make our brain processes put us into epistemic contact with our e-qualia, it would equally make them put us into epistemic contact with our cranial currents. But -

given the scenario sketched earlier - nothing could put us into epistemic contact with our cranial currents; therefore nothing could put us into epistemic contact with our e-qualia. (See also Objection 3 in the next section.)

We have considered all the resources by means of which e-qualists might have been able to escape the cranial currents counter-example. It turns out that they cannot do the job. It is the overall structure of the e-qualia story, not the details of the realm of e-qualia, still less the details of the physical realm, that would prevent the physical processes in an E- world from putting anyone into epistemic contact with e-qualia. I conclude that, quite generally, clauses (E1)-(E4) of the e-qualia story are inconsistent with (E5).

E-qualists will be keen to press objections. After summarizing the argument so far, I will reinforce it by discussing those I have come across.

8 The argument so far

1. E-qualia are inert (E3). 2. Being in epistemic contact with one's e-qualia involves activities such as noticing,

attending to, thinking about, remembering, and comparing them ((E5) and Sect. 4). 3. These activities involve causation and the persistence of unconscious items and

structures, and therefore (in an E-world) can be performed only by physical processes (Sect. 4).

4. That raises the question (Q) of how physical processes could put anyone into epistemic contact with e-qualia (Sects. 5 and 6).

5. One suggestion is that the causation of e-qualia by, and their isomorphism to, certain physical processes might combine to put E-worlders into epistemic contact with their e-qualia (Sects. 5 and 6).

6. The example of the patterns of electric currents induced in my brain by the relevant physical processes is a counter-example (Sect. 7).

7. It is also a counter-example to the idea that the intrinsic properties of e-qualia or of the physical component of an E-world might provide for epistemic contact, and to the suggestion that counterfactual dependence of particular e-qualia on particular preceding physical events might do the trick (Sect. 7).

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8. The same example helps to bring out the fact that it is the overall structure of the e-

qualia model of consciousness which prevents physical processes from putting anyone into epistemic contact with e-qualia (Sect. 7).

9. Thus no one in an E-world could be in epistemic contact with e-qualia in the sense of clause (E5) of the e-qualia story, and the e-qualia story involves a contradiction: it is inconceivable in the relevant sense (Sect. 7).

9 Objections

Objection 1: 'It looks as if the argument presupposes a causal account of aboutness and reference. If so, you're after all just pitting your intuitions against those of your oppo- nents - and begging the question.'

Reply 1: That's a misunderstanding. The argument doesn't presuppose or in any way depend on a causal account of aboutness. It depends on the fact that e-qualia cannot do

cognitive processing and are epistemically insulated from the physical processes which do such processing. Certainly, part of the trouble is that e-qualia have no effects (and that mere counterfactual dependence would not be enough). But another is that they cannot

provide for the persisting unconscious structures involved in cognitive processing. The

upshot is that the relevant cognitive processing would have to be done by physical processes; while the cranial currents example helps to show how the structure of the e-qualia model of consciousness is guaranteed to prevent such processes from being able to put an individual into epistemic contact with e-qualia. Reflection on the reasoning might incline one to favor a causal account of aboutness and reference; but the argument doesn't depend on a causal account.

Objection 2: 'Your presentation reflects a tendentious view of the subject of experience. You imply it is the functioning body or brain: a conception in which e-qualia are add-on extras. From that perspective the notion of e-qualia can seem mysterious. But for e-qualists a different perspective is more natural, according to which the primary locus of the subject is in the stream of consciousness itself. Far from the subject being constituted by physical processes, those processes are in a way peripheral.'

Reply 2: Suppose for argument's sake that the e-qualia caused by physical processes in an individual's body did form a stream of consciousness - even a subject. By the argument in Sect. 7, those e-qualia' s inertness would still ensure that they and the putative subject were epistemically insulated from all cognitive processes. No subject could think about, notice, attend to, or remember items in that stream of consciousness: no one could be in

epistemic contact with them in the relevant sense. So this objection does nothing to undermine the argument based on the cranial currents case.13

Objection 3: 'You overlook a crucial possibility: we might stand in an epistemic relation of acquaintance to e-qualia. That would break the analogy between e-qualia and cranial currents. Its mere possibility blocks the inference from cranial currents to e-qualia, rendering the cranial currents case irrelevant and demolishing the whole argument of Sect. 7. When e-qualia are caused by bodily processes, the acquaintance relation puts the subject into epistemic contact with them, within the stream of consciousness. Beneath the surface

13 It would not help e-qualists to say e-qualia are subjectless: that would only support my claim that no one could be in epistemic contact with them. On the other hand, I see no objection to the notion of integrated, nonrelational processes of having-qualia (see Kirk, 2005, pp. 154-158)- but that is obviously inconsistent with the e-qualia story.

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of consciousness there are various kinds of information processing. By virtue of the counterfactual dependence of these underlying processes on e-qualia, and the causal impact they have back on the stream of consciousness, the subject is after all in a position to notice, compare, and remember e-qualia.'14

Reply 3: Keep in mind that what e-qualists have to explain is how, in an E-world, physical processes in an individual body could contribute to the relevant sort of epistemic contact with e-qualia: that is question (Q). I will argue that the suggested notion of 4 acquaintance' goes no way towards answering that question.

Evidently the relata in the acquaintance relation are supposed to be e-qualia on the one hand and 'the subject' on the other. But something must underlie this relation: must make the difference between its holding and its not holding. What could that something be? We know the only resources available are: (a) intrinsic properties of the physical component; (b) intrinsic properties of e-qualia; (c) the ways in which natural necessity might relate those properties. None of these can provide for the relevant kind of epistemic contact.

(a) is ruled out directly by the cranial currents example. Since the intrinsic properties of the physical component of an E-world could not put me into epistemic contact with my cranial currents, they could not put me into epistemic contact with anything else that was related to my physical cognitive processes only by being caused by and isomorphic to them.

The objection seems to envisage that (b) what underlies the relation is an intrinsic

property of e-qualia. But this thought was anticipated and dismissed in Sect. 7. My cranial currents are a counter-example to the suggestion that if something - regardless of its nature - were caused by and isomorphic to some of my physical processes (while not affecting them), then these would put me into epistemic contact with that something in the relevant sense (Sect. 7). It follows, as we saw, that the intrinsic properties of the item in question make no difference to the argument. Whatever they may be, they are locked up beyond the epistemic reach of my cognitive processing. So if what underlies the acquaintance relation is an intrinsic property of e-qualia, then both that property and the subject are marooned within that realm, and cannot put me into any relevant kind of epistemic contact with e-qualia.

It is suggested that (c) acquaintance operates in combination with the counterfactual dependence of underlying physical processes on relations among e-qualia. This dependence, together with the effects of those physical processes on the stream of consciousness, is supposed to put the subject 'in a position to notice, compare, and remember e-qualia'. However, I argued in Sect. 7 that counterfactual dependence could not provide for that kind of epistemic contact; and we have just seen that the suggested relation of acquaintance does not block that argument.

Objection 4: 'You're construing the special epistemic access that e-qualists claim we have to our qualia on the model of familiar kinds of epistemic access such as sense perception. But according to e-qualists, our epistemic access to qualia is private in this sense: to have them is to know them. The accessibility of electric currents is just observability: they are observable by anyone suitably equipped. So the cranial currents example doesn't work because patterns of electric currents are nothing like what e-qualia are supposed to be.'

Reply 4: The argument shows there is no way anyone could 'have' e-qualia in the relevant sense: no one could be their subject and know them. E-qualia are certainly very

Thanks to a reviewer for this and the preceding objection, and for the phrasing of the suggested perspective on consciousness in an E-world.

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different from electric currents, and our relation to them would indeed be unlike our relation to such things. But the argument exploits a fact which holds regardless of how different those relations and their relata may be in other respects: that the cognitive activities involved in epistemic contact cannot be performed by e-qualia and must be

performed by physical processes (Sect. 4) - which cannot be epistemically linked with e-

qualia (Sect. 7). Objection 5: 'E-qualists need only point out that a different conception of e-qualia

might be devised, according to which they were causally active.'

Reply 5: That is not in the e-qualia story; and I am arguing only that the e-qualia story itself, not some variant of it, is inconceivable. (See also Sect. 10 below.)

I conclude that the conclusion stands. E-worlders' physical processes cannot put anyone into epistemic contact with their e-qualia in the sense of (E5), so that (E5) is inconsistent with the rest of the e-qualia story. That completes my case for thesis (A): in the relevant

sense, the e-qualia story is inconceivable. I now have to show (B): that the conceivability of zombies would entail the conceivability of the e-qualia story. The argument has two

phases, set out in the next two sections.

10 Conceivability of zombies would entail conceivability of inert conscification

The claim is not that the conceivability of zombies would entail that the e-qualia story was

true,15 only that it was conceivable. So I have to show that the conceivability of zombies would entail that the e-qualia story cannot be known a priori to be false (see Sect. 3 above). The broad idea is simple: if zombies were conceivable, then conceivably what started off as a zombie twin of our world could be transformed into a world where the e-qualia story held. I expect many readers will concede that straight off, but it is vital to see that some

apparent escape routes are blocked; this needs some care. Not all zombists are dualists: some are physicalists, some idealists, some panpsychists.

However, as I will explain shortly, all zombists are committed at least to the conceivability of dualism: the view that the world consists of a physical component and a non-physical component which are 'separate existences' in the sense that neither entails the existence of the other; and the non-physical component makes us conscious. (The non-physical com-

ponent might consist of our minds, or non-physical qualia, or a single cosmic center of consciousness. For convenience I will refer to the non-physical component in the singular.)

Here is why zombists are committed to the conceivability of dualism. Given the definition of zombies (Sect. 2), the conceivability of zombies entails the conceivability of a

purely physical world that is closed under causation, and whose inhabitants are not only behaviorally like us, but physically just as physicalists suppose we are - yet not conscious. I assume it is at least conceivable that we are in physical respects just as physicalists suppose we are. So, since zombies would lack something we have, what we have and they would lack must be non-physical. This non-physical component of our world, which made us conscious, would be logically independent of its physical component.16 That is dualism. So if zombies are conceivable, so is dualism.

15 See n. 8 above. 16

'Logically' independent in the sense that its existence was not entailed (or a priori necessitated) by the

physical component.

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84 R. Kirk

I will now argue that zombist dualism commits you to the conceivability of something very close to the e-qualia story: 'inert conscification'. (In the next section I will go on to

argue that the conceivability of inert conscification entails that of the e-qualia story itself.) We can take it that dualism is either interactionist, epiphenomenalist, or parallelist.

Now, epiphenomenalists believe the physical component of our world is a causally closed

system, while the non-physical component is caused by the physical component but inert. Just as their position entails that, conceivably, the extinction of this inert non-physical component would transform our world into a world of zombies, so it entails that, con-

ceivably, the genesis of a suitable inert non-physical item in a zombie twin of our world would result in the existence of a (phenomenally) conscious human-like population.17 Thus

epiphenomenalists are committed to the conceivability of inert conscification in the fol-

lowing sense:

(C) A causally inert non-physical item if/ could be associated with a zombie twin z of our world so as to transform it into a world z* whose inhabitants enjoyed our kind of

phenomenal consciousness, including epistemic contact, ij/ is caused by the physical component of z*.

Parallelists too, although they hold there is no causal action between the physical and the

non-physical, are committed to the conceivability of (C). For they must hold that causal relations are contingent, so that even if a does not in fact cause b, it must be conceivable that it should do so.

Zombist interactionists, however, may think they can escape commitment to (C), first because they reject the causal closure of the physical; second because they insist that the

non-physical component of our world is causally active (ert?). I will argue that, even so, they must concede that (C) is conceivable. Before defining zombist interactionism I will refine the definition of a 'zombie twin' world. A world z is a zombie twin of ours just in case:

z is a purely physical, causally closed system; z is physically as like the actual world as possible (with physical causes substituted for non-physical ones where necessary); the human-like inhabitants of z lack phenomenal consciousness.

Zombist interactionism can now be defined:

(ZI) Zombies are conceivable and the actual world consists of a physical component and a non-physical component. The latter includes (or consists of) something i/^* such that: (i) ij/* makes us phenomenally conscious and keeps us in epistemic contact with our experiences; (ii) i/^* has effects on the physical component of the world and is affected by it.

I will argue that (ZI) entails that conceivably our world as thus characterized - I will refer to it as i - could be transformed into a world like z*; hence that (C) is conceivable. What forces zombist interactionists into this position is their peculiar conception of consciousness. For (ZI) entails the following three propositions.

(1) Conceivably the laws of nature governing i (the world according to (ZI)) could change at a certain time so that from that time on: (i) no non-physical items in i had effects; (ii) whatever had been directly or indirectly caused by non-physical items

17 Conscification need not be thought of as zombies becoming conscious, but only as the coming into existence of conscious subjects whose physical components had been zombies.

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was instead caused by physical items, so that every physical effect now had a physical cause; (iii) all non-physical items other than \j/* ceased to exist, so that ij/* was the

only non-physical item.

(2) If the changes described in (1) are conceivable, then it is also conceivable that after the changes ij/*, in spite of being inert, should continue to make the inhabitants of i conscious and keep them in epistemic contact with their experiences.

(3) Given (1) and (2), (C) is conceivable.

If (ZI) entails (1), (2), and (3), obviously it entails that (C) is conceivable. I will argue that

(ZI) does indeed entail those premisses. Premiss (1): Condition (i) is unproblematic. Causation is contingent, so zombists cannot

deny that conceivably anything that actually has effects - including i/^* - might have ex- isted without having effects.18

(ZI) also entails that condition (ii) is straightforwardly conceivable together with (i), at least for the case of physical effects. For (ZI) has it that zombies are conceivable; and in a zombie world all physical events in i would be caused physically. (One result would be that our successors in i behaved exactly like us. Some interactionists might deny that physical events could cause human-like behavior, but they could not be zombists.)

As to won-physical effects, the question is whether there are any which could not

conceivably be produced physically; and interactionists can have no a priori objections to that. So (ZI) entails that conceivably conditions (i) and (ii) are jointly satisfiable.

(iii): since (ZI), being a variety of dualism, entails that the non-physical component of

reality is logically independent of the physical component, it entails that that all non-

physical items in i might conceivably cease to exist; a fortiori that conceivably all non-

physical items other than \j/* could cease to exist, leaving ^* (now inert) as the only surviving non-physical item. (In that case, notice, if i//* ceased to exist the result would be a zombie twin of our world.) Clearly there is no inconsistency between that and conditions

(i) and (ii); so (ZI) entails (1). Premiss (2): Evidently, one consequence of the changes envisaged in conditions (i) and

(ii) of premiss (1) would be that the physical component of i was closed under causation.

Also, because all those kinds of physical items that had previously been caused non-

physically would continue to be caused (though physically), i would remain physically similar to what it had been; in fact the physical component of i would be exactly like a zombie twin of our world. The question now is whether (ZI) entails that conceivably i/f* would continue to make the inhabitants of i conscious and keep them in epistemic touch with their experiences, or whether the changes would absolutely19 prevent that.

Consider, then, what differences the changes would make to i. They are that (in i after the changes) (i) no non-physical items have effects; (ii) physical items have all the kinds of effects that were originally produced by non-physical items; (iii) the only non-physical item is i/^*. (i) and (ii) are crucial, of course, since they stop i from being an interactionist

18 Cartesian zombists might demur. If thinking - a kind of activity - is essential to the soul's existence, then

i//* cannot cease to be a cause without ceasing to exist. Also, some interactionist zombists might maintain it is a priori necessary that consciousness involves non-physical causes. See, however, the discussion of

premiss (2) below. (Note that zombists cannot resist condition (i) by invoking causal essentialism, according to which a thing's causal dispositions are essential to it. For if that doctrine is taken to entail that what

physicalists count as physical items cannot conceivably exist without causing or being caused by conscious states, then zombies are inconceivable for that reason; while if it lacks that entailment, then it lets in (i).) 19 It would not be enough to claim that conceivably the changes might prevent ij/* from continuing to make our successors conscious; (2) says only that (ZI) entails it is conceivable that it should do so.

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86 R. Kirk

world and put it on the way to being like z*. But do they absolutely prevent ifr* from

contributing to making fs inhabitants conscious? If so, that must be on account either of

(a) what {//* and any other non-physical items were doing before the changes - what they caused, what functions they performed - or/and of (b) what those non-physical items were: their nature.

(a): what causes what is contingent, at least for zombists. The laws of nature may preclude the physical causation of items supposedly caused by non-physical items; but zombist interactionists cannot deny it is conceivable that those things should be done

physically. So (a) appears to be ruled out. (b): since the changes leave t/f* untouched, they cannot affect its nature. So we can

dismiss (b) too. Since \j/* includes whatever 'makes us conscious and keeps us in epistemic touch with

our experiences', it is hard to see how the third difference - that after the changes there are no non-physical items other than t/f* - could prevent \j/* from continuing to make the inhabitants of i conscious.

Thus there is at least a strong case for the claim that (ZI) entails the conceivability of

premiss (2). However, zombist interactionists might think I have not done them justice. They might urge that it is not by merely nomic necessity that what provides for consciousness is causally efficacious: they might claim it is a priori necessary; so that even if

\j/* continued to exist after the changes, its inertness would prevent its continuing to make

anyone conscious.20 Now, I agree with the widespread view that whatever makes us conscious cannot be inert. But I will argue that zombists are committed to a conception of consciousness which entails that the contrary is at least conceivable. (Interactionist zombists might endorse the correct view about consciousness and causality; but if I am

right that would make them inconsistent: their zombism commits them to an incorrect view as well. In that case, appealing to the correct view cannot protect their incorrect view from the arguments of Sects. 3-9.)

Like most of us, zombists claim to know they are conscious. Unlike some of us, they cannot justify this claim by reference to observation of physical facts such as behavior -

because they think zombies would be physically indistinguishable from us, at least

superficially. This means that interactionist zombists must think their knowledge that they are conscious, hence their knowledge that i//* exists, comes from the fact that they have conscious experiences themselves, not from knowing any physical facts. A consequence is that they cannot consistently claim it is a priori that if/* must have effects in order to make us conscious. If we can know we are conscious by actually having conscious experiences, then we can know it without also knowing whether or not consciousness has effects. So zombist interactionists cannot maintain it is inconceivable that \j/* should make us conscious in spite of being inert. They may point out that we observe what we take to be effects of our being conscious. But we cannot observe that they are effects of i//*: that is

part of a theory which, for them, might conceivably be mistaken. (Consistently with our

experience, those effects and \j/* might be joint effects of some common cause, for

example.) Zombist interactionists might raise another objection. They might accept that epistemic

contact requires consciousness to have effects, but maintain that even if ij/* continued to make our successors conscious, its lack of causal efficacy would prevent it from continuing to sustain epistemic contact.21 I find it hard to make sense of that suggestion. Significantly,

20 Here I take into account the worries mentioned in note 18. 21 Thanks to an anonymous reviewer for this suggestion.

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Chalmers has said there is "not even a conceptual possibility" that a subject should have a red experience "without any epistemic contact with it".22 And surely he is right about that. After all, we are discussing phenomenal consciousness, the idea that there is something it is like for the subject. If there is nothing it is like, the subject is not phenomenally conscious. Now, could there be anything it was like for a subject who had phenomenally conscious

experiences but could not notice, attend to, or compare them? How could that be different from what it was like to be completely unconscious: like nothing? Zombists typically base their claims about what is conceivable on what is imaginable. But can we even imagine the situation described? When I imagine myself being phenomenally conscious, I automati-

cally imagine myself having experiences that I can notice, attend to, remember, or com-

pare. When I try to imagine not being able to do those things, imagination fails: I cannot take the first step of an argument for the conceivability of the situation described. Nor does that seem to be a psychological defect. It seems more like a symptom of the fact that being in epistemic contact with one's conscious experiences is part of what it is to have them. Absent any argument to the contrary, I conclude it is not conceivable that our successors in i should be conscious without being in epistemic contact with their experiences. If that is

right, the objection fails.23 Premiss (3): We noted that after the changes, the physical component of i would be

closed under causation and resemble a zombie twin of our world. The difference between i and a zombie world is of course ^*, which, though caused by something physical (or at least conceivably so caused24) is inert. As we have just seen, iff* would make i's inhabitants conscious and ensure they were in epistemic contact with their experiences. A look at (C) confirms that if we take i//* = ^, those features of i after the changes are the defining features of z*. Hence if (ZI) commits its exponents to (1) and (2), it commits them to the

conceivability of (C). Earlier I argued that all zombists are committed to the conceivability of dualism, and

that epiphenomenalists and parallelists are committed to the conceivability of inert conscification as defined by (C). I have just argued that interactionist zombists are committed to

(1), (2), and (3), and conclude they too are committed to the conceivability of (C). Given that all dualists belong to one or other of those classes, it follows that all zombists are committed to the conceivability of inert conscification, or (C).

11 Conceivability of inert conscification would entail conceivability of the e-qualia story

It is easy to see that the definitions of a 'zombie twin' world and of (C) ensure that the

conceivability of (C) entails the conceivability of a world z* satisfying the following conditions, which mirror (E1)-(E5):

(Zl) z* is partly physical and its physical component is closed under causation: every physical effect has a physical cause.

22 Chalmers (1996, p. 197). 23 Zombist interactionists might consider maintaining that i^*'s loss of causal efficacy would cut off not

only epistemic contact but consciousness too. But by the argument of the last paragraph that would prevent them from being zombists. 24 (ZI) does not appear to entail that t/f* is caused by physical items in i, only that it is affected by them. However, given causation is contingent (n. 18), (ZI) does entail it is conceivable that i/f* should be caused

physically.

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88 R. Kirk

(Z2) The human-like organisms in z* are related to a special kind of non-physical item \j/. \j/ makes them phenomenally conscious.

(Z3) \j/ is wholly caused by physical processes but inert: it has no effects either on the

physical world or internally. (Z4) The human-like inhabitants of z* consist of nothing but functioning bodies and i//. (Z5) The human-like inhabitants of z* are able to notice, attend to, think about, compare,

and (on occasions) remember the experiences provided for by i//.

So the conceivability of (C) entails the conceivability of (Z1)-(Z5). But that entails the

conceivability of (E1)-(E5). For there are only three differences between (Z1)-(Z5) and (E1)-(E5): "z" occurs in place of "the world"; "human-like inhabitants" in place of "human beings"; "a special kind of non-physical item (or items) ^" in place of "e-

qualia". The first two are not significant, since they arise from the fact that while (El)- (E5) are a story about how the actual world and its human inhabitants might conceivably have been, (Z1)-(Z5) are about a special kind of world and its human-like inhabitants -

which is also how the actual world and its human inhabitants might have been on the

assumption that zombies are conceivable. Obviously those differences do not prevent the

conceivability of (Z1)-(Z5) from entailing the conceivability of (E1)-(E5). The third difference may appear troublesome. What compels zombists to concede that

conceivably a world satisfying (Z1)-(Z5) might have e-qualia as its special non-physical component ij/, rather than something different? There is no real difficulty here. By (C), \f/ makes z*'s inhabitants conscious and thereby ensures that their experiences have, in Chalmers's words, "those properties of mental states that type those states by what it is like to have them" (see Sect. 3). That means i// underlies their qualia in a broad, neutral sense of Qualia'. By (C), \j/ is also inert and caused by physical items, so the same must go for these qualia. Moreover, there is nothing in i other than its physical component and ij/. But then these qualia satisfy those parts of the e-qualia story which define qualia (clauses (E2), (E3), and (E4)) and must be counted as e-qualia.

So the conceivability of inert conscification, or (C), entails the conceivability of (El)- (E5). We saw in the last section that the conceivability of zombies entails that of (C). So the conceivability of zombies entails that of (E1)-(E5), which gives us:

(B) If zombies were conceivable, the e-qualia story would be conceivable.

But we already have:

(A) The e-qualia story is not conceivable.

By contraposition, zombies are not conceivable.25

25 Special thanks to Bill Fish for much detailed discussion and correspondence, and to David Chalmers for

comments, suggestions, and encouragement through several revisions.

References

Chalmers, D. (1996). The conscious mind. New York: Oxford University Press. Chalmers, D. (1999). Materialism and the metaphysics of modality. Philosophy and Phenomenological

Research, 59, 475^96. Chalmers, D. (2002). Does conceivability entail possibility? In T. Gendler, & J. Hawthorne (Eds.), Con-

ceivability and possibility (pp. 145-200). Oxford: Oxford University Press. Dennett, D. C. (1991). Consciousness explained. Boston: Little, Brown.

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Dennett, D. C. (1995). The unimagined preposterousness of zombies. Journal of Consciousness Studies, 2, 322-326.

Gendler, T., & Hawthorne, J. (Eds.) (2002). Conceivability and possibility. Oxford: Oxford University Press. Kirk, R. (1974). Zombies v. materialists. Aristotelian Society Proceedings, supplementary vol. 48, 135-152. Kirk, R. (2005). Zombies and consciousness. Oxford University Press. Kripke, S. (1980). Naming and necessity. Cambridge: Harvard. Perry, J. (2001). Knowledge, possibility, and consciousness. Cambridge, MA: MIT Press. Ryle, G. (1949). The concept of mind. London: Hutchinson. Shoemaker, S. (1981). Absent qualia are impossible. Philosophical Review, 90, 581-599. Shoemaker, S. (1999). On David Chalmers's The Conscious Mind. Philosophy and Phenomenological

Research, 59, 475^96. Stout, G. F. (1931). Mind and matter. Cambridge: Cambridge University Press. Tye, M. (2006). Absent qualia and the mind-body problem. Philosophical Review, 115, 139-168. Wittgenstein, L. (1953). Philosophical investigations (G. E. M. Anscombe, Trans). Oxford: Blackwell.

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