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https://plato.stanford.edu/entries/zombies/ | 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: either epiphenomenal or parallel to the correlated physical processes. It therefore seems that if a zombie world is conceivable then so is epiphenomenalism. (Note that 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 is the familiar and powerful claim that experiences have effects on behavior (Perry 2001). A less obvious objection to epiphenomenalism 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 or refer to must have effects on us, even if only indirectly (Kripke 1972/80). On that basis, our counterparts in epiphenomenalistic worlds could not know about or refer to their qualia. If that is right, epiphenomenalistic worlds are not conceivable, in which case neither are zombies. | zombies |
https://plato.stanford.edu/entries/zombies/ | 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). | zombies |
https://plato.stanford.edu/entries/zombies/ | 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, remembering, and comparing our experiences: activities that bring us into ‘epistemic contact’ with them. Such activities involve cognitive processing, which in turn involves changes causing other changes. Since epiphenomenal qualia are causally inert, they themselves could not do that processing; so if they actually constitute our experiences (as epiphenomenalism and parallelism imply) then the necessary processing must be done by the body. The trouble is that the zombie story makes 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 (Kirk argues) cannot do the necessary cognitive work. 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. | zombies |
https://plato.stanford.edu/entries/zombies/ | ‘Powerful qualities’. An interesting recent 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 whatever qualities are 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. | zombies |
https://plato.stanford.edu/entries/zombies/ | For other attacks on the conceivability of zombies see Balog 1999; Cottrell 1999; Harnad 1995; Marcus 2004; Shoemaker 1999; Stoljar 2001; Tye 2006. | zombies |
https://plato.stanford.edu/entries/zombies/ | Premise (2) of the conceivability argument is: whatever is conceivable is possible. Although this appears to be a defensible claim, it has been attacked from several angles. | zombies |
https://plato.stanford.edu/entries/zombies/ | 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 that 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). | zombies |
https://plato.stanford.edu/entries/zombies/ | 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 what he calls ‘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; Perry 2001, 169–208; Piccinini 2017; Sebastián 2017; Shoemaker 1999; Soames 2005; Yablo 1999.) | zombies |
https://plato.stanford.edu/entries/zombies/ | 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. | zombies |
https://plato.stanford.edu/entries/zombies/ | Thus it is argued that even if a zombie world is indeed 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.) | zombies |
https://plato.stanford.edu/entries/zombies/ | 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 about the ‘intrinsic’ properties which supposedly underlie and account for their 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. | zombies |
https://plato.stanford.edu/entries/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. | zombies |
https://plato.stanford.edu/entries/zombies/ | Philip Goff (2010) 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.) | zombies |
https://plato.stanford.edu/entries/zombies/ | 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. | zombies |
https://plato.stanford.edu/entries/zombies/ | 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. See Stalnaker 2002 for a related point, and for criticism, Alter 2007; Chalmers 2010, pp. 159–59; Crane 2006). | zombies |
https://plato.stanford.edu/entries/zombies/ | 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). | zombies |
https://plato.stanford.edu/entries/zombies/ | 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). | zombies |
https://plato.stanford.edu/entries/zombies/ | 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). | zombies |
https://plato.stanford.edu/entries/zombies/ | If zombies are genuinely possible, then not only is physicalism problematic, so are some widely held views on other topics. Here are three notable examples. | zombies |
https://plato.stanford.edu/entries/zombies/ | 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 — requiring 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 favor ‘panprotopsychism’, according to which what is metaphysically fundamental is not physical properties, but phenomenal or ‘protophenomenal’ ones (Chalmers 1991, 297—299; 1999, 492) — a view arguably compatible with the causal closure of the physical. But neither of those options is easy. Abandoning causal closure conflicts with empirical evidence; while the idea that phenomenal or quasi-phenomenal properties are fundamental is obscure. | zombies |
https://plato.stanford.edu/entries/zombies/ | 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 poses no special problem (Chalmers 1996, 171). | zombies |
https://plato.stanford.edu/entries/zombies/ | 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, however, may regard 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. | zombies |
https://plato.stanford.edu/entries/zombies/ | 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: | zombies |
https://plato.stanford.edu/entries/zombies/ | On the other hand, some physicalists believe the zombie idea exerts an irrational grip on anti-physicalist thinking, so that | zombies |
https://plato.stanford.edu/entries/zombies/ | 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. | zombies |
https://plato.stanford.edu/entries/zombies/ | animal: consciousness | behaviorism | -->conceivability--> | consciousness | dualism | epiphenomenalism | functionalism | -->knowledge argument--> | -->Kripke--> | mental causation | mind/brain identity theory | -->neutral monism--> | other minds | physicalism | private language | qualia | skepticism | supervenience | zombies |
https://plato.stanford.edu/entries/zombies/ | Many thanks to David Chalmers and to Bill Fish for valuable detailed comments and suggestions on drafts of this entry. | zombies |
https://plato.stanford.edu/entries/zombies/ | Copyright © 2019 by Robert Kirk <Robert.Kirk@nottingham.ac.uk> | zombies |
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https://plato.stanford.edu/entries/zombies/ | The Stanford Encyclopedia of Philosophy is copyright © 2021 by The Metaphysics Research Lab, Department of Philosophy, Stanford University | zombies |
https://plato.stanford.edu/entries/zombies/ | Library of Congress Catalog Data: ISSN 1095-5054 | zombies |