> http://en.wikipedia.org/wiki/Philosophical_zombie
>
> A philosophical zombie or p-zombie is a hypothetical being that is
> indistinguishable from a normal human being except that it lacks
> conscious experience or qualia or sentience. When a zombie is poked
> with a sharp object, for example, it does not feel any pain. It behaves
> as if it does feel pain (it may say "Ouch!" and so forth), but it does
> not actually have the experience of pain as a person normally does.
>
> The notion of a philosophical zombie is mainly used in arguments (often
> called zombie arguments) in the philosophy of mind, particularly
> arguments against forms of physicalism or materialism-the view that
> the universe is entirely physical. ("Physical" here is usually taken to
> mean something along the lines of that which the language of physics
> describes; however, the term can be used in several ways.)
>
> Types of zombies
> The notion of a philosophical zombie is widely used in thought
> experiments, however the use is not always entirely uniform. There are,
> in effect, different types of p-zombies. What differs between the types
> of p-zombies that philosophers discuss is what they have in common with
> normal human beings. P-zombies were introduced primarily to argue
> against specific types of physicalism. Take behaviorism, for example.
> According to behaviorism, mental states are nothing over and above
> behavior; thus, belief, desire, thought, consciousness, and so on, are
> simply certain kinds of behavior. One might invoke the notion of a
> p-zombie that is behaviorally identical to normal human beings, but
> that lacks conscious experiences. Such a being is impossible, according
> to the behaviorist, since conscious experiences are nothing over and
> above certain behaviors. Thus, insofar as a p-zombie so described is
> possible, then behaviorism is false. Likewise, if such a behavioral
> zombie is impossible, then behaviorism is true.
>
> Thus, one might distinguish between various types of zombies, as they
> are used in different thought experiments, such as the following:
>
> A behavioral zombie is behaviorally identical with humans and yet has
> no conscious experience.
> A neurological zombie has a human brain and is otherwise physically
> identical to humans; nevertheless, it has no conscious experience.
> A soulless zombie lacks a soul but is otherwise indistinguishible from
> an ordinary person; the concept is used to question what, if anything,
> the soul does.
> However, philosophical zombies are primarily discussed in the context
> of arguments against physicalism in general. Thus, a p-zombie is
> typically taken to be a being that is physically identical to a normal
> human being but that lack conscious experience.
>
> Zombie arguments
> According to any version of physicalism, a p-zombie that is physically
> identical yet lacks conscious experience is impossible. This is because
> physicalists maintain that the universe is entirely physical; so as the
> physical constitution of the p-zombie is fixed, there is nothing left
> to be absent, even consciousness. Therefore, insofar as such a being is
> possible, physicalism is false. Zombie arguments attempt to motivate
> the belief that zombies are possible, and, thus, that physicalism is
> false. The argument, then, is taken to support dualism-the view that
> the universe is made up of two substances: the mental and the physical.
>
> The zombie argument against physicalism is, thus, a version of a
> general modal argument against physicalism, such as that of Saul
> Kripke's in "Naming and Necessity" (1972)[1]. The notion of a p-zombie,
> as used to argue against physicalism, was notably developed 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 that
> lacks conscious experiences. Every being in such a world would be a
> p-zombie. The structure of Chalmers's version of the zombie argument
> can be outlined as follows:
>
> 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 and have any additional facts left over. (This is because,
> according to P, there are only physical things, so there is nothing to
> be different; any world that is physically identical to our world is
> just simply identical to our world, so nothing can be different about
> it.)
> But there is a possible world in which the physical facts are the same
> as those of our world but that there are additional facts that are
> different. (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.)
> Therefore, physicalism is false. (The conclusion follows by modus
> tollens.)
> The argument is valid, in that if its premises are true, then the
> conclusion must be true. However, whether its premises are true is what
> philosophers dispute. For example, concerning premise two: Is such
> zombie world really possible? Chalmers states that "it certainly seems
> that a coherent situation is described; I can discern no contradiction
> in the description."[2] Since such a world is conceivable, Chalmers
> claims, it is possible; and if such a world is possible, then
> physicalism is false. Chalmers agrees that such p-zombies are
> (probably) not naturally or physically possible, just logically or
> metaphysically possible. Chalmers states: "Zombies are probably not
> naturally possible: they probably cannot exist in our world, with its
> laws of nature."[3]
>
> Thus, one is lead to the following questions: What is the relevant
> notion of possibility here? And, is such a scenario in premise two
> possible in the same sense as in premise one? Most philosophers agree
> that the relevant possiblity is not so weak as logical possibility;
> surely a zombie world is logically possible (there is no logical
> contradiction in the scenario). But, such a weak notion is not crucial
> to a metaphysical thesis such a 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."[4] Chalmers claims that this inference from
> conceivability to metaphysical possibility is not generally legitimate,
> but is so for phenomenal concepts such as consciousness.[5]
>
> Criticism
> A physicalist might respond to the zombie argument in several ways.
> Most responses deny premise two (of Chalmers's version above); that is,
> they deny that a zombie scenario is possible. Physicalists tend not to
> deny premise one, because it requires denying something that most
> physicalists think is part of or an implication of physicalism.
>
> One response is to claim that the idea of qualia and related phenomenal
> notions of the mind are not coherent concepts, and thus the zombie
> scenario is incoherent. Daniel Dennett and others take this line. They
> argue that while consciousness qualia, 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 you
> can just strip off a person's mental life without any behavioral or
> physiological differences. Dennett coined the term zimboes to argue
> that the idea of a philosophical zombie is incoherent-see Dennett
> (1995) and (1999), for example. Dennett 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."[6]
>
> Another physicalist response is to provide some an error theory for the
> intuitions of the possibility of a zombie scenario. Philosophers, such
> as Stephen Yablo (1998), have taken this line and argued that concepts
> about what is physical and is possibly physical adapt over time, so
> while conceptual analysis is reliable in some areas of philosophy, it
> is not reliable here. Yablo states, "I am 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."[7]
>
> 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 (for example,
> refuting physicalism). However, others, such as Dennett, Paul
> Churchland, W.V. Quine, and so on, have fundamentally different views
> from Chalmers about the nature and scope of philosophical analysis.
> Thus, discussion of the zombie argument remains vigorous in philosophy.