## 1. The idea of zombies



Descartes held that non-human animals are automata: their behavior is
wholly explicable in terms of physical mechanisms. But human behavior
(he argued) could not be explained like that. Exploring the idea of a
machine that would look and behave like a human being, he thought two
things would unmask it: it could not use language creatively, and it
could not produce appropriate non-verbal behavior in arbitrarily
various situations (*Discourse V*). For him, therefore, no
machine could behave like a human being. Knowing only seventeenth
century technology, he concluded that to explain distinctively human
behavior required something beyond the physical: an immaterial mind,
interacting with processes in the brain and the rest of the body.
(Importantly, he also had *a priori* arguments for the same
conclusion, one of which anticipates the 'conceivability
argument' discussed in Section 3 below.) If he is right, there
could not be a world physically like the actual world but lacking such
minds: human bodies would not work properly. If we suddenly lost our
minds our bodies might continue to run on for a while: our hearts
might continue to beat, we might breathe while asleep and digest food;
we might even walk or sing in a mindless sort of way (so he implies in
his *Reply to Objections IV*). But without the contribution
made by minds, behavior could not show characteristically human
features. So although Descartes did everything short of spelling out
the idea of zombies, the question of their possibility did not arise
for him. The nearest thing was automata whose behavior was easily
recognizable as not fully human.



In the nineteenth century scientists began to think that physics was
capable of explaining all physical events that were explicable at all.
It seemed that every physical effect has a physical cause: that the
physical world is 'closed under causation'. The developing
science of neurophysiology was set to extend such explanations to
human behavior. But if human behavior is explicable physically, how
does consciousness fit into the story? One response --
physicalism (or materialism) -- is to insist that consciousness
too involves only physical processes. However, the phenomena of
consciousness are hard to account for in those terms, and some
thinkers concluded with Descartes that something nonphysical must be
involved. Given they accepted the causal closure of the physical, they
were forced to conclude that consciousness has no effects on the
physical world. On this view human beings are 'conscious
automata', as T. H. Huxley put it: all physical events, human
behavior included, are explicable in terms of physical processes; and
the phenomena of consciousness are causally inert by-products --
epiphenomena (see James 1890, Chapter 5). It eventually became clear
that this view entailed there could be purely physical organisms
exactly like us except for lacking consciousness. G. F. Stout argued
that if epiphenomenalism (the more familiar name for the
'conscious automaton' theory) is true,



> 
> 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. Human bodies would still
> have gone through the motions of making and using bridges, telephones
> and telegraphs, of writing and reading books, of speaking in
> Parliament, of arguing about materialism, and so on. There can be no
> doubt that this is *prima facie* incredible to Common Sense
> (Stout 1931: 138f.).
> 



What Stout describes here and finds *prima facie* incredible is
a zombie world: an entire world whose physical processes are closed
under causation (as the epiphenomenalists he was attacking held) and
exactly duplicate those in the actual world, but where there are no
conscious experiences.



Similar ideas were current in discussions of physicalism in the 1970s.
As a counterexample to the psychophysical identity theory there was an
'imitation man', whose 'brain-states exactly
paralleled ours in their physico-chemical properties' but who
felt no pains and saw no colors (Campbell 1970). Zombies were put
forward as a counterexample to physicalism in general, and arguments
devised to back up the intuition that they are possible (Kirk 1974a,
1974b). However, these arguments fell short of their target because
they depended on much the same cluster of intuitions as the original
idea.



Other kinds of systems were envisaged which behaved like normal human
beings, or were even functionally like human beings, but lacked the
'qualia' we have (Block 1980a, 1980b, 1981; Shoemaker
1975, 1981). (Roughly, qualia are the properties by which we classify
experiences according to 'what they are like': what it is
like to smell roasting coffee beans, for example. Even physicalists
can use this expression, although unlike dualists they take qualia to
be physical.) The most systematic use of the zombie idea against
physicalism is by David Chalmers 1996, whose important contributions
to the debate will be considered shortly.



If zombies are to be counterexamples to physicalism, it is not enough
for them to be behaviorally and functionally like normal human beings:
plenty of physicalists accept that merely behavioral or functional
duplicates of ourselves might lack qualia. Zombies must be like normal
human beings in *all* physical respects, and they must have the
physical properties that physicalists suppose we have. This requires
them to be subject to the causal closure of the physical, which is why
their supposed lack of consciousness is a challenge to physicalism. If
instead they were to be conceived of as creatures whose behavior could
not be explained physically, physicalists would have no reason to
bother with the idea: there is plenty of evidence that, as
epiphenomenalists hold, our movements actually are explicable in
physical terms (see e.g. Papineau 2002).



The usual assumption is that none of us is actually a zombie, and that
zombies cannot exist in our world. The central question, however, is
not whether zombies can exist in our world, but whether they, or a
whole zombie world (which is sometimes a more appropriate idea to work
with), are possible in some broader sense.


## 2. Zombies and physicalism



A metaphor of Saul Kripke's helps to show how the zombie idea
threatens physicalism (Kripke 1972/80, 153f.). Imagine God creating
the world and deciding to bring into existence the whole of the purely
physical universe. Having created this physical universe, did he have
to do any more work to provide for consciousness? Answering yes to
this question implies there is more to consciousness than the physical
facts alone can supply. If nothing else, it implies that consciousness
depends at least partly on nonphysical properties, ones that would not
exist in a purely physical world; it would be a zombie world.
Physicalists, on the other hand, are committed to answering no. They
have to say that by fixing the purely physical facts, God did
everything necessary to fix the mental facts about the organisms
thereby created, including their thoughts, feelings, emotions, and
experiences. In other words, it seems that physicalists must say that
in some sense the purely physical truths entail the mental truths
(Kirk 1974a, 1974b argued that physicalism requires an
'Entailment Thesis' to that effect). If indeed fixing the
physical facts alone is enough to fix the mental facts, then a zombie
world is impossible. 



Not everyone agrees that physicalism entails the impossibility of
zombies. One suggestion is that physicalists can concede there are
possible worlds which are exact duplicates of our world in all purely
physical respects, but where the physical properties which give rise
to consciousness in our world are prevented from doing so by
nonphysical items which block consciousness. That would let
physicalists consistently allow the possibility of zombie worlds
(Leuenberger 2008. On such 'blockers' see Hawthorne 2002b;
Chalmers 2010, 163-165). This approach, however, is inconsistent
with maintaining that actual conscious states are either identical
with or constituted by physical or functional states. If my conscious
state is the same as or constituted by a physical state, then there is
no possible world where the latter exists without the former. It is
therefore not clear that physicalists can consistently allow the
possibility of consciousness-blockers. Lei Zhong 2021 takes a very
different approach, challenging the widely held view that physicalism
commits one to the supervenience of the mental on the physical. 



But what kind of impossibility is relevant here? Physicalists cannot
just say zombies are ruled out by the laws of nature, since even
dualists can agree they are impossible in that sense: that it is by
*nomological* necessity that the physical facts about us bring
consciousness with them. Physicalism therefore needs something
stronger. 



Two further kinds of necessity are usually considered: logical and
metaphysical. Now, many philosophers (largely influenced by the zombie
idea) believe the connection from physical facts to consciousness
cannot be logical even in a broad sense. And certainly the conceptual
scheme of physics does not *appear* to leave room for logical
links from physical to phenomenal (see e.g. Kriegel 2011; Stoljar
2006). However, some argue that nevertheless zombies are not really
conceivable at all (Kirk 2005, 2008, 2013; Tye 2006); Kirk 2013 also
maintains that although the physical facts do not entail the truth
about conscious experience *a priori*, they nevertheless entail
it by logical necessity. 



Still, many physicalists hold that what guarantees the impossibility
of zombies is 'metaphysical' necessity. Typically they
maintain that states of phenomenal consciousness are identical with
physical states, and that these identities are necessary a posteriori
as argued by Kripke (see e.g. McLaughlin 2005, and for criticism,
Stoljar 2000). But the vocabulary of possibility and necessity is
slippery. For example there is disagreement over whether logical and
metaphysical possibility are different (section 3.1 below); when
Kripke (1972/80) writes of 'logical' and
'metaphysical' possibility he seems to use those words
interchangeably (Yablo 1999: 457n.), and some use
'logical' where others prefer 'conceptual'
(Chalmers 1999: 477); compare Latham 2000, 72f.).



Many think that if the physical facts entail consciousness by
metaphysical necessity, then physicalists can maintain that even
though zombies are metaphysically impossible, they are still
*conceivable* (Balog 2012; Loar 1990/97; McLaughlin 2005;
Sections 5.1, 5.2 below). To the contrary, Chalmers argues that
conceivability actually entails metaphysical possibility. If he is
right, then that popular brand of physicalism is mistaken. The
so-called 'conceivability argument' for the possibility of
zombies will provide a focus for discussing some of the main problems
raised by the zombie idea.


## 3. The conceivability argument for the possibility of zombies



The simplest version of this argument goes:


1. Zombies are conceivable.
2. Whatever is conceivable is possible.
3. Therefore zombies are possible.



(Kripke used a similar argument in his 1972/80. For versions of it see
Chalmers 1996, 93-171; 2010, 141-205; Levine 2001; Nagel
1974; Stoljar 2001. Michael Pelczar (2021) argues for the same
conclusion without appealing to conceivability.) Clearly the
conceivability argument is valid. However, both its premisses are
problematic. They are unclear as stated, and controversial even when
clarified. A key question is how we should understand
'conceivable' in this context.



Many philosophers are willing to concede that zombies are conceivable
in some sense (e.g. Hill 1997; Hill and McLaughlin 1999; Loar 1999;
Yablo 1999). However, that sense is sometimes quite broad. For
example, a claim that 'there are no substantive a priori ties
between the concept of pain and the concept of C-fiber
stimulation' has been backed up by the point that 'it is
in principle possible to master either of these concepts fully without
having mastered the other' (Hill 1997, 76). By that standard,
though, it would be conceivable that the ratio of a circle's
circumference to its diameter is a rational number, when it
isn't. If conceivability in that sense entailed possibility, it
would be both possible and impossible for the ratio to be rational;
which would make such conceivability useless for the purposes of the
conceivability argument. So understood, premiss (1) would be easy to
swallow; but (2) would have to be rejected. Evidently, the lower the
threshold for conceivability, the easier it is to accept (1) --
but the harder it is to accept (2). So the kind of conceivability
invoked in premisses (1) and (2) needs to be strongly constrained. A
common and useful definition, which will be followed here, is:
*something is conceivable if and only if it cannot be ruled out a
priori.* (For sophistication of these and related ideas see
Chalmers 1999, 477; 2002; 2007; 2010; and 5.1 below.) 



Joseph Levine discusses a version of the conceivability argument,
seeing the conceivability of zombies as 'the principal
manifestation of the explanatory gap' (2001: 79). In his view,
what creates this gap is the *epistemological* problem of
explaining how the phenomenal is related to the physical. He sees no
way to solve this problem, and thinks it remains even if zombies are
impossible.



Campbell, Copeland and Deng 2017 argue that, quite generally, for any
conceivability argument there is a corresponding 'mirror
argument' which can be rejected only at the cost of undermining
the main argument, and conclude that all conceivability arguments are
'logically bankrupt'. 



We now face two key questions: Are zombies conceivable in the sense
explained? If they are conceivable, does it follow that they are
possible? Only if the answer to both questions is yes will the
conceivability argument succeed. We can take them in that order.


## 4. Are zombies conceivable?



Those who exploited the zombie idea in the 1970s typically assumed
without argument that zombies are not just conceivable but possible
(e. g. Campbell 1970, Nagel 1970). When Chalmers reactivated the idea
he found the conceivability of zombies 'obvious',
remarking that 'it certainly seems that a coherent situation is
described; I can discern no contradiction in the description'
(1996, p. 96). However, he also recognized that this intuition cannot
be relied on. The nature of conscious experience is after all hard to
understand: what strikes some people as obviously possible could still
turn out to harbour hidden contradictions (Nagel 1998; Stoljar 2001).
Clearly, those who maintain that zombies are conceivable must provide
justification, recognizing that, being an epistemic claim dependent on
our cognitive abilities, it is defeasible.


### 4.1 Arguments for the conceivability of zombies



Chalmers (1996) set out five arguments against the view that there is
an *a priori* entailment from physical facts to mental facts
-- and so *for* the view that zombies are conceivable.
Each argument would directly or indirectly reinforce the intuitive
appeal of the zombie idea. The first will be considered shortly; the
other four appeal respectively to the alleged possibility of
'inverted spectrum' without physical difference; the
alleged impossibility of learning about conscious experience on the
basis of purely physical information; Jackson's (1982)
'knowledge argument' (related to the last); and what
Chalmers calls 'the absence of analysis': the point being
that his opponents 'will have to give us some idea of
*how* the existence of consciousness might be entailed by the
physical facts', when (assuming the other arguments work)
'any attempt to demonstrate such an entailment is doomed to
failure' (1996, p. 104). 



His first argument goes roughly as follows. Suppose a population of
tiny people disable your brain and replicate its functions themselves,
while keeping the rest of your body in working order (see Block
1980a); each homunculus uses a cell phone to perform the
signal-receiving and -transmitting functions of an individual neuron.
Would such a system be conscious? Intuitively one may be inclined to
say not. Some, notably functionalists, bite the bullet and answer yes.
However, the argument does not depend on assuming that the
homunculus-head would not be conscious. It depends only on the
assumption that its not being conscious is *conceivable*
-- which many people find reasonable. In Chalmers's words,
all that matters here is that when we say the system might lack
consciousness, 'a meaningful possibility is being expressed, and
it is an open question whether consciousness arises or not'
(1996, p. 97). If he is right, then conceivably the system is not
conscious. In that case it is already very much like a zombie, the
only difference being that it has little people where a zombie has
neurons. And why should that make a difference to whether the
situation is conceivable? Why should switching from homunculi to
neurons necessarily switch on the light of consciousness? (For doubts
about the assumption that it is conceivable that the homunculus-head
lacks consciousness, see e.g. Loar 1990/1997, pp. 613f.)



Other considerations favoring the conceivability of zombies can be
found in Block 1995, 2002; Levine 2001; Searle 1992. Chalmers 2010
develops his defense further. Brian Cutter 2020 offers an
anti-materialist modal argument which does not rely on the assumption
that the physical truths are compatible with the absence of
consciousness.


### 4.2 Arguments against the conceivability of zombies



Although in the past it was quite widely accepted that zombies are
conceivable, skepticism has grown. Before considering direct attacks
on the idea, let us briefly recall three views which once appeared to
support the claim that we can know *a priori* that dualism is
false -- hence, on reasonable assumptions, that zombies are not
conceivable. 



The first is verificationism, according to which a (declarative)
sentence is meaningful just in case its truth or falsity can be
verified. This entails that unverifiable sentences are literally
meaningless, so that no metaphysical claim according to which
unobservable nonphysical items exist can be true. However, since our
ability to think and talk about our experiences is itself a problem
for verificationism, to presuppose this view when attacking the zombie
idea would beg the question. The second view appeals to
Wittgenstein's private language argument. Although not crudely
verificationistic, it depends on the assumption that in order for
words to be meaningful, their use must be open to public checking. But
since this checkability assumption, if sound, would prove that we
cannot talk about qualia in the ways defenders of the zombie
possibility think we can, it too seems question-begging in the present
context. According to the third view, behaviorism, there is no more to
having mental states than being disposed to behave in certain ways. As
a possible basis for attacking the zombie idea, behaviorism is in a
similar situation to verificationism and the private language
argument. Zombies would satisfy all behavioral conditions for full
consciousness, so that if we could know a priori that behaviorism was
correct, zombie worlds would be inconceivable for that reason. It
seems unlikely, though, that behaviorism can be shown to be correct.
(Dennett 1991 defends a position with strong affinities to
behaviorism, though it might be better classified as a variety of
functionalism). 



Functionalism is a much more widely supported approach to the mental.
According to it, mental states are not just a matter of behavior and
dispositions, but of causal or other functional relations among
sensory inputs, internal states, and behavioral outputs. (It is
important to take account of internal functions not necessarily
reflected in behavioral dispositions, otherwise functionalism falls to
the usual objections to behaviorism, such as the
'homunculus-head' described in the last section (Kirk
2005, 2013, 2017).) Now, since zombies would satisfy all the
functional conditions for full consciousness, functionalism entails
that zombies are impossible -- though it would obviously be
question-begging to presuppose it when attacking the zombie idea.
Increasingly sophisticated versions of functionalism are being
developed, however, and any arguments for it are a fortiori arguments
against the possibility of zombies. (For defenses of functionalism
against zombies see Dennett 1991; 1995; 1999; Kirk 2017; Shoemaker
1999; Tye 2006; 2008; for doubts about functionalism's capacity
to deal with zombies see for example Harnad 1995.)



Apart from broad-front functionalist theories of the mental, there are
more narrowly focused attacks on the conceivability of zombies, some
of which are noted below. 



*Can we really imagine zombies?* Daniel Dennett thinks those
who accept the conceivability of zombies have failed to imagine them
thoroughly enough: 'they invariably underestimate the task of
conception (or imagination), and end up imagining something that
violates their own definition' (1995, p. 322. Marcus 2004 makes
a related point). Given his broadly functionalist model of
consciousness, he argues, we can see why the 'putative contrast
between zombies and conscious beings is illusory' (325. See also
his 1991; 1999). Consciousness is 'not a single wonderful
separable thing ... but a huge complex of many different
informational capacities' (1995, 324. Cottrell 1999 supports
this approach).



*The 'epistemic approach'.* Stoljar (2006, 2020)
emphasizes that the conceivability argument presupposes we have a
complete knowledge of the relevant physical facts, when it is likely
that we don't. If that is right, we cannot properly conceive of
the possibilities in question, in which case premiss (1) of the
conceivability argument is false. A bonus of this view is that it
leaves us free to suppose there is a reductive explanation of
consciousness -- that the physical facts are such that there is
consciousness in all possible worlds where those facts obtain --
even when we don't know what those facts are.



*Zombies' utterances*. Suppose I smell roasting coffee
beans and say, 'Mm! I love that smell!'. Everyone would
rightly assume I was talking about my experience. But now suppose my
zombie twin produces the same utterance. He too seems to be talking
about an experience, but in fact he isn't because he's
just a zombie. Is he mistaken? Is he lying? Could his utterance
somehow be interpreted as true, or is it totally without truth value?
Nigel Thomas (1996) argues that 'any line that zombiphiles take
on these questions will get them into serious trouble'.



*Knowing about and referring to qualia*. Recall that by
definition a zombie world is just like our world as physicalists
suppose it to be, but without consciousness. Since this implies that
consciousness depends on something nonphysical, it follows that
zombies (assuming they are possible in the first place) could be made
conscious by the addition of something nonphysical, which might as
well be qualia. And given that a zombie world would be causally
closed, these qualia would have to be causally inert: perhaps still
caused by the correlated physical processes, perhaps just parallel to
them. It therefore seems that if a zombie world is conceivable, so is
epiphenomenalism. (Of course this does not require epiphenomenalism to
be *actually* true as well as conceivable.) If that is correct,
objections to the conceivability of epiphenomenalism are also
objections to the conceivability of zombies, the most obvious of these
being simply that experiences have effects on behavior. A less obvious
objection starts from the fact that we *refer to and know
about* our conscious experiences -- which can hardly be
denied, since otherwise we could not be discussing these ideas in the
first place. The objection appeals to the widely held view that
whatever we can know about or refer to must have effects on us, if
only indirectly (Kripke 1972/80). On that basis our counterparts in
epiphenomenalistic worlds could not know about or refer to their
qualia, with the consequence that (given the above reasoning) neither
epiphenomenalistic worlds nor zombie worlds are conceivable.



To this attack Chalmers replies that the crucial consideration is that
we are 'acquainted' with our experiences. This
'intimate epistemic relation' both ensures that we can
refer to experiences and also justifies our claims to know about them.
Since, in contrast, our zombie twins have no experiences, what appear
to be their judgments about experience are unjustified. Chalmers
suggests that even if qualia have no causal influence on our
judgments, their mere presence in the appropriate physical context
ensures that our thoughts are about those qualia. He thinks it also
constitutes justification for our knowledge claims even if experiences
are not explanatorily relevant to making the judgments in question
(Chalmers 1996, 172--209; 1999, 493f; see also his 2003,
2010).



*The problem of epistemic contact*. Just now it seemed that if
zombies are conceivable, then epiphenomenalist and parallelist worlds
are also conceivable. In that case the friends of zombies must explain
how the epiphenomenal qualia in such worlds could possibly be objects
of acquaintance, or indeed make any sort of intimate contribution to
people's lives; and here Kirk (2005; 2008) suggests the zombie
idea faces a further difficulty. This emerges when we consider such
things as attending to, thinking about, comparing and --
especially -- remembering our experiences. These activities bring
us into 'epistemic contact' with them and involve
cognitive processing, which in turn involves changes causing other
changes. Being causally inert, the epiphenomenal qualia themselves
could not do that processing; so if they actually constitute our
experiences (as epiphenomenalism and parallelism imply) then the
necessary processing must be purely physical. The trouble is that the
zombie story appears to make it impossible for such processing to put
us into epistemic contact with epiphenomenal qualia. This is because
the only resources it can appeal to for that purpose are the assumed
causation of qualia by neural processes and their isomorphism with
them: factors which cannot do the necessary cognitive work (Kirk 2005;
2008). If that is right, the notions of epiphenomenal qualia and
zombies lead to a contradiction. They imply a conception of
consciousness which requires people to be in epistemic contact with
their qualia, while at the same time ruling out the possibility of
such contact.



*'Powerful qualities'*. Another interesting
objection to the zombie idea is based on the (controversial) idea of
'powerful qualities': the view that all properties are
both dispositional and qualitative, and indeed that a thing's
dispositions are identical with its qualities. Alexander Carruth
(2016), for example, argues that the conceivability argument
presupposes that while physical properties are dispositional,
phenomenal ones are qualitative. On that basis a zombie duplicate of
our world would instantiate our world's dispositional properties
but not its phenomenal ones. The powerful qualities view rules that
out *a priori*, making it not even conceivable. For if a
thing's dispositions are identical with its qualities, nothing
can instantiate certain dispositional properties without also
instantiating all qualities supposedly identical with them. Countering
this line of argument, Henry Taylor (2017) claims it depends on an
implausible account of the distinction between the physical and the
phenomenal, arguing in particular that the physical cannot be confined
to the dispositional. 



For other attacks on the conceivability of zombies see Balog 1999;
Cottrell 1999; Harnad 1995; Kirk 2005, 2008, 2013; Marcus 2004;
Shoemaker 1999; Stoljar 2001; Tye 2006. 


## 5. Does conceivability entail possibility?



Premise (2) of the conceivability argument is: Whatever is conceivable
is possible. It has been attacked from several angles, as follows.



### 5.1 Objections based on *a posteriori* necessity



A number of philosophers argue that Kripke's ideas about a
posteriori necessary truth facilitate the defense of physicalism. They
urge that even if a zombie world is conceivable, that does not
establish it is possible in the way that matters. Conceivability is an
epistemic notion, they say, while possibility is a metaphysical one:
'It is false that if one can in principle conceive that P, then
it is logically possible that P; ... Given psychophysical
identities, it is an 'a posteriori' fact that any physical
duplicate of our world is exactly like ours in respect of positive
facts about sensory states' (Hill and McLaughlin 1999, 446. See
also Hill 1997; Loar 1990/1997; 1999; McLaughlin 2005; Webster 2006).
Some philosophers reject even the assumption that conceivability is a
*guide* to possibility, challenging the view that the burden of
proof is on those who deny the zombie possibility (Block and Stalnaker
1999; Hill and McLaughlin 1999; Yablo 1993).



Chalmers has responded in several places (1996, 131-134; 1999,
476-7; 2010, 141-205). His most detailed version of the
conceivability argument (2010) uses the framework of two-dimensional
semantics. This enables him to distinguish two kinds of possibility
and two corresponding kinds of conceivability. In the
'primary' sense conceivability entails possibility; for
example it is conceivable that water should have been a substance
chemically different from H2O. In the other, 'secondary'
sense, it is neither conceivable nor possible that water should have
been chemically different. The difficulty for the conceivability
argument can be expressed by saying that even if zombie worlds are
primarily conceivable and therefore primarily possible, it does not
follow that they are also secondarily possible. And a posteriori
physicalists will typically deny that it follows, on the ground that
only the secondary possibility of zombie worlds would entail the
falsity of physicalism. At this point Chalmers in effect presents his
opponents with a dilemma, which is (crudely summarizing his
conclusions) that either the primary conceivability of zombies does
after all entail their secondary possibility, in which case the
conceivability argument works and materialism is false; or else
'Russellian monism', briefly considered at Section 5.3
below, is true. (See also Jackson 1998; and for discussions, Brueckner
2002; Loar 1999; Hill and McLaughlin 1999; Piccinini 2017;
Sebastian 2017; Shoemaker 1999; Soames 2005; Yablo
1999.) 


### 5.2 The phenomenal concept strategy



Many physicalists hold that both the zombie idea and Frank
Jackson's knowledge argument can be dealt with through a proper
understanding of the nature of phenomenal concepts (roughly, the
concepts we use when conveying the character of our experiences: for
example 'sweet', 'the way I see blue').
Exponents of the conceivability argument hold that the supposed
'explanatory gap' between the physical and the phenomenal
-- which is expressed in the idea that zombies are conceivable
-- brings with it an ontological gap. According to the
'phenomenal concept strategy' (Stoljar 2005) there is
really only a conceptual gap: phenomenal concepts have features which
mislead us into supposing there is an ontological gap in addition to
an epistemic one, when there isn't.



Thus it is argued that even if a zombie world is conceivable, it does
not follow that there are nonphysical properties in our world. If that
is right, physicalists can concede the conceivability of zombies while
insisting that the properties we pick out in terms of phenomenal
concepts are physical. 'Given that properties are constituted by
the world and not by our concepts', Brian Loar comments,
'it is fair of the physicalist to request a justification of the
assumption that conceptually distinct concepts *must* express
metaphysically distinct properties' (Loar 1999, 467; see also
his 1997). He also argues that phenomenal concepts are
'recognitional', in contrast to physical concepts, which
are 'theoretical'. Phenomenal concepts, Loar says,
'express the very properties they pick out, as Kripke observed
in the case of 'pain'' (1999, 468). He thinks these
points explain the conceivability of a zombie world, while maintaining
that there is no possible world in which the relevant physical
properties are distinct from consciousness. Chalmers objects that
Loar's account does not justify the view that physical concepts
refer to phenomenal properties (1999, 488). He argues further (2007)
that exponents of this approach face a dilemma. Let C be whichever
psychological 'key features' we have but zombies lack.
Then if it is conceivable that the purely physical facts about us
should have held without C, then C is not physicalistically
explicable. On the other hand, if that is not conceivable, then in his
view C cannot explain our epistemic situation as contrasted with that
of zombies. So either C is not physicalistically explicable, or it
cannot explain our epistemic situation. (For discussions see Ball
2009; Balog 2012; Carruthers 2005; Chalmers 1999; 2007; 2010; Crane
2005; Loar 1990/97; Papineau 2002; Pereboom 2011; Stoljar 2000; Tye
2008.) 


### 5.3 Russellian monism



Following Russell (1927), some philosophers suggest that physics tells
us only about the 'structural' properties of things
-- such as their dispositions and nomic relations -- rather
than the 'intrinsic' properties which supposedly underlie
and account for those structural properties. Thus Daniel Stoljar
(2001) argues that there are two distinct notions of the physical and
correspondingly of physicalism, depending on whether one appeals only
to what is provided for by physics or also to the intrinsic properties
of physical objects. He suggests that even if one of the corresponding
two versions of the conceivability argument is sound, the other is not
-- because (roughly) physicalists can always object that, since
we do not know enough about the physical world (in particular, about
its intrinsic properties), we cannot 'strongly' conceive
of the possibility of zombies.



These ideas are exploited in what Chalmers calls 'Russellian
monism' (a variety of neutral monism). In our world, he
suggests, the underlying intrinsic properties might be
'phenomenal properties, or they might be protophenomenal
properties: properties that collectively constitute phenomenal
properties when organized in the appropriate way' (2010: p.
151); while in some other worlds the corresponding intrinsic physical
properties did not provide for consciousness. If the intrinsic
properties which supposedly provide for our consciousness are
nevertheless classified as physical, exponents can deny the
possibility of zombies if these are understood to be our
'full' physical duplicates. At the same time they can
concede the possibility of zombies which duplicate us only in their
structural properties. As he points out, this view is 'a highly
distinctive form of physicalism that has much in common with property
dualism and that many physicalists will want to reject'
(Chalmers 2010, p. 152; see also Pereboom 2011). One obstacle to
counting it as physicalism is that it seems unable to explain why the
special intrinsic properties in our world should provide for
consciousness, while those which perform the same functions in those
other worlds do not: this has to be accepted as a brute fact. 



Philip Goff (2010; see also his 2017) suggests that this loophole for
Russellian versions of physicalism weakens the zombie argument. He
recommends instead an argument from ghosts: pure subjects of
experience without any physical nature. He argues that such ghosts are
conceivable and possible, and that they provide an argument against
physicalism which leaves no loophole for Russellian monism.
(Physicalists are likely to object that arguments against the
conceivability of zombies can also be mobilized against ghosts.) 


### 5.4 Other objections



*Special factors*. It has been suggested that there are special
factors at work in the psychophysical case which have a strong
tendency to mislead us. For example it is claimed that what enables us
to imagine or conceive of states of consciousness is a different
cognitive faculty from what enables us to conceive of physical facts:
'there are significant differences between the cognitive factors
responsible for Cartesian intuitions [such as those about zombies] and
those responsible for modal intuitions of a wide variety of other
kinds' (Hill and McLaughlin 1999, p. 449. See also Hill 1997).
The suggestion is that these differences help to explain the ease with
which we seem able to conceive of zombies, and the difficulty we have
in understanding the claim that they are nevertheless impossible.



*Conditional analysis*. Another line of objection rests on
conditional analyses of the concept of qualia. Roughly, the idea is
that *if* there actually are certain nonphysical properties
which fit our conception of qualia, then that is what qualia are, in
which case zombies are conceivable; but *if* there are no such
nonphysical properties, then qualia are whichever physical properties
perform the appropriate functions, and zombies are not conceivable. It
is argued that this approach enables physicalists to accept that the
possibility of zombies is conceivable, while denying that zombies are
conceivable (Hawthorne 2002a; Braddon-Mitchell 2003. For criticism see
Alter 2007; Chalmers 2010, pp. 159-59).



*Causal essentialism*. According to the theory of causal
essentialism, the causal properties of physical properties are
essential to them. Brian Garrett (2009) exploits this theory to argue
that the zombie argument against physicalism depends on broadly Humean
assumptions about the laws of nature and property identity which
presuppose the falsity of causal essentialism. If we reject those
assumptions and accept that some physical properties have essentially
the capacity to produce consciousness, then 'we cannot accept
the genuine possibility of zombie worlds' even if such worlds
are conceivable (see also Aranyosi 2010).



*More on zombies' utterances*. Consider a zombie world
that is an exact physical duplicate of our world and contains zombie
twins of all philosophers, including some who appeal to the
conceivability argument. Katalin Balog (1999) argues that while their
utterances would be meaningful, their sentences would not always mean
what they do in our mouths. She further argues -- to oversimplify
-- that if the conceivability argument were sound in actual
philosophers' mouths, then it would be sound in the mouths of
zombie philosophers too. But since by hypothesis physicalism is true
in their world, their argument is not sound. Therefore the
conceivability argument used by actual philosophers is not sound
either. If this argument works, it has the piquant feature that
'the zombies that antiphysicalists think possible in the end
undermine the arguments that allege to establish their
possibility' (502. Chalmers offers brief replies in his 2003;
2010, pp. 159-60).



*The anti-zombie argument for physicalism*. The conceivability
argument -- which assumes physicalism entails that zombies are
impossible -- purports to refute it by showing they are possible.
As we saw, the simplest version of this argument goes: (1) zombies are
conceivable; (2) whatever is conceivable is possible; (3) therefore
zombies are possible. However, 'anti-zombies' --
duplicates of ourselves made conscious by the purely physical facts
(Frankish 2007) -- also seem conceivable. So we have a parallel
argument: (1\*) anti-zombies are conceivable; (2) whatever is
conceivable is possible; (3\*) therefore anti-zombies are possible. But
(3) and (3\*) cannot both be true, since if the purely physical facts
about anti-zombies make them conscious, then the exactly similar
physical facts about zombies make them conscious too, and they are not
zombies after all (Frankish 2007; Marton 1998; Piccinini 2017;
Sturgeon 2000, pp. 114-116). One moral is that we should reject
the inference from conceivability to possibility. (Brown 2010 argues
that if anti-zombies are conceivable, then zombies are inconceivable.)
The most promising reply for exponents of the conceivability argument
seems to be to deny that anti-zombies are conceivable (Chalmers 2010,
180).


## 6. Other issues



The list of 'Related Entries' below indicates the range
and depth of the issues raised by the zombie idea, only some of which
have been touched on in this entry. If zombies are really possible,
then not only is physicalism problematic, so are widely held views on
other topics. Here are three striking examples.


### 6.1 Mental causation



Descartes accepted the common assumption that not only do physical
events have mental effects, but mental events have physical effects
(for example, thinking about the political situation makes me write a
letter). The difficulty for his dualism, it was thought, was to
understand how the nonphysical could have effects on the physical. But
if zombies are possible -- which requires the physical world to
be causally closed -- there is no work for nonphysical qualia to
do. In that case the difficulty is to understand how, in spite of
appearances, the nonphysical could *fail* to have effects on
the physical. Still supposing zombies are possible, it then becomes
hard to see any alternative to parallelism or epiphenomenalism, with
the radical revision of common assumptions about mental causation that
those views entail. True, the friends of zombies do not seem compelled
to be epiphenomenalists or parallelists about the *actual*
world. They may be interactionists, holding that our world is not
physically closed, and that as a matter of actual fact nonphysical
properties do have physical effects. Or they may adopt some variety of
panpsychism, according to which what is metaphysically fundamental is
not physical properties, but phenomenal or perhaps
'protophenomenal' ones (Chalmers 1991, 297--299;
1999, 492; Goff 2017; Strawson 2008) -- a view arguably
compatible with the causal closure of the physical. But neither of
those options is easy. Abandoning causal closure appears to conflict
with empirical evidence; while the idea that phenomenal or
quasi-phenomenal properties are fundamental remains obscure.


### 6.2 The function of consciousness



The apparent possibility of zombies also seems to pose a problem for
evolutionary theory. Why did creatures with qualia survive rather than
those creatures' zombie counterparts? If zombies could have
survived, what's the use of consciousness? Owen Flanagan and
Thomas Polger have used the apparent possibility of zombies to support
the claim that 'There are as yet no credible stories about why
subjects of experience emerged, why they might have won -- or
should have been expected to win -- an evolutionary battle
against very intelligent zombie-like information-sensitive
organisms' (1995, 321): a problem not faced by those who reject
the possibility of zombies. One response on behalf of those who do
accept it is to suggest that there might be fundamental laws linking
the phenomenal to the physical. Such laws would not depend on whether
conscious creatures ever happened to evolve, in which case, arguably,
evolution raises no special problem (Chalmers 1996, 171) --
although the existence of such laws would pose its own problems.


### 6.3 Other minds



If qualia have no physical effects, then nothing will enable anyone to
establish for certain that anyone else actually has qualia.
Philosophers who believe they have a solid response to skepticism
about other minds may therefore conclude that this consequence of the
zombie idea is enough to condemn it. Others may draw the opposite
conclusion and take the skeptical consequence as 'a
confirmation', on the ground that we really are ignorant of
others' minds (Campbell 1970, 120). (Of course not all responses
to other minds skepticism imply that zombies are inconceivable.)


## 7. Conclusion



The intuitive appeal of the zombie idea can be overwhelming. But that
was true once of the idea that the earth stands still, and is true now
of the idea that science can explain events without appealing to
anything nonphysical. Some anti-physicalists believe their
opponents' commitment makes them turn a blind eye to the
difficulties:



> 
> Some may be led to deny the possibility [of zombies] in order to make
> some theory come out right, but the justification of such theories
> should ride on the question of possibility, rather than the other way
> round (Chalmers 1996, 96).
> 



On the other hand, some physicalists believe the zombie idea exerts an
irrational grip on anti-physicalist thinking, so that



> 
> it is tempting to regard anti-physicalist arguments as
> rationalizations of an intuition whose independent force masks their
> tendentiousness (Loar 1990/1997, 598).
> 



In spite of the fact that the arguments on both sides have become
increasingly sophisticated -- or perhaps because of it --
they have not become more persuasive. The pull in each direction
remains strong.