Final draft. Published in Philosophical Studies 139 (2008): pp. 73-89.

The Inconceivability of Zombies

Robert Kirk

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.

I. 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 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

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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 sections III-IX; (B) in sections X and XI.

II. ‘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

parallelism 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 them are produced physically. They can be defined as follows: creatures

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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

conceivability 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.

III. 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.

(E1) 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 (E1) 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

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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 (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 (E1)-(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 (E1)-(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.

IV. 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

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(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

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.

V. 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 (E1) 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,

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that a single temporally extended complex of e-qualia, ψ, is introduced into this zombie

world. ψ 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 ψ: think about, attend to, compare, or remember some of the e-qualia in it?11

No. ψ 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 ψ with one particular E-worlder rather than with any other. Therefore nothing

could put any particular E-worlder into epistemic contact with ψ. 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 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-

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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.

VI. 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.

VII. 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

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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 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 isomorphic 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.

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(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 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 imagined) 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

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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.

VIII. 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 section IV).

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

(IV).

4. That raises the question (Q) of how physical processes could put anyone into

epistemic contact with e-qualia (V and VI).

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 (VI, VII).

6. The example of the patterns of electric currents induced in my brain by the relevant

physical processes is a counter-example (VIII).

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 (VIII).

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 (VIII).

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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 (VIII).

IX. 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

opponents – 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 section VII, 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

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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

section VII. 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 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 ‘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 section

VII. 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 (section VII). It follows, as we saw, that the

intrinsic properties of the item in question make no difference to the argument.

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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 section VII 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

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 (section IV) – which cannot be epistemically linked

with e-qualia (section VII).

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 X below.)

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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.

X. 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 III

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 component 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 (section II), 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.

14

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, conceivably, 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 following sense:

(C) A causally inert non-physical item ψ 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. ψ 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.

15

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 ψ* such that:

(i) ψ* makes us phenomenally conscious and keeps us in epistemic contact with our

experiences; (ii) ψ* 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 was

instead caused by physical items, so that every physical effect now had a physical cause;

(iii) all non-physical items other than ψ* ceased to exist, so that ψ* was the only non-

physical item.

(2) If the changes described in (1) are conceivable, then it is also conceivable that after

the changes ψ*, 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 ψ* – might have

existed 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.

16

(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 non-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 ψ* could cease to exist, leaving ψ* (now

inert) as the only surviving non-physical item. (In that case, notice, if ψ* 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 ψ* 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 ψ*. (i) and (ii) are crucial, of course, since they stop i from being

an interactionist world and put it on the way to being like z*. But do they absolutely

prevent ψ* from contributing to making i’s inhabitants conscious? If so, that must be on

account either of (a) what ψ* 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.

17

(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 ψ* untouched, they cannot affect its nature. So

we can dismiss (b) too.

Since ψ* 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 ψ* – could prevent ψ* 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 ψ* 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 sections III-IX.)

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 ψ* 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 ψ* 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 ψ* 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

18

they are effects of ψ*: that is part of a theory which, for them, might conceivably be

mistaken. (Consistently with our experience, those effects and ψ* 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 ψ*

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, 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

automatically imagine myself having experiences that I can notice, attend to, remember,

or compare. 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 ψ*, which, though caused by something

physical (or at least conceivably so caused24) is inert. As we have just seen, ψ* 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 ψ* = ψ, 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).

19

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).

XI. 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):

(Z1) z* is partly physical and its physical component is closed under causation: every

physical effect has a physical cause.

(Z2) The human-like organisms in z* are related to a special kind of non-physical item

ψ. ψ makes them phenomenally conscious.

(Z3) ψ 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 ψ.

(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 ψ.

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) ψ” in place of

“e-qualia”. The first two are not significant, since they arise from the fact that while

(E1)-(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).

20

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 ψ, rather than something different? There is no real difficulty

here. By (C), ψ 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 section III). That means ψ underlies

their qualia in a broad, neutral sense of ‘qualia’. By (C), ψ 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 ψ. 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 (E1)-(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

_____________________

NOTES

1 For the view that the zombie possibility entails the falsity of physicalism see e.g.

David Chalmers, The Conscious Mind (New York: Oxford University Press, 1996); R.

Kirk, Zombies v. Materialists, Aristotelian Society Proceedings, supplementary vol. 48

(1974): 135-152 and Zombies and Consciousness (Oxford University Press, 2005):

21

7-23. For the conceivability argument see e.g. Saul Kripke, Naming and Necessity,

(Cambridge: Harvard, 1980); Chalmers, Does Conceivability Entail Possibility? in T.

Gendler and J. Hawthorne, eds., Conceivability and Possibility (Oxford: Oxford

University Press, 2002): 145-200; and other essays in Gendler and Hawthorne, eds.2 Indirect objections: Gilbert Ryle, The Concept of Mind (London: Hutchinson, 1949,

and Ludwig Wittgenstein, Philosophical Investigations, tr. G. E. M. Anscombe (Oxford:

Blackwell, 1953); direct ones: e.g. Daniel C. Dennett, Consciousness Explained

(Boston: Little, Brown, 1991) and The Unimagined Preposterousness of Zombies,

Journal of Consciousness Studies, 2 (1995): 322-6; R. Kirk, Zombies and

Consciousness: 37-57; Sydney Shoemaker, Absent Qualia are Impossible, Philosophical

Review, 90 (1981): 581-599, and On David Chalmers’s The Conscious Mind,

Philosophy and Phenomenological Research, 59 (1999): 439-444; Michael Tye, Absent

Qualia and the Mind-Body Problem, Philosophical Review, 115 (2006): 139-68.

3 This outline mirrors that of Kirk, Zombies and Consciousness: 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 G. F. Stout, Mind and Matter (Cambridge: Cambridge University Press, 1931): 138f.

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: 438-440): Kirk, Zombies and

Consciousness: 97-118. The definition of zombie twin worlds is refined in section X

below.

6 See for example John Perry, Knowledge, Possibility, and Consciousness, (Cambridge

Mass: MIT Press, 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” (Materialism

and the Metaphysics of Modality, Philosophy and Phenomenological Research, 59

22

(1999): 475-496, p. 493. See also his The Conscious Mind: 150-160), and sections X

and XI 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 The Conscious Mind, p. 359, n.2.

9 Chalmers uses ‘epistemic contact’ for what he calls ‘acquaintance’ with qualia (The

Conscious Mind, p. 197). I will consider this notion later (section IX, 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

section VII.

11 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).

12 Epiphenomenalists typically assume isomorphism. See e.g. Chalmers, The Conscious

Mind, p. 243.

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

Zombies and Consciousness: 154-8) – but that is obviously inconsistent with the e-

qualia story.

14 Thanks to a reviewer for this and the preceding objection, and for the phrasing of the

suggested perspective on consciousness in an E-world.

15 See n. 8 above.

23

16 ‘Logically’ independent in the sense that its existence was not entailed (or a priori

necessitated) by the physical component.

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.

18 Cartesian zombists might demur. If thinking – a kind of activity – is essential to the

soul’s existence, then ψ* 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 ψ* from

continuing to make our successors conscious; (2) says only that (ZI) entails it is

conceivable that it should do so.

20 Here I take into account the worries mentioned in note 18.

21 Thanks to an anonymous reviewer for this suggestion.

22 The Conscious Mind, p. 197.

23 Zombist interactionists might consider maintaining that ψ*’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

24 (ZI) does not appear to entail that ψ* 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 ψ* should be caused physically.

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

David Chalmers for comments, suggestions, and encouragement through several

revisions.

25