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philosophical fire. But precisely for that reason, it is important to pay attention to the particulars in using

zombies as a tool of imagination in thought experiments" (1995: 327).

Güzeldere provides a taxonomy of zombies based on the question of how zombies are identical to

conscious creatures: Are they supposed to be behaviorally, functionally, or physically3 identical toconscious beings? (1995) Behaviorally identical zombies make all the overt movements and utterances

that conscious creatures do, but they may have any internal structure and may be composed of whatever

material.4 Behaviorally identical zombies needn't be hollow shells, they could be quite sophisticated;however, in considering behaviorally identical zombies their internal organization is left unspecified.

Functionally identical zombies not only make the movements that conscious creatures do, they also

have in some sense the same internal organization that conscious creatures do. Physically identicalzombies are identical to conscious creatures cell for cell, molecule for molecule, atom for atom. These

three ways in which zombies could be stipulated as identical to conscious creatures parallel three

families of theories: behaviorism, functionalism, and Identity Theory.5 These are some of the theoriesthat zombies can help us explore.

Owen Flanagan and I have drawn also a modal distinction regarding zombies, as has DavidChalmers.6 We distinguish not only the sort of similarity that zombies might bear to human beings, butalso the sense in which zombies are supposed to be possible: Are zombies logically, metaphysically, or

naturally possible? (Flanagan and Polger 1995; Chalmers 1996). Logically possible, I take it, just

means not contradictory .7 There is some question about whether there is an even weaker sort of possibility, something like conceivability (Horgan 1987) or epistemic possibility (Kripke 1972).

Naturally possible I take to be something like compatible with all and only the actual substances and

laws of nature.8 There is no general agreement about what metaphysical possibility is. Whether or not

metaphysical possibility is connected to conceivability is a point of contention; although there is de factoconsensus that conceivability (or imaginability, if that is different) is our only guide to metaphysical

possibility.

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Figure 1. Zombie Scorecard.

Taken together these distinctions form the axes of what I call the Zombie Scorecard (Polger 2000).

Crudely put, the zombies that we are asked to conceive increase in strength (i.e., how much you arerequired to imagine) from left to right and from bottom to top of the Zombie Scorecard. The strongest

claim is of the natural possibility of physically identical zombies. The weakest claim is of the logical

possibility of behaviorally identical zombies. The Zombie Scorecard is a way of organizing questions

about zombies. For example, in box (7) we ask the question:

(Q7) Is it logically possible that there be zombies that are behaviorally identical to human beings?9



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Can we answer this question? Although no claim about zombies is entirely uncontroversial, most

philosophers these days would regard as harmless the logical possibility of behaviorally identical

zombies. Nevertheless, someone who is inclined towards analytic behaviorism, for example, would notaccept this possibility; and Dennett rejects (7) on the grounds that behavioral identity would require

functional identity, and he denies the coherence of functionally identical zombies (1995; see my

"Zombies Explained").

2. Getting to Know Zombies

Below I provide a tour of the zombie questions, but it is not my purpose to recount every reason for

which any thinker accepts or rejects each of the nine possibilities. My goal, rather, is to illuminate the

structure of the problem and the surrounding debates. So before we look at the zombies themselves, letus attend to some nuances of the debate.

1. Comparative Anatomy of Qualia Mutants

If the zombie questions are asked in the form, "Is it y-ly possible that there be creatures that are x-lyidentical to human beings but which lack consciousness?" (where y is a mode of possibility and x is a

degree or kind of identity) then someone might object that this begs the question as to whether human

beings are conscious. To avoid any appearance of impropriety, the questions should be rephrased, "Is

it y-ly possible that there be two creatures that are x-ly identical to one another but differ in that one isconscious and the other is not?"

This formulation has the additional advantage of making transparent how the form of the zombieconstruct is related to absent, inverted, alien, and dancing qualia thought experiments:

Zombies and Absent Qualia: Is it y-ly possible that there be two creatures that are x-ly identical toone another but differ in that one is conscious and the other is not?

Inverted Qualia: Is it y-ly possible that there be two creatures that are x-ly identical to one another but differ in that one's conscious states are "inverted" with respect to the other's?



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Alien Qualia: Is it y-ly possible that there be two creatures that are x-ly identical to one another but

differ in that one has conscious states that are entirely different in quality from those had by theother?

Dancing Qualia: Is it y-ly possible that there be two creatures that are x-ly identical to one another but differ in that one is always conscious and the other sometimes has the same sort of

consciousness as the first and other times has a different sort (inverted or alien qualia) or none atall (absent qualia)?

These questions are all phrased in terms of interpersonal comparisons. If you think that the interpersonal

cases are always or sometimes ill-defined (what Shoemaker (1982) calls the Frege-Schlick view), youcan still ask all of these questions in their intrapersonal form. To do so, simply consider the two

creatures in the above formulations as two creature-stages; that is, "Is it y-ly possible that a creature at

time t2 be x-ly identical to the same creature at t1 but differ in that…?" It is a short step from the

intrapersonal form to the first-person form, "Is it y-ly possible that I at time t2 be x-ly identical to myself att1 but differ in that…?" Both of the intrapersonal and first personal variations are envisioned by Robert

Kirk (1974a).

2. Zombies v. Swampman: Physical and Functional Identity

If the "functional organization" of a thing is thought of in a way that is both narrow (local, system

internal) and ahistorical (synchronic, not dependent on the system's history) then physical identity entails

functional identity. (Similarly, physical identity entails behavioral identity, but that is trivial on a thin notionof behavior.) It will be useful, in the discussion that follows, to exploit the asymmetric connection between

physical and functional identity. The trouble with this strategy is that there are available notions of

function, some of which are used by philosophers of mind, that do not fit these constraints-they areneither narrow nor ahistorical. For example, synchronic organismic physical identity does not entail

functional identity according to "wide content" or teleological versions of functionalism. The functional

organization of a thing at a time may depend on the history of the thing and its ancestors. In this way,



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functional identity can differ from behavioral and physical identity, which are narrowly individuated. This

makes things very messy.

To avoid the complication presented by wide or historical notions of function, we must be willing to

expand the scope of the stipulated functional identity to whole worlds or whole world-histories rather thanindividual creatures. (Ned Block makes a similar move in his "Inverted Earth".) The question then

becomes: "Is it y-ly possible that our world be diachronically (through its entire history) x-ly identical to

how it is in fact but that some creature in the world differ with respect to its consciousness?" This mayseem an ad hoc move, but there is some support for it. The zombie problem admits the indexical

formulation, "Could it be the case that this thing, right here, right now, have a different state of

consciousness than it actually does, viz., none at all, while remaining identical in x way?" This version of the question licenses the whole-world identity formulation of the zombie problem.

Another reason for allowing the whole-world formulation is that denying the possibility of physically

identical zombies is supposed to be a benchmark for materialism; it ought not be the case that anyobviously materialist theory is grouped with non-materialist theories due to a technicality in how the

zombie question is put. So for the purposes of the zombie question, physical identity entails functionalidentity. By broadening the temporal and spatial scope considered in framing the stipulated identity of

zombies, the zombie problem is kept distinct from its intentional cousin, Swampman. Questions about

Swampman are specifically questions about two things that are currently identical but that have distinct histories. If the zombie question is interpreted with the widest temporal and spatial scope, the zombie

question is about whether there are possible differences between two things which are currently

identical and have identical histories. Thus, on this reading, Fred Dretske (1995), for example, could

answer differently to questions about the natural possibility of physically identical zombies (no) and thepossibility of Swampman (yes). For teleological functionalists and teleological representationalists

about qualia, the only difference between zombies and swamp-people will be the temporal scope over which the modal possibilities are to range. In other ways, they are the same question. If you reject the

wide historical scope formulation of the zombie question then some wide-content theorists (e.g.,

Dretske) will have to admit that physically identical zombies are naturally possible. In this case it will turn-out that the natural possibility of physically identical zombies is no challenge to materialism-for wide-

content representationalists like Dretske count as materialists, if anyone does!



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3. Zombies and Conceivability

Zombies enter contemporary debates over consciousness in two ways. Some philosophers beginwith the intuition that zombies are (or are not) possible, and then proceed to draw out the consequences

and construct their theories appropriate to this intuition. Other philosophers begin with their preferred

theories of consciousness and, on that basis, conclude that zombies are (or are not) possible.

Those who (like David Chalmers) begin with the intuition that zombies are possible usually regardthis as showing that some or all versions of materialism (physicalism, naturalism) in philosophy of mind

are inadequate. Robert Kirk (1974) introduced the logical or metaphysical possibility10 of functionally or physically identical11 zombies as prima facie reason for rejecting materialism. Likewise, Chalmers'

(1996) takes the possibility of functionally and physically identical zombies to show the inadequacy of

physicalist theories of consciousness. Kirk and Chalmers offer conceivability arguments: We canconceive of zombies of thus-and-such sort, therefore such zombies are possible, therefore such-and-

such consequences follow for what are metaphysically viable theories of consciousness.

In contrast, those who (like Daniel Dennett) are committed to materialism (physicalism, naturalism)in philosophy of mind usually regard this stance as foreclosing the possibility of some or all varieties of

zombies. Dennett denies the natural possibility of any sort of zombies because of his dedication to a

loose behavioral functionalism and his wariness of the notion of consciousness that might be needed bythe friend of zombies (1991, 1995). Fred Dretske, as mentioned, takes himself to be committed to the

natural possibility of physically identical zombies on the basis of a prior commitment to a teleofunctional

theory of consciousness (1995).12 Robert Kirk now finds zombies impossible because he thinks that

their description is incompatible with our own grasp of psychological concepts (1999). And I haveargued that an Identity Theory permits the natural possibility of functionally or behaviorally identical

zombies (2000).

The point is that not all zombie friends and foes are running conceivability arguments.13 This isimportant for at least two reasons. First, we need to see what sort of argument we are faced with if we

are to know how to respond to it. No progress is to be found from a clash of intuitions in which each side

simply insists that it is better at conceiving or imagining. Both sides bear the burden of at leastaccounting for the opposing intuition; and if someone offers a theory that permits/limits the possibilities



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of zombies, then it is no good to simply reiterate one's intuition. Second, those who advocate zombies

on the basis of conceivability arguments tend to hold that if behavioral zombies are possible then so are

functional zombies, and if functional zombies then so too physical zombies. This makes the types of

zombies seem like an all-or-nothing possibility.14 But those of us whose belief in zombies is aconsequence of prior metaphysical commitments are able to discriminate between the possibilities

more readily. We are able to do this because our preferred theories provide us with ways of justifying

principles for making distinctions whereas the zombie conceivability advocates cannot appeal to suchprinciples-for if they do, they are revealed as having prior theoretical commitments after all.

4. Zombies and Skepticism

Zombies arose from skepticism about other minds. Yet this is not the only way that some kind of skepticism figures into claims about zombies. To sort out the connections is beyond the scope of this

Guide; but I'll indicate some considerations. At various points, some zombie debaters appear to be

arguing from the position of skepticism about certain kinds of scientific explanation. Chalmers, who

supports the possibility of zombies, doubts that any possible physicalist explanation can yield theconceptual necessity between body and mind that is required of a theory of consciousness. But

Dennett's anti-zombie argument also has a skeptical component. He sees the claim of identity over-and-above correlation, for example, as unwarranted and philosophically extravagant because he is

skeptical that non-functional differences can make a difference in the world. I do not hope to offer any

wisdom concerning what we should think about these moves other than to note that the skepticalelement in debates about zombies and qualia in general has not been much attended to.

3. A Guide to Species of Zombies

Talking about zombies just so, if we are not quite careful about how we take the details to be filled

out, will only lead to confusion. But if we are careful, then zombies can be the useful tool that Kirkintended for exploring what we would like out of a theory of consciousness.

For some years now I have defended the natural possibility of functionally identical zombies. This is

not because I have strong intuitions about zombies, it is because I take this possibility to be an



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acceptable consequence of the metaphysical view about consciousness that I find independently

attractive. That is, my commitment (such as it is) to functionally identical zombies does not result from a

conceivability argument. Of course a great deal of what passes for philosophy of mind depends on

one's intuitions; I mean only to indicate that the intuitions with which I begin are not intuitions aboutzombies.

Descartes, I hazard to say, did not have intuitions about zombies. But we can be confident that if

you are Descartes, then you are committed to the logical possibility of behavioral, functional, and

physical zombies. Whether you would also take all zombies to be naturally or metaphysically possibledepends on whether you take the soul-body union to have any necessity; perhaps it is anachronistic to

expect this detail from you, Msr. Descartes. This is how the non-conceivabi lity use of zombies proceeds. Once you have a theory of

consciousness in hand, you can determine whether or which zombies your theory allows for. Dualisms

are the easy cases; the particular details tend to matter more for versions of materialism (physicalism,naturalism) because they deliberately distinguish themselves according to notions of identity and modal

strength. Here then are some suggestions concerning how the advocates of some theories of

consciousness should regard the various permutations of zombies. (If you are beginning with aconceivability argument to the effect that some or other zombies are possible then this will tell you which

sorts of theories are available to you, and which your intuitions may render problematic.)

Questions (Q7)-(Q9). Is it logically possible that there be two creatures that are behavioral ly,

functionally, or physically identical to one another but differ in that one is conscious and the other is

not?



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Yes, all three kinds of zombies are logically possible. Two creatures that are physically identical are also

functionally and behaviorally identical. So if physically identical zombies are logically possible then sotoo are functionally (remember the caveats!) and behaviorally identical zombies. Unless dualism is not

even logically possible, physically identical zombies are logically possible.

The troubles with dualism are famous; but it has yet to be decisively shown false as a matter of logic alone, to be logically impossible.15 So for the momentwe cannot be certain that dualism is logically

impossible. We must conclude that all three kinds of zombies are logically possible.

There are other ways to secure the logical possibility of functionally and behaviorally identicalzombies even if it turns-out that dualism is not logically possible. But this brief sketch is sufficient for

present purposes. Logical possibility (non-contradiction) is a rather weak claim. The fact that various

kinds of zombies are logically possible has nary an implication for the metaphysical theories of consciousness.16

Questions (Q1), (Q4), and (Q7). Is it logically, metaphysically, or naturally possible that there be twocreatures that are behaviorally identical to one another but differ in that one is conscious and the



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other is not?

Yes, all three kinds of zombies are possible.17 We can take advantage of the relation between the

modes of possibility to once again answer several questions at once. If it is naturally possible that there

be two creatures that are behaviorally identical to one another but differ in that one is conscious and theother is not, then it must also be metaphysically and logically possible.

And, as noted earlier, there is little doubt that behavioral zombies are naturally possible.18

Remember that it is irrelevant how the effect is achieved. So, for example, a life-size robotic puppetremotely controlled by a giant super-computer, or by my nervous system, would do the trick. When we

see what a weak claim is being made, it is striking. It's not merely that a human behavioral duplicate is

highly improbable but would violate no laws of nature. Rather, such a thing is not so improbable at all.We should all be rather surprised if a future George Lucas or Steven Spielberg or Rodney Brooks didn't

actually manufacture some such thing in the not too distant future.

The case is stronger still if we remind ourselves that the zombie question can be considered with



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respect not only to humans but to any putatively conscious creature, including conscious animals. You

may doubt that perfect behavioral duplicate human puppets are close at hand; it's more difficult to

ignore the likelihood of robotic dogs or cats, say. Don't put RoboPuppy on lay-away yet-but it would be a

tough road to argue that such a thing is precluded by laws of nature. 19

What is naturally possible is also metaphysically and logically possible; so the answers to (Q1),

(Q4), and (Q7) are each "yes."

Question (Q3). Is it naturally possible that there be two creatures that are physically identical to oneanother but differ in that one is conscious and the other is not?

If dualism is false then there cannot be two creatures that are physically identical but that differ with

respect to whether or not they are conscious.20

The analytic or metaphysical behaviorist is committed to any behaviorally identical organismsbeing ipso facto mentally identical. Adding "more" identity makes no difference as long as physically

identical things are also behaviorally identical. So the analytic or metaphysical behaviorist is committed



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to the natural impossibility of physically identical zombies. Functionalists are committed to the view that

two things that are functionally identical are identical in consciousness; and two things that are physicallyidentical are also functionally identical. So functionalists deny the possibility of physically identical

zombies. Finally, for the Identity Theorist mental states are identical to physical states, so two things with

the same physical states necessarily have the same mental states-physically identical zombies areimpossible.

An exception to the negative answer to question (Q3) might be some sorts of anomalism, dual

aspect theory or property dualism. But in order to accept the natural possibility of physically identicalzombies one would have to deny any lawful or lawlike connection between the two aspects or kinds of

properties-even a one-way connection.

David Chalmers' double-aspect view of information, for example, requires that there will be noveluniversal bridge principles connecting the physical properties and the conscious (informational)

properties in any possible world. If natural possibility covers the expanded physical theory (physics*, or

whatever the new expanded physical theory is to be called) that Chalmers advocates, then he mayanswer negatively to (Q3) (1995, 1996).21 But if Chalmers' view does not fall within the bounds of

naturalism, then physically identical zombies will be naturally (but not naturally*) possible for him.22

Questions (Q5) and (Q6). Is it metaphysically possible that there be two creatures that are functionally

or physically identical to one another but differ in that one is conscious and the other is not?



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I don't know whether functionally or physically identical zombies are metaphysically possible. Thetrouble, mentioned above, is that there is no agreement about the nature of metaphysical possibility. I

will leave questions (Q5) and (Q6) as officially unanswerable at this time. Even so, perhaps I can

gingerly offer a few thoughts on how one might approach these questions. On its most natural reading, Identity Theory denies that (Q6) is possible. That is, if sensations are in

actuality natural kinds that are identical to natural kinds of brain processes, then there are no

metaphysically possible worlds in which sensations and brain processes are not identical. This is

exactly parallel to Kripke's (1972) widely accepted denial that there could be any metaphysicallypossible worlds in which water is not H20, given that water is in fact H20 in the actual world. Whatever

metaphysical possibility comes to, it will be metaphysically impossible for things that are in fact identical

to be non-identical. Thus, the Identity Theorist denies the metaphysical possibi lity of physically identicalzombies.

But even with this standard example there are troubles, for if metaphysical possibility is (contra

Kripke) more like conceivability23 then the answer to (Q6) may be, "yes, physically identical zombies are

metaphysicallypossible " David Chalmers sometimes interprets the zombie question in this way (1996)



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metaphysically possible." David Chalmers sometimes interprets the zombie question in this way (1996).

Question (Q6) then becomes, "Might we have discovered that it is metaphysically possible for two

things to be physically identical to one another but differ in that one is conscious and the other is not?"

On Kripke's reading, that question has the same answer as question (Q3), whether physically identicalzombies are logically possible; and they are indeed logically possible so long as dualism is logically

possible.

The third question in the metaphysical possibility row of the Zombie Scorecard, (Q4), regarding

behavioral zombies, received a positive answer thanks to a reasonably uncontroversial answer to themodally stronger question, (Q1) that behaviorally identical zombies are naturally possible. Can a similar

strategy be employed to answer (Q6)? Suppose we assume a negative answer to (Q3): physicallyidentical zombies are not naturally possible. Still, the negative answer to the question of the

metaphysical possibility of physically identical zombies does not follow. The impossibility of the

stronger claim, (Q3), tells us nothing about the possibil ity of the weaker claim at stake in (Q6). It takes astronger claim than the denial of (Q3) to secure a negative answer to (Q6). It would take the force of

necessity, as would flow from the claim of type-identity or logical supervenience with respect to

consciousness. As indicated, in view of the confusion over what exactly it is for something to be metaphysically

possible, it seems best to set questions (Q5) and (Q6) aside for the time being. This is not particularly

satisfying, but it's just as well. For Güzeldere (1995) has argued that the real zombie debate these daysis over the remaining question, whether functionally identical zombies are naturally possible.

The Seventh Inning Stretch

All this wrangling over the details makes clear that, when it comes to zombies, the details matter.Imagining zombies is just a vivid way of forcing ourselves to face the consequences of views that we

already hold. Some of the consequences of a view, or the caveats necessary to maintain it, may not be

palatable to all philosophers. But those consequences and caveats don't come from the notion of a

zombie, they are merely highlighted by asking the zombie questions. Güzeldere writes,

Belief in zombies has become a litmus test for intuitions in recent philosophy of mind.… The set of

answers one chooses to give to questions of this sort is usually a good indicator of where one



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answers one chooses to give to questions of this sort is usually a good indicator of where one

stands with respect to a variety of issues regarding consciousness-its ontology, nature, function,

evolutionary role, and so on. (1995: 326-327)

Before going on, let's take stock of where we've been. If you have been following along with your Zombie

Scorecard i t should now look like this:

Figure 2. Zombie scorecard for (Q1), (Q3)-(Q8).

The Scorecard shows that all kinds of zombies are logically possible, behaviorally identical zombies are

possible under all modes of possibility physically identical zombies are not naturally possible and we



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possible under all modes of possibility, physically identical zombies are not naturally possible, and we

have no idea what to say about the metaphysical possibility of functionally or physically identical

zombies. Whereas a large consensus allowed us to answer with some confidence questions (Q1), (Q4),

and (Q7) through (Q9) with little difficulty, it became evident with (Q3), (Q5) and (Q6) that we must not somuch expect to answer questions about zombies as to use the zombies to sort our intuitions and views.

Answering these questions required us to take stances on controversial theories.

One position remains unexamined.

Questions (Q2). Is it naturally possible that there be two creatures that are functionally identical to one

another but differ in that one is conscious and the other is not?

I'm not here going to defend the view that functionally identical zombies are naturally possible. 24 To showthat functionally identical zombies are naturally possible I would have to provide either an argument for

Identity Theory or dualism (either of which entail the natural possibility of functionally identical zombies),

or an argument against functionalism (which entails the natural impossibility of functionally identical



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or an argument against functionalism (which entails the natural impossibility of functionally identical

zombies.)

As a matter of fact I am attracted to an Identity Theory. So on my view there might be some-many,

innumerably many-systems that are (in some sense) functionally identical to human beings but thatwould lack consciousness. Although the ranks of Identity Theorists are growing, we are still the minority

by far. So most philosophers of mind who fall roughly in the functionalist tradition (saying just what that is,

of course, is a difficult matter) will reject the possibility of functionally identical zombies.

This is what is at issue between Dennett and myself, and there is no sense in recounting it here. ButI will note that even functionalists ought to admit that there are some notions of function and functional

equivalence according to which two things may be (in that sense) functionally equivalent but differ withrespect to consciousness. Until they tell us which notion or notions they prefer, it is difficult to assess

their claims with respect to the possibility of zombies. My suspicion is that many functionalists are

helping themselves to a very broad notion of function-one that includes all causal relations, a view better called "mechanism" (Brandon 1996). In that case, when they deny the possibility of "functionally"

identical zombies they are really denying the possibility of physically (i.e., causally) identical zombies.

And that I can agree to, though I arrive at the view by a different route. Further exploring this suggestionis beyond the scope of the current Guide.

4. The Current Status of Zombies

For better or for worse, zombies are a hot topic in philosophy of mind. There are two particularlylively discourses. (Of course they are related.) The better-known strand stars David Chalmers and his

numerous critics and supporters. It has been carried out in Chalmers' book The Conscious Mind , in

The Journal of Consciousness Studies, and recently in Philosophy and Phenomenological Research.The lesser known strand, of which I am rather fond myself, stars Daniel Dennett. It began in The Journal of Consciousness Studies but has developed in a series of talks and book chapters.

The Chalmers strand of the zombie debates involves the logical and metaphysical possibility of

physically identical zombies, and therefore the viability of materialism itself. It is fair to say that thisstrand is the direct descendent of Kirk's argument; indeed he has recently weighed-in again (Kirk

1999). Chalmers argues that physically identical zombies are metaphysically possible on the basis of a

conceivability argument His critics and supporters concern themselves with whether such zombies are



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conceivability argument. His critics and supporters concern themselves with whether such zombies are

possible, and also with whether that possibility is an obstacle to any form of materialism (physicalism,

naturalism) as Chalmers argues.

The Dennett strand of the debates concerns the natural (and perhaps metaphysical) possibility of functionally identical zombies, and therefore the viability of certain forms of materialism (physicalism,

naturalism). Dennett argues that if functionally identical zombies are naturally possible then

consciousness must have no functional role in psychology, and must therefore be epiphenomenal or

systematically mysterious. Dennett regards this consequence as a reductio ad absurdum of the veryidea of zombies-if conceiving of zombies requires that consciousness is epiphenomenal, then the idea

of zombies is to be rejected altogether. One reason that Dennett thinks this is that he takes all zombiesupporters to be arguing in Chalmers' way, running conceivability arguments against materialism.

Counter to this line of reasoning, I have maintained that views that allow for the natural possibility of

functionally identical zombies do not render consciousness epiphenomenal; I offer Identity Theory as anexample of one such view.

5. Timeline: The Evolution of Zombies

Here are a few milestones in the short life of zombies in philosophy of mind.

1975 Robert Kirk invents zombies

[Zombies in hibernation]

1991 Daniel Dennett discusses zombies in his Consciousness Explained.

[Later I argue that, far from being mere curiosities, zombies play a central role in Dennett's defense of

his Multiple Drafts Model.]

1994 Todd Moody publishes "Conversations with Zombies," also in The Journal of Consciousness

Studies.

1995 David Chalmers summarizes his arguments in a Precis to The Conscious Mind in The Journal of



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1995 David Chalmers summarizes his arguments in a Precis to The Conscious Mind in The Journal of

Consciousness Studies. A symposium on Moody's paper gives zombies some momentum, and

launches the disagreement between Dennett and myself (with Owen Flanagan).

1996 Chalmers' The Conscious Mind is published. Zombies are a hot topic at the Tucson II conference.

Chalmers mentions them in his keynote talk. Robert Kirk chairs a session including myself, GüvenGüzeldere, and NNNN.

1998 Dennett's response to Flanagan and I, "The Unimagined Preposterousness of Zombies," appearsin his Brainchildren anthology, garnering the attention of Scientific American and Salon, among others. I

reply to Dennett with my talk, "Zombies Explained," at a conference in Dennett's honor in Newfoundland;

Dennett responds.

1999 Dennett gives Royal Institute talk on zombies. A symposium on Chalmers' The Conscious Mind in

Philosophy and Phenomenological Research discusses zombies.

2000-2001 John Perry's discussion of zombies in his latest book, no doubt many zombies at Tucson IV.

Notes

* Portions of this Guide are drawn from my "Zombies Explained" (2000).

1 _C. B. Martin and M. Deutscher used the term in passing in a related way (1966), but clearly did not

intend the consequences that Kirk explicates.

2 _It is worth noting that Kirk, though he did not dwell on the permutations of zombie thought experiments,

described or mentioned nearly all of the varieties that now have currency. More recent innovations areDennett's "zimboes" (1991) and Chalmers' "dancing qualia" (1996).

3 _Güzeldere considers behavioral, functional, and physiological zombies. I shall consider zombies that

are physically identical to any arbitrary degree of specificity. The purpose of stipulating zombies in this

way is to ensure that if the distinction between the physical and the functional can be maintained at all

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y p y

(and several have argued that it cannot; e.g., Lycan 1987, Hill 1991) then physically identical zombies

are a distinct construct from functionally identical zombies. Specifically, the distinction holds even if the

biological (physiological) structure of organisms is essentially functional in nature (as per Millikan 1989,1993; Lycan 1987, 1996; Neander 1991).

4 _This is supposed to be neutral about whether behavioral zombies have psychological states in

general, and in particular about whether their movements qualify as actions.

5 _For the present purposes behaviorism, functionalism, and Identity Theories, as well as other views

mentioned in the discussion to follow, are considered as theories of consciousness. Although

functionalism, for example, is often held as a theory of mind (cognition and intentionality, say) but not of consciousness (e.g., Ned Block 1980a), what is at stake here are just those functionalist theories that

are intended to explain consciousness (e.g., William Lycan 1987, 1996).

6

_See also Don Locke (1976).

7 _Some philosophers, including some in this debate, take logical possibility to be a more robust notionakin to what I am calling metaphysical possibility.

8 _This is what Terence Horgan calls physical possibil ity , which he distinguishes from a slightly weaker nomological possibility (1987). Complications about the locality of laws in space-time can be avoided;

see remarks regarding Chalmers with respect to (Q3) (note ##, below).

9 _Güzeldere suggests that an equivalent, and more convenient, way of talking about the ways that

zombies could be identical is by considering them as distinct kinds of zombies: behavioral zombies,

functional zombies, and physical zombies (1995). I will sometimes use this way of talking. So when Iask, say, whether behaviorally identical zombies are naturally possible, this is shorthand for question

(Q1), "Is it naturally possible that there be zombies that are behaviorally identical to human beings?"

10 _Kirk says he is using logical possibility in a loose sense, so it is more like what I am calling

metaphysical possibility.



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p y p y

11 _Kirk sees physical states as causally individuated. The theories he mentions as representing"materialism" are Lewis' and Armstrong's causal-specification theories. Whether these are to be

classified as functionalist or Identity theories is an issue I set aside for the moment. (Lewis now says

that he does not know if he is a functionalist; see his entry in Guttenplan 1994.)

12

_Keep in mind that Dretske, once we make the caveat that I suggested, could deny that he allows for physically identical zombies.

13 _It is a criticism that I level at Dennett that he persistently treats any zombie advocate as running a

conceivability argument.

14 _Chalmers has pushed me on this in correspondence.

15 _But see Douglas Long (1977) for a contemporary argument against the coherence of disembodied

minds. Hilary Putnam (1997) seems to suggest that while dualism may make sense in a religious

context, it is not given a meaning in the scientific-cum-philosophical context.

16 _It is very revealing, however, if what is at stake are certain views of semantics and reference, such as

logical or analytical behaviorism, or the (capital-F) 'Functionalism' that Block (1980a) attributes to, for example, David Lewis. Putnam's X-worlders (super-super-Spartans) in his classic critique of logical

behaviorism, "Brains and Behavior," are forerunners of the zombies (1968).

17 _Notice that there is an overlap with the previous set of questions. Even if dualism turns out to not be

logically possible, a "yes" answer to (Q7) can be secured by this second line of reasoning.

18 _Adherents of some versions of behaviorism, however, would reject the possibility of behaviorally

identical zombies. The logical or analytic behaviorist holds that mentalistic terms, such as

'consciousness', refer only to behaviors and dispositions to behave. An analytic behaviorist wouldtherefore deny that behaviorally identical zombies are logically possible-and thus also deny that they are

metaphysically or naturally possible. A metaphysical behaviorist might allow that behaviorally identical



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zombies are logically possible, but deny that they are metaphysically or naturally possible on the

grounds that consciousness simply is behavior. These views do not figure prominently in the zombie

debates.

19 _The Sony Corporation is well on their way. See here.

20

_This answer also depends on the caveat made to accommodate wide-content and historical theories.

21 _Even if those fundamental bridge principles are local to time-space, Chalmers could at least deny that

physically identical zombies are naturally possible at a location l and time t . If we allow comparison

across space and time, so that the laws might change, then Chalmers view might allow for such apossibility as (Q3). But it is reasonable to construe the zombie debate as restricting the possibilities by

time and space. For each imagined comparative question, the point could just as well be made by

asking, "Could this thing here and now, such as myself, have different consciousness than it does in

actuality have, while remaining identical in x way?"

This time-space restriction that allows proponents of views like Chalmers' to deny the natural possibilityof physically identical zombies is compatible with the time-space permissiveness discussed with

respect to Dretske's wide content view, above. In the wide-content case, we are considering how far

into the past a things' history is relevant to our considerations. By including an organisms completeevolutionary history, we ensure that the wide-content materialist can deny the natural possibility of

zombies. But in order for Chalmers to deny the natural possibility of zombies we need yet more-for it is

compatible with his view that two things that are physically identical through their entire history differ withrespect to consciousness if the fundamental bridge principles joining matter and consciousness were

different . To counter this possibility we can either (i) insist that the fundamental bridge principles are

universal in time and space for any given world, or (ii) make the comparison only between things thatare identical both in their complete physical and nomological histories (including bridge principles.) The

latter is accomplished, for example, by asking the zombie question in the first personal form. So the

same form of the question that permits the Dretske-type wide-content qualia theorist to deny the natural

possibility of physically identical zombies also allows Chalmers-type dual-aspect theorist to deny the

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natural possibility of physically identical zombies, by accounting for any effects of intra-world variation in

fundamental bridge principles.

22 _We need to be careful about how such cases are described. Chalmers sometimes denies that

physically identical zombies are naturally possible. But in those cases he is invoking his new bridgelaws, so it is really naturally* that he is discussing.

23 _Kripke's (1972) "epistemic" possibility; Horgan's (1987) vague conceivability-but Horgan denies aconnection between that kind of conceivability and metaphysical possibility.

24 _Against Daniel Dennett I argue that the possibility of functionally identical zombies neither requiresnor entails an epiphenomenalist conception of consciousness.

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