The philosophical zombie or p-zombie argument is a thought experiment in philosophy of mind and philosophy of perception that imagines a being that, if it could conceivably exist, logically disproves the idea that physical substance is all that is required to explain consciousness. Such a zombie would be indistinguishable from a normal human being but lack conscious experience, qualia, or sentience. For example, if a philosophical zombie were poked with a sharp object it would not inwardly feel any pain, yet it would outwardly behave exactly as if it did feel pain. The thought experiment sometimes takes the form of imagining a zombie world, indistinguishable from our world, but lacking first person experiences in any of the beings of that world.
Philosophical zombie arguments are used in support of mind-body dualism against forms of physicalism such as materialism, behaviorism and functionalism. It is an argument against the idea that the "hard problem of consciousness" (accounting for subjective, intrinsic, first person, what-it's-like-ness) could be answered by purely physical means. Proponents of the argument, such as philosopher 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, because it would establish the existence of conscious experience as a further fact. However, some physicalists like Daniel Dennett counter that philosophical zombies are logically incoherent and thus impossible; other physicalists like Christopher Hill argue that philosophical zombies are coherent but not metaphysically possible.
Philosophical zombies are associated with David Chalmers, but it was philosopher Robert Kirk who first used the term "zombie" in this context in 1974. Prior to that, Karlyn Campbell made a similar argument in his 1970 book Body and Mind, using the term "Imitation Man." Chalmers further developed and popularized the idea in his work.
There has been a lively debate about what the zombie argument shows. Critics who primarily argue that zombies are not conceivable include Daniel Dennett, Nigil J. T. Thomas, David Braddon-Mitchell, and Robert Kirk; critics who assert mostly that conceivability does not entail possibility include Katalin Balog, Keith Frankish, Christopher Hill, and Stephen Yablo; and critics who question the logical validity of the argument include George Bealer.
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.
A 2013 survey of professional philosophers conducted by Bourget and Chalmers produced the following results:
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 behaviorism, according to which mental states exist solely as behavior. Belief, desire, thought, consciousness, and so on, are only behavior (whether external behavior or internal 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 metaphysically or, in some cases, 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.
- A zombie universe that is identical to our world in all physical ways, except no being in that world has qualia.
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. 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 and the 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 metaphysically 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;
- 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.
- 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 metaphysically possible.
- Therefore, physicalism is false. (The conclusion follows from 2. and 3. by modus tollens.)
The above is a strong formulation of the zombie argument. There are other formulations of the zombies-type argument which follow the same general form. The premises of the general zombies argument are implied by the premises of all the specific zombie arguments. A general zombies 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 substitute 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 would imply that physical truths don't metaphysically necessitate phenomenal truths. To formulate the general form of the zombies argument, take the sentence 'P' to be true if and only if the conjunct of all microphysical truths of our world obtain, 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.
- Therefore, 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).
If one accepts two-dimensional semantics, Chalmers' argument is logically valid. Some philosophers accept its validity but dispute its soundness, arguing that its premises are false. Zombies might not actually be conceivable or, if they are, just because they are conceivable, that might not mean that they are possible. 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." This leads to the questions of the relevant notion of "possibility": if something is conceivable, does that mean it is possible? Most physicalist responses deny that the premise of a zombie scenario is possible.
Many physicalist philosophers 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.
One can hold that zombies are a logical possibility but not a metaphysical possibility. If logical possibility does not entail metaphysical possibility across the domain of relevant truths, then the mere logical possibility of zombies is not sufficient to establish their 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 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". 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.
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 was claiming to prove. 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 not nonphysical, i.e., is physical. Paralleling the argument from Chalmers: It's conceivable that zoombies exist, so it's possible they exist, so dualism is false. Given the symmetry between the zombie and zoombie arguments, we can't arbitrate the physicalism/dualism question a priori.
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.
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 they're not guaranteed to work when both used at the same time. Hence Chalmers's argument needn't go through.: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 doesn't mean we must accept it. And indeed, the fact that Chalmers concludes we have epiphenomenal mental states that don't cause our physical behavior seems one reason to reject his principle.:449–51
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.
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 (mental phenomena are the effects, but not the causes, of physical phenomena) 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.
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.
- Begging the question
- Dual-aspect theory
- Explanatory gap
- Functionalism (philosophy of mind)
- Hard problem of consciousness
- Inverted spectrum
- Map–territory relation
- Mind-body problem
- Neutral monism
- No true Scotsman
- Philosophy of mind
- Problem of other minds
- Quantum Night
- Reverse engineering
- Ship of Theseus
- Turing test
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- Field Guide to the Philosophy of Mind
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- Skepdic entry on p-zombies
- A Chaospet comic on the subject of philosophical zombies
- On The Conceivability of Zombies Paper argues that Philosophical Zombies are not conceivable