Philosophical zombie  

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A philosophical zombie, p-zombie or p-zed is a hypothetical being that is indistinguishable from a normal human being except 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. While 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), it does not actually have the experience of pain as a putative 'normal' person does.

The notion of a philosophical zombie is mainly a thought experiment used in arguments (often called zombie arguments) in the philosophy of mind, particularly arguments against forms of physicalism, such as materialism and behaviorism.


Types of zombies

Philosophical zombies are widely used in thought experiments, though the detailed articulation of the concept is not always the same. There are, in effect, different types of p-zombies. What differs is how much exactly they have in common with normal human beings. P-zombies were introduced primarily to argue against specific types of physicalism, such as behaviorism. According to behaviorism, mental states exist solely in terms of behavior: belief, desire, thought, consciousness, and so on, are simply certain kinds of behavior or tendencies towards behaviors. One might invoke the notion of a p-zombie that is behaviorally indistinguishable from a normal human being, but that lacks conscious experiences. According to the behaviorist, such a being is not logically possible, since consciousness is defined in terms of behavior. So an appeal to the intuition that a p-zombie so described is possible furnishes an argument that behaviorism is false. Behaviorists tend to respond to this that a p-zombie is not possible and so the theory that one might exist is false.

One might distinguish between various types of zombies, as they are used in different thought experiments, as follows:

  • A behavioral zombie is behaviorally indistinguishable from a human and yet has no conscious experience.
  • A neurological zombie has a human brain and is otherwise physically indistinguishable from a human; nevertheless, it has no conscious experience.
  • A soulless zombie lacks a soul but is otherwise indistinguishable from a human; this concept is used to inquire to what, if anything, the soul might amount.

However, philosophical zombies are primarily discussed in the context of arguments against physicalism (or functionalism) in general. Thus, a p-zombie is typically understood as a being that is physically indistinguishable from a normal human being but that lacks conscious experience.

Zombie arguments

According to physicalism, the physical facts determine all other facts; it follows that, since all the facts about a p-zombie are fixed by the physical facts, and these facts are the same for the p-zombie and for the normal conscious human from which it cannot be physically distinguished, physicalism must hold that p-zombies are not possible, or that p-zombies are the same as normal humans. Therefore, zombie arguments support lines of reasoning that aim to show that zombies are possible.

Most arguments ultimately lend support to some form of dualism—the view that the world includes two kinds of substance (or perhaps two kinds of property): the mental and the physical.

The zombie argument against physicalism is, therefore, a version of a general modal argument against physicalism, such as that of Saul Kripke's in "Naming and Necessity" (1972). The notion of a p-zombie, as used to argue against physicalism, was notably advanced in the 1970s by Thomas Nagel (1970; 1974) and Robert Kirk (1974).

However, the zombie argument against physicalism in general 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 indiscernible from our world, but entirely lacking conscious experience. In such a world, the counterpart of every being that is conscious in our world would be a p-zombie. The structure of Chalmers' version of the zombie argument can be outlined as follows:

  1. If physicalism is true, then it is not possible for there to be a world in which all the physical facts are the same as those of the actual world but in which there are additional facts. (This is because, according to physicalism, all the facts are fully determined by the physical facts; so any world that is physically indistinguishable from our world is entirely indistinguishable from our world.)
  2. But there is a possible world in which all the physical facts are the same as those of our world but in which there are additional facts. (For example, it is possible that there is a world exactly like ours in every physical respect, but in it everyone lacks certain mental states, namely any phenomenal experiences or qualia. The people there look and act just like people in the actual world, but they don't feel anything; when one gets shot, for example, he yells out as if he is in pain, but he doesn't feel any pain.)
  3. Therefore, physicalism is false. (The conclusion follows by modus tollens.)

The argument is logically valid, in that if its premises are true, then the conclusion must be true. However, philosophers dispute that its premises are true. For example, concerning premise 2: Is such a zombie world really possible? Chalmers states that "it certainly seems that a coherent situation is described; I can discern no contradiction in the description." Since such a world is conceivable, Chalmers claims, it is possible; and if such a world is possible, then physicalism is false. Chalmers is arguing only for logical possibility, and he maintains that this is all that his argument requires. He states: "Zombies are probably not naturally possible: they probably cannot exist in our world, with its laws of nature."

This leads to the following questions: What is the relevant notion of possibility here? Is the scenario in premise 2 possible in the sense that is suggested in premise 1? Some philosophers maintain that the relevant kind of possibility is not so weak as logical possibility. They argue that, while a zombie world is logically possible (that is, there is no logical contradiction in any full description of the scenario), such a weak notion is not relevant in the analysis of a metaphysical thesis such as physicalism. Most philosophers agree that the relevant notion of possibility is some sort of metaphysical possibility. What the proponent of the zombie argument claims is that one can tell from the armchair, just 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." Chalmers claims that this inference from conceivability to metaphysical possibility is not generally legitimate, but it is legitimate for phenomenal concepts such as consciousness.


A physicalist might respond to the zombie argument in several ways. Most responses deny premise 2 (of Chalmers' version above); that is, they deny that a zombie scenario is possible.

One response is to claim that the idea of qualia and related phenomenal notions of the mind are not coherent concepts, and the zombie scenario is therefore incoherent. Daniel Dennett and others take this line. They argue that while consciousness, subjective experiences, and so forth exist in some sense, they are not as the zombie argument proponent claims they are; 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 coined the term zimboes (philosophical zombies that have second-order beliefs) to argue that the idea of a philosophical zombie is incoherent. He states: "Philosophers ought to have dropped the zombie like a hot potato, but since they persist in their embrace, this gives me a golden opportunity to focus attention on the most seductive error in current thinking." In a related vein, Nigel Thomas argues that the zombie concept is inherently self-contradictory: Because zombies, ex hypothesis, behave just like regular humans, they will claim to be conscious. Thomas argues that any construal of this claim (that is, whether it is taken to be true, false, or neither true nor false) inevitably entails either a contradiction or a manifest absurdity.

Another physicalist response is to provide an error theory to account for intuition that zombies are possible. Philosophers such as Stephen Yablo (1998) have taken this line and argued that notions of what counts as physical, and what counts as physically possible, change over time; so while conceptual analysis is reliable in some areas of philosophy, it 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 that philosophers have about the method and scope of philosophy itself. It gets to the core of disagreements about the nature and abilities of conceptual analysis. Proponents of the zombie argument, such as Chalmers, 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, W.V.O. Quine, and so on, have fundamentally different views from Chalmers about the nature and scope of philosophical analysis. For this reason, discussion of the zombie argument remains vigorous in philosophy.

Under physicalism, it has been claimed that one must either believe 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 a product of the physical world and is therefore no different from anyone else's. This argument has been expressed by Daniel Dennett who argues that "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!" . While it has been argued that zombies are metaphysically impossible under the assumption of physicalism, it has also been argued that zombies are not conceivable under the assumption of physicalism. It has been claimed that under physicalism, when a distinction is made in ones mind between a hypothetical zombie and oneself (assumed not to be a zombie), and noting that the concept of oneself is under physicalism a product of physical reality, the concept of the hypothetical zombie can only be a subset of the concept of oneself and will in this nature also entail a deficit in observables (cognitive systems) thereby contradicting the original definition of a zombie. This argument has been expressed by Daniel Dennett who 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" .

See also

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