For example, if a philosophical zombie were poked with a sharp object, it would not feel any pain, but it would react exactly the way any conscious human would. Philosophical zombie arguments are used against forms of physicalism and in defense of the hard problem of consciousness, which is the problem of accounting in physical terms for subjective, intrinsic, first-person, what-it's-like-ness experiences. Proponents of philosophical zombie arguments, such as the philosopher David Chalmers, argue that since a philosophical zombie is by definition physically identical to a conscious person, even its logical possibility refutes physicalism. This is because it establishes the existence of conscious experience as a further fact.[2] Philosopher Daniel Stoljar points out that zombies need not be utterly without subjective states, and that even a subtle psychological difference between two physically identical people, such as how coffee tastes to them, is enough to refute physicalism.[3] Such arguments have been criticized by many philosophers. Some physicalists, such as Daniel Dennett, argue that philosophical zombies are logically incoherent and thus impossible, or that all humans are philosophical zombies;[4][5] others, such as Christopher Hill, argue that philosophical zombies are coherent but metaphysically impossible.[6]
History
Philosophical zombies are associated with David Chalmers, but it was philosopher Robert Kirk who first used the term "zombie" in this context, in 1974. Before that, Keith Campbell made a similar argument in his 1970 book Body and Mind, using the term "imitation man".[7] Chalmers further developed and popularized the idea in his work.
There has been a lively debate about what the zombie argument shows.[7] Critics who primarily argue that zombies are not conceivable include Daniel Dennett, Nigel J. T. Thomas,[8] David Braddon-Mitchell,[9] and Robert Kirk.[10] Critics who assert mostly that conceivability does not entail possibility include Katalin Balog,[11]Keith Frankish,[12]Christopher Hill,[6] and Stephen Yablo.[13] Critics who question the argument's logical validity include George Bealer.[14]
In his 2019 update to the article on philosophical zombies in the Stanford Encyclopedia of Philosophy, Kirk summed up the current state of the debate:
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.[15]
A 2013 survey of professional philosophers by Bourget and Chalmers found that 36% said p-zombies were conceivable but metaphysically impossible; 23% said they were metaphysically possible; 16% said they were inconceivable; and 25% responded "other".[16] In 2020, the same survey yielded almost identical results: "inconceivable" 16%, conceivable but impossible 37%, "metaphysically possible" 24%, and "other" 23%.[17]
Types of zombies
Though philosophical zombies are widely used in thought experiments, the detailed articulation of the concept is not always the same. P-zombies were introduced primarily to argue against specific types of physicalism such as materialism and behaviorism, according to which mental states exist solely as behavior. Belief, desire, thought, consciousness, and so on, are conceptualized as behavior (whether external behavior or internal behavior) or tendencies towards behaviors. A p-zombie behaviorally indistinguishable from a normal human being but lacking conscious experiences is therefore not logically possible according to the behaviorist,[18] so an appeal to the logical possibility of a p-zombie furnishes an argument that behaviorism is false. Proponents of zombie arguments generally accept that p-zombies are not physically possible, while opponents necessarily deny that they are metaphysically or, in some cases, even logically possible.
The unifying idea of the zombie is that of a human completely lacking conscious experience. It is possible to distinguish various zombie subtypes used in different thought experiments as follows:
A behavioral zombie is behaviorally indistinguishable from a human.
A neurological zombie has a human brain and is generally physiologically indistinguishable from a human.[19]
An imperfect zombie or imp-zombie is like a p-zombie but behaves differently than a human. It is important in the context of the mind-evolution problem.[20]
A zombie universe is identical to our world in all physical ways, except no being in it has qualia.
Zombie arguments
Zombie arguments often support lines of reasoning that aim to show that zombies are metaphysically possible in order to support some form of dualism—in this case the view that the world includes two kinds of substance (or perhaps two kinds of property): the mental and the physical.[21]
In physicalism, material facts determine all other facts. Since any fact other than that of consciousness may be held to be the same for a p-zombie and for a normal conscious human, it follows that physicalism must hold that p-zombies are either not possible or are the same as normal humans.
According to Chalmers, one can coherently conceive of an entire zombie world, a world physically indistinguishable from this one but entirely lacking conscious experience. Since such a world is conceivable, Chalmers claims, it is metaphysically possible, which is all the argument requires. Chalmers writes: "Zombies are probably not naturally possible: they probably cannot exist in our world, with its laws of nature."[23] The outline structure of Chalmers's version of the zombie argument is as follows:
According to physicalism, all that exists in our world (including consciousness) is physical.
Thus, if physicalism is true, a metaphysically possible world in which all physical facts are the same as those of the actual world must contain everything that exists in our actual world. In particular, conscious experience must exist in such a possible world.
Chalmers argues that we can conceive of a world physically indistinguishable from our world but in which there is no consciousness (a zombie world). From this it follows that such a world is metaphysically possible.
The above is a strong formulation of the zombie argument. There are other formulations of zombie-type arguments that follow the same general form. The premises of the general zombie argument are implied by the premises of all the specific zombie arguments.
A general zombie argument is in part motivated by potential disagreements between various anti-physicalist views. For example, an anti-physicalist view can consistently assert that p-zombies are metaphysically impossible but that inverted qualia (such as inverted spectra) or absent qualia (partial zombiehood) are metaphysically possible. Premises regarding inverted qualia or partial zombiehood can replace premises regarding p-zombies to produce variations of the zombie argument.
The metaphysical possibility of a physically indistinguishable world with either inverted qualia or partial zombiehood implies that physical truths do not metaphysically necessitate phenomenal truths.
To construct the general form of the zombie argument, take the sentence P to be true if and only if the conjunct of all microphysical truths of our world obtain, and take the sentence Q to be true if some phenomenal truth that obtains in the actual world obtains. The general argument goes as follows.
It is conceivable that P is true and Q is not true.
If it is conceivable that P is true and Q is not true then it is metaphysically possible that P is true and Q not true.
If it is metaphysically possible that P is true and Q is not true then physicalism is false.
Q can be false in a possible world if any of the following obtains: (1) there exists at least one invert relative to the actual world; (2) there is at least one absent quale relative to the actual world; (3) all actually conscious beings are p-zombies (all actual qualia are absent qualia).
Another way to construe the zombie hypothesis is epistemically—as a problem of causal explanation, rather than as a problem of logical or metaphysical possibility. The "explanatory gap"—also called the "hard problem of consciousness"—is the claim that (to date) no one has provided a convincing causal explanation of how and why we are conscious. It is a manifestation of the very same gap that (to date) no one has provided a convincing causal explanation of how and why we are not zombies.[25]
The philosophical zombie argument can also be seen through the counterfeit bill example brought forth by Amy Kind. Kind's example centers around a counterfeit 20-dollar bill made to be exactly like an authentic 20-dollar bill. This is logically possible. Yet the counterfeit bill would not have the same value.
According to Kind, in her book Philosophy of Mind: The Basics, The Zombie Argument can be put in this standard form from a dualist point of view:
Zombies, creatures that are microphysically identical to conscious beings but that lack consciousness entirely, are conceivable. If zombies are conceivable then they are possible. Therefore, zombies are possible. If zombies are possible, then consciousness is non-physical. Therefore, consciousness is non-physical.[26]
Responses
Galen Strawson argues that it is not possible to establish the conceivability of zombies, so the argument, lacking its first premise, can never get going.[27]
Chalmers has argued that zombies are conceivable, saying, "it certainly seems that a coherent situation is described; I can discern no contradiction in the description."[28]
Many physicalist philosophers[who?] have argued that this scenario eliminates itself by its description; the basis of a physicalist argument is that the world is defined entirely by physicality; thus, a world that was physically identical would necessarily contain consciousness, as consciousness would necessarily be generated from any set of physical circumstances identical to our own.
The zombie argument claims that one can tell by the power of reason that such a "zombie scenario" is metaphysically possible. Chalmers writes, "From the conceivability of zombies, proponents of the argument infer their metaphysical possibility"[23] and argues that this inference, while not generally legitimate, is legitimate for phenomenal concepts such as consciousness since we must adhere to "Kripke's insight that for phenomenal concepts, there is no gap between reference-fixers and reference (or between primary and secondary intentions)."
That is, for phenomenal concepts, conceivability implies possibility. According to Chalmers, whatever is logically possible is also, in the sense relevant here, metaphysically possible.[29]
Another response is the denial of the idea that qualia and related phenomenal notions of the mind are in the first place coherent concepts. Daniel Dennett and others argue that while consciousness and subjective experience exist in some sense, they are not as the zombie argument proponent claims. The experience of pain, for example, is not something that can be stripped off a person's mental life without bringing about any behavioral or physiological differences. Dennett believes that consciousness is a complex series of functions and ideas. If we all can have these experiences the idea of the p-zombie is meaningless.
Dennett argues that "when philosophers claim that zombies are conceivable, they invariably underestimate the task of conception (or imagination), and end up imagining something that violates their own definition".[4][5] He coined the term "zimboes"—p-zombies that have second-order beliefs—to argue that the idea of a p-zombie is incoherent;[30] "Zimboes thinkZ they are conscious, thinkZ they have qualia, thinkZ they suffer pains—they are just 'wrong' (according to this lamentable tradition), in ways that neither they nor we could ever discover!".[5]
Michael Lynch agrees with Dennett, arguing that the zombie conceivability argument forces us to either question whether we actually have consciousness or accept that zombies are not possible. If zombies falsely believe they are conscious, how can we be sure we are not zombies? We may believe we are experiencing conscious mental states when in fact we merely hold a false belief. Lynch thinks denying the possibility of zombies is more reasonable than questioning our own consciousness.[31]
Furthermore, when the concept of self is deemed to correspond to physical reality alone (reductive physicalism), philosophical zombies are denied by definition. When a distinction is made in one's mind between a hypothetical zombie and oneself (assumed not to be a zombie), the hypothetical zombie, being a subset of the concept of oneself, must entail a deficit in observables (cognitive systems), a "seductive error"[5] contradicting the original definition of a zombie.
Thomas Metzinger dismisses the zombie argument as no longer relevant to the consciousness community, calling it a weak argument that covertly relies on the difficulty in defining "consciousness" and an "ill-defined folk psychological umbrella term".[32]
According to verificationism,[1] for words to have meaning, their use must be open to public verification. Since it is assumed that we can talk about our qualia, the existence of zombies is impossible.
Artificial intelligence researcher Marvin Minsky saw the argument as circular. The proposition of the possibility of something physically identical to a human but without subjective experience assumes that the physical characteristics of humans are not what produces those experiences, which is exactly what the argument claims to prove.[33]
Richard Brown agrees that the zombie argument is circular. To show this, he proposes "zoombies", which are creatures nonphysically identical to people in every way and lacking phenomenal consciousness. If zoombies existed, they would refute dualism because they would show that consciousness is indeed physical. Paralleling the argument from Chalmers: It is conceivable that zoombies exist, so it is possible they exist, so dualism is false. Given the symmetry between the zombie and zoombie arguments, we cannot arbitrate the physicalism/dualism question a priori.[34]
Similarly, Gualtiero Piccinini argues that the zombie conceivability argument is circular. Piccinini questions whether the possible worlds where zombies exist are accessible from our world. If physicalism is true in our world, then physicalism is one of the relevant facts about our world for determining whether a possible zombie world is accessible from our world. Therefore, asking whether zombies are metaphysically possible in our world is equivalent to asking whether physicalism is true in our world.[35]
Stephen Yablo's (1998) response is to provide an error theory to account for the intuition that zombies are possible. Notions of what counts as physical and as physically possible change over time so conceptual analysis is not reliable here. Yablo says he is "braced for the information that is going to make zombies inconceivable, even though I have no real idea what form the information is going to take."[36]
The zombie argument is difficult to assess because it brings to light fundamental disagreements about the method and scope of philosophy itself and the nature and abilities of conceptual analysis. Proponents of the zombie argument may think that conceptual analysis is a central part of (if not the only part of) philosophy and that it certainly can do a great deal of philosophical work. But others, such as Dennett, Paul Churchland and W.V.O. Quine, have fundamentally different views. For this reason, discussion of the zombie argument remains vigorous in philosophy.
Some accept modal reasoning in general but deny it in the zombie case. Christopher S. Hill and Brian P. McLaughlin suggest that the zombie thought experiment combines imagination of a "sympathetic" nature (putting oneself in a phenomenal state) and a "perceptual" nature (imagining becoming aware of something in the outside world). Each type of imagination may work on its own but not work when used at the same time. Hence Chalmers's argument need not go through.[37]:448
Moreover, while Chalmers defuses criticisms of the view that conceivability can tell us about possibility, he provides no positive defense of the principle. As an analogy, the generalized continuum hypothesis has no known counterexamples, but this does not mean we must accept it. Indeed, according to Hill and McLaughlin, the fact that Chalmers concludes we have epiphenomenal mental states that do not cause our physical behavior seems to be a reason to reject his principle.[37]:449–51
Related thought experiments
Frank Jackson's knowledge argument is based around a hypothetical scientist, Mary, who is forced to view the world through a black-and-white television screen in a black and white room. Mary is a brilliant scientist who knows everything about the neurobiology of vision. Even though she knows everything about color and its perception (e.g. what combination of wavelengths makes the sky seem blue), she has never seen color. If Mary were released from this room and experienced color for the first time, would she learn anything new? Jackson initially believed this supported epiphenomenalism (mental phenomena are the effects, but not the causes, of physical phenomena) but later changed his view to physicalism, suggesting that Mary is simply discovering a new way for her brain to represent qualities that exist in the world.
Swampman is an imaginary character introduced by Donald Davidson. If Davidson goes hiking in a swamp and is struck and killed by a lightning bolt while nearby another lightning bolt spontaneously rearranges a bunch of molecules so that, entirely by coincidence, they take on exactly the same form that Davidson's body had at the moment of his untimely death, then this being, "Swampman", has a brain structurally identical to Davidson's and will thus presumably behave exactly like Davidson. He will return to Davidson's office and write the same essays he would have written, recognize all of his friends and family, and so forth.[38]
John Searle's Chinese room argument deals with the nature of artificial intelligence: it imagines a room in which a conversation is held by means of written Chinese characters that the subject cannot actually read, but is able to manipulate meaningfully using a set of algorithms. Searle holds that a program cannot give a computer a "mind" or "understanding", regardless of how intelligently it may make it behave. Stevan Harnad argues that Searle's critique is really meant to target functionalism and computationalism, and to establish neuroscience as the only correct way to understand the mind.[39]
Physicist Adam Brown has suggested constructing a type of philosophical zombie using counterfactual quantum computation, a technique in which a computer is placed into a superposition of running and not running. If the program being executed is a brain simulation, and if one makes the further assumption that brain simulations are conscious, then the simulation can have the same output as a conscious system, yet not be conscious.[40]
Epiphenomenalism is a position in the philosophy of mind on the mind–body problem. It holds that subjective mental events are completely dependent for their existence on corresponding physical and biochemical events within the human body, but do not themselves influence physical events. According to epiphenomenalism, the appearance that subjective mental states influence physical events is an illusion, with consciousness being a by-product of physical states of the world. For instance, fear seems to make the heart beat faster, but according to epiphenomenalism, the biochemical secretions of the brain and nervous system —not the experience of fear—is what raises the heartbeat. Because mental events are a kind of overflow that cannot cause anything physical, yet have non-physical properties, epiphenomenalism is viewed as a form of property dualism.
In philosophy, physicalism is the view that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. It is opposed to idealism, according to which the world arises from mind. Physicalism is a form of ontological monism—a "one substance" view of the nature of reality, unlike "two-substance" or "many-substance" (pluralism) views. Both the definition of "physical" and the meaning of physicalism have been debated.
Consciousness Explained is a 1991 book by the American philosopher Daniel Dennett, in which the author offers an account of how consciousness arises from interaction of physical and cognitive processes in the brain. Dennett describes consciousness as an account of the various calculations occurring in the brain at close to the same time. He compares consciousness to an academic paper that is being developed or edited in the hands of multiple people at one time, the "multiple drafts" theory of consciousness. In this analogy, "the paper" exists even though there is no single, unified paper. When people report on their inner experiences, Dennett considers their reports to be more like theorizing than like describing. These reports may be informative, he says, but a psychologist is not to take them at face value. Dennett describes several phenomena that show that perception is more limited and less reliable than we perceive it to be.
Intentionality is the mental ability to refer to or represent something. Sometimes regarded as the mark of the mental, it is found in mental states like perceptions, beliefs or desires. For example, the perception of a tree has intentionality because it represents a tree to the perceiver. A central issue for theories of intentionality has been the problem of intentional inexistence: to determine the ontological status of the entities which are the objects of intentional states.
David John Chalmers is an Australian philosopher and cognitive scientist specializing in the areas of the philosophy of mind, and the philosophy of language. He is a professor of philosophy and neural science at New York University, as well as co-director of NYU's Center for Mind, Brain and Consciousness. In 2006, he was elected a Fellow of the Australian Academy of the Humanities. In 2013, he was elected a Fellow of the American Academy of Arts & Sciences.
In the philosophy of mind, the explanatory gap is the difficulty that physicalist philosophies have in explaining how physical properties give rise to the way things feel subjectively when they are experienced. It is a term introduced by philosopher Joseph Levine. In the 1983 paper in which he first used the term, he used as an example the sentence, "Pain is the firing of C fibers", pointing out that while it might be valid in a physiological sense, it does not help us to understand how pain feels.
Eliminative materialism is a materialist position in the philosophy of mind. It is the idea that the majority of mental states in folk psychology do not exist. Some supporters of eliminativism argue that no coherent neural basis will be found for many everyday psychological concepts such as belief or desire, since they are poorly defined. The argument is that psychological concepts of behavior and experience should be judged by how well they reduce to the biological level. Other versions entail the nonexistence of conscious mental states such as pain and visual perceptions.
The knowledge argument is a philosophical thought experiment proposed by Frank Jackson in his article "Epiphenomenal Qualia" (1982) and extended in "What Mary Didn't Know" (1986).
In the philosophy of mind, the hard problem of consciousness is to explain why and how humans and other organisms have qualia, phenomenal consciousness, or subjective experience. It is contrasted with the "easy problems" of explaining why and how physical systems give a (healthy) human being the ability to discriminate, to integrate information, and to perform behavioral functions such as watching, listening, speaking, and so forth. The easy problems are amenable to functional explanation—that is, explanations that are mechanistic or behavioral—since each physical system can be explained purely by reference to the "structure and dynamics" that underpin the phenomenon.
The inverted spectrum is the hypothetical concept, pertaining to the philosophy of color, of two people sharing their color vocabulary and discriminations, although the colors one sees—one's qualia—are systematically different from the colors the other person sees.
In philosophy of mind, Cartesian materialism is the idea that at some place in the brain, there is some set of information that directly corresponds to our conscious experience. Contrary to its name, Cartesian materialism is not a view that was held by or formulated by René Descartes, who subscribed rather to a form of substance dualism.
The philosophy of mind is a branch of philosophy that deals with the nature of the mind and its relation to the body and the external world.
"What Is It Like to Be a Bat?" is a paper by American philosopher Thomas Nagel, first published in The Philosophical Review in October 1974, and later in Nagel's Mortal Questions (1979). The paper presents several difficulties posed by phenomenal consciousness, including the potential insolubility of the mind–body problem owing to "facts beyond the reach of human concepts", the limits of objectivity and reductionism, the "phenomenological features" of subjective experience, the limits of human imagination, and what it means to be a particular, conscious thing.
Michael Tye is a British philosopher who is currently the Dallas TACA Centennial Professor in Liberal Arts at the University of Texas at Austin. He has made significant contributions to the philosophy of mind.
In philosophy of mind, qualia are defined as instances of subjective, conscious experience. The term qualia derives from the Latin neuter plural form (qualia) of the Latin adjective quālis meaning "of what sort" or "of what kind" in relation to a specific instance, such as "what it is like to taste a specific apple — this particular apple now".
The Conscious Mind: In Search of a Fundamental Theory was published in 1996, and is the first book written by David Chalmers, an Australian philosopher specialising in philosophy of mind. Although the book has been greatly influential, Chalmers maintains that it is "far from perfect", as most of it was written as part of his PhD dissertation after "studying philosophy for only four years".
Externalism is a group of positions in the philosophy of mind which argues that the conscious mind is not only the result of what is going on inside the nervous system, but also what occurs or exists outside the subject. It is contrasted with internalism which holds that the mind emerges from neural activity alone. Externalism is a belief that the mind is not just the brain or functions of the brain.
The concept of absent qualia is one of two major functionalist objections to the existence of qualia, the other being the inverted spectrum hypothesis. Qualia is a philosophical term used to refer to an individual's subjective experience, that is to say, the way something feels to that individual at that particular moment.
Joseph Levine is an American philosopher at the University of Massachusetts Amherst who received his PhD from Harvard University in 1981.
The phenomenal concept strategy (PCS) is an approach within philosophy of mind to provide a physicalist response to anti-physicalist arguments like the explanatory gap and philosophical zombies. The name was coined by Daniel Stoljar. As David Chalmers put it, PCS "locates the gap in the relationship between our concepts of physical processes and our concepts of consciousness, rather than in the relationship between physical processes and consciousness themselves." The idea is that if we can explain why we think there is an explanatory gap, this will defuse the motivation to question physicalism.
References
Notes
1 2 Kirk, Robert (2009). "Zombie". In Zalta, Edward N. (ed.). The Stanford Encyclopedia of Philosophy (Summer 2009sed.).
↑ Chalmers, D. (1996): The Conscious Mind, Oxford University Press, New York.
1 2 Hill, Christopher (1997). "Imaginability, conceivability, possibility, and the mind-body problem". Philosophical Studies. 87 (1): 61–85. doi:10.1023/A:1017911200883. S2CID170606123.
↑ Hauser, Larry (2006). "Zombies, Blade Runner and the Mind-Body Problem". In Greene, Richard; Mohammad, K. Silem (eds.). The Undead and Philosophy. Open Court. p.55. ISBN978-0-8126-9601-1. Philosophy zombies "eat brains" by seeming to be counterexamples to every attempt to identify mental states with adaptive behavioral output. Behavioralists propose to identify mental states with adaptive behavioral output.
↑ Lynch, Michael P. (2006). Zombies and the case of the phenomenal pickpocket. Synthese 149 (1):37-58.
↑ Harris, Sam; Metzinger, Thomas (September 10, 2017). "The Nature of Consciousness". Making Sense. Sam Harris. Retrieved 27 August 2020. (25.45) TM:I think it will not be a mystery. Life is not a mystery anymore, but a hundred and fifty years ago many people thought that this is an irreducible mystery. (25:57) SH:So you're not a fan anymore, if you ever were, of the framing by David Chalmers of the Hard Problem of Consciousness? TM: No, that's so boring. I mean, that's last century. I mean, you know, we all respect Dave [Chalmers], and we know he is very smart and has got a very fast mind, no debate about that. But Conceivability Arguments are just very, very weak. If you have an ill-defined folk psychological umbrella term like "consciousness", then you can pull off all kinds of scenarios and zombie thought experiments. It doesn't really… It helped to clarify some issues in the mid 90's, but the consciousness community has listened to this and just moved on. I mean nobody of the serious researchers in the field thinks about this anymore, but it has taken on like a folkloristic life of its own. A lot of people talk about the Hard Problem who wouldn't be able to state what it consists in now.
1 2 Christopher S. Hill; Brian P. Mclaughlin (Jun 1999). "There are Fewer Things in Reality Than are Dreamt of in Chalmers's Philosophy". Philosophy and Phenomenological Research. 59 (2): 445–454. doi:10.2307/2653682. JSTOR2653682.
↑ Harnad, Stevan (2001), "What's Wrong and Right About Searle's Chinese Room Argument", in M.; Preston, J., Essays on Searle's Chinese Room Argument, Oxford University Press.
Chalmers, David. 1995. "Facing Up to the Problem of Consciousness", Journal of Consciousness Studies, vol. 2, no. 3, pp.200–219. Online PDF
Chalmers, David. 1996. The Conscious Mind: In Search of a Fundamental Theory, New York and Oxford: Oxford University Press. Hardcover: ISBN0-19-511789-1, paperback: ISBN0-19-510553-2
Chalmers, David. 2003. "Consciousness and its Place in Nature", in the Blackwell Guide to the Philosophy of Mind, S. Stich and F. Warfield (eds.), Blackwell. Also in Philosophy of Mind: Classical and Contemporary Readings, D. Chalmers (ed.), Oxford, 2002. ISBN0-19-514581-X, Online PDF
Kripke, Saul. 1972. "Naming and Necessity", in Semantics of Natural Language, ed. by D. Davidson and G. Harman, Dordrecht, Holland: Reidel, pp.253–355. (Published as a book in 1980, Harvard University Press.)
Nagel, Thomas. 1970. "Armstrong on the Mind", Philosophical Review, vol. 79, pp.394–403. JSTOR2183935
Thomas, N.J.T. 1998. "Zombie Killer", in S.R. Hameroff, A.W. Kaszniak, & A.C. Scott (eds.), Toward a Science of Consciousness II: The Second Tucson Discussions and Debates (pp.171–177), Cambridge, MA: MIT Press. Online
Yablo, Stephen. 2000. "Textbook Kripkeanism and the Open Texture of Concepts", Pacific Philosophical Quarterly, vol. 81, pp.98–122. Online textdoi:10.1111/1468-0114.00097
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