Philosophical zombies

A philosophical zombie or p-zombie in the philosophy of mind and perception is a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience. When a zombie is poked with a sharp object, for example, it does not feel any pain though it behaves exactly as if it does feel pain (it may say "ouch" and recoil from the stimulus, or tell us that it is in intense pain).

The notion of a philosophical zombie is used mainly in thought experiments intended to support arguments (often called "zombie arguments") against forms of physicalism such as materialism, behaviorism and functionalism. Physicalism is the idea that all aspects of human nature can be explained by physical means: specifically, all aspects of human nature and perception can be explained from a neurobiological standpoint. Some philosophers, like David Chalmers, argue that since a zombie is defined as physiologically indistinguishable from human beings, even its logical possibility would be a sound refutation of physicalism. However, physicalists like Daniel Dennett counter that Chalmers's physiological zombies are logically incoherent and thus impossible.

Types of zombie
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 behaviorism, according to which mental states exist solely as behavior: belief, desire, thought, consciousness, and so on, are simply certain kinds of behavior or tendencies towards behaviors. A p-zombie that is behaviorally indistinguishable from a normal human being but lacks conscious experiences is therefore not logically possible according to the behaviorist, 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 even logically possible.

The unifying idea of the zombie is of a human that has no conscious experience, but one might distinguish various types of zombie used in different thought experiments as follows:


 * A behavioral zombie that is behaviorally indistinguishable from a human.
 * A neurological zombie that has a human brain and is generally physiologically indistinguishable from a human.
 * A soulless zombie that lacks a "soul".

Zombie arguments
Zombie arguments often support lines of reasoning that aim to show that zombies are logically 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. According to physicalism, physical 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 a normal conscious human, it follows that physicalism must hold that p-zombies are either not possible or are the same as normal humans.

The zombie argument is a version of general modal arguments against physicalism such as that of Saul Kripke against that kind of physicalism known as type-identity theory. Further such arguments were notably advanced in the 1970s by Thomas Nagel (1970; 1974) and Robert Kirk (1974) but the general argument was most famously developed in detail by David Chalmers in The Conscious Mind (1996). According to Chalmers one can coherently conceive of an entire zombie world, a world physically indistinguishable from this world but entirely lacking conscious experience. The counterpart of every conscious being in our world would be a p-zombie. Since such a world is conceivable, Chalmers claims, it is logically possible, which is all the argument requires. Chalmers states: "Zombies are probably not naturally possible: they probably cannot exist in our world, with its laws of nature." The outline structure of Chalmers' version of the zombie argument is as follows;


 * 1) According to physicalism, all that exists in our world (including consciousness) is physical.
 * 2) Thus, if physicalism is true, a logically 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.
 * 3) In fact we can conceive of a world physically indistinguishable from our world but in which there is no consciousness (a zombie world). From this (so Chalmers argues) it follows that such a world is logically possible.
 * 4) Therefore, physicalism is false. (The conclusion follows from 2. and 3. by modus tollens.)

Responses
Chalmers' argument is logically valid: if its premises are true then the conclusion must be true. However, other philosophers dispute that its premises are true. For example, is such a world really possible? Chalmers states that "it certainly seems that a coherent situation is described; I can discern no contradiction in the description." This leads to the questions of the relevant notion of "possibility": is the scenario described in premise 3 possible in the sense that is suggested in premise 2? Most physicalist responses deny that the premise of a zombie scenario is possible.

Many physicalist philosophers argued that this scenario eliminates itself by its description; the basis of 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.

Some philosophers maintain that a possibility stronger than logical possibility is required and that, while a zombie world is logically possible, such a weak notion is not relevant in the analysis of a metaphysical thesis such as physicalism. Most agree that the relevant notion of possibility is some sort of metaphysical possibility. The zombie argument claims that one can tell by the power of reason that such a "zombie scenario" is metaphysically possible. Chalmers states; "From the conceivability of zombies, proponents of the argument infer their metaphysical possibility" 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.

Another response is 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". He coined the term zimboes (p-zombies that have second-order beliefs) to argue that the idea of a p-zombie is incoherent; "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!". Under (reductive) physicalism, one is inclined to believe either that anyone including oneself might be a zombie, or that no one can be a zombie – following from the assertion that one's own conviction about being, or not being a zombie is (just) a product of the physical world and is therefore no different from anyone else's. P-zombies in an observed world would be indistinguishable from the observer, even hypothetically (when the observer makes no assumptions regarding the validity of their convictions). Furthermore, when 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" contradicting the original definition of a zombie.

Verificationism states that, 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. A related argument is that of "zombie-utterance". If someone were to say they love the smell of some food, a zombie producing the same reaction would be perceived as a person having complex thoughts and ideas in their head indicated by the ability to vocalize it. If zombies were without awareness of their perceptions the idea of uttering words could not occur to them. Therefore, if a zombie has the ability to speak, it is not a zombie. Nigel Thomas argues that the zombie concept is self-contradictory in that, since zombies ex hypothesi behave just like regular humans, they will claim to be conscious, which, whether this claim is taken to be true, false, or neither true nor false, inevitably entails either a contradiction or a manifest absurdity.

Artificial intelligence researcher Marvin Minsky sees 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 was claiming to prove. 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."

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. However 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.

Another way to construe the zombie hypothesis, however, 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.

Related thought-experiments
Frank Jackson's Mary's room 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 Mary 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 were to experience color for the first time, would she learn anything new? Jackson initially believed this supported epiphenomenalism but later changed his views 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 that which Davidson had 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.

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.

References and further reading

 * 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: ISBN 0-19-511789-1, paperback: ISBN 0-19-510553-2
 * 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. ISBN 0-19-514581-X, Online PDF
 * Chalmers, David. 2004. "Imagination, Indexicality, and Intensions", Philosophy and Phenomenological Research, vol. 68, no. 1. Online text
 * Dennett, Daniel. 1995. "The Unimagined Preposterousness of Zombies", Journal of Consciousness Studies, vol. 2, no. 4, pp. 322–326. Online abstract
 * Dennett, Daniel. 1999. "The Zombic Hunch: Extinction of an Intuition?", Royal Institute of Philosophy Millennial Lecture. Online text
 * Kirk, Robert. 1974. "Sentience and Behaviour", Mind, vol. 83, pp. 43–60.
 * 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.
 * Nagel, Thomas. 1974. "What is it Like to Be a Bat?" Philosophical Review, vol. 83, pp. 435–450.
 * 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 text