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What nerve!

Brain in a vat

In philosophy, the brain in a vat is any of a variety of thought experiments intended to draw out certain features of our ideas of knowledge, reality, truth, mind, and meaning. It is drawn from the idea, common to many science fiction stories, that a mad scientist might remove a person's brain from the body, suspend it in a vat of life-sustaining liquid, and connect its neurons by wires to a supercomputer which would provide it with electrical impulses identical to those the brain normally receives. According to such stories, the computer would then be simulating a virtual reality (including appropriate responses to the brain's own output) and the person with the "disembodied" brain would continue to have perfectly normal conscious experiences without these being related to objects or events in the real world.

The simplest use of brain-in-a-vat scenarios is as an argument for philosophical skepticism and Solipsism. A simple version of this runs as follows: Since the brain in a vat gives and receives the exact same impulses as it would if it were in a skull, and since these are its only way of interacting with its environment, then it is not possible to tell, from the perspective of that brain, whether it is in a skull or a vat. Yet in the first case most of the person's beliefs may be true (if he believes, say, that he is walking down the street, or eating ice-cream); in the latter case they are false. Since, the argument says, you cannot know whether you are a brain in a vat, then you cannot know whether most of your beliefs might be completely false. Since, in principle, it is impossible to rule out your being a brain in a vat, you cannot have good grounds for believing any of the things you believe; you certainly cannot know them.

This argument is a contemporary version of the argument given by Descartes in Meditations on First Philosophy (which he eventually rejects) that he could not trust his perceptions on the grounds that an evil demon might, conceivably, be controlling his every experience. It is also more distantly related to Descartes' argument that he cannot trust his perceptions because he may be dreaming (Descartes's dream argument is preceded by Zhuangzi in "Chuang Chou dreamed he was a butterfly".). In this latter argument the worry about active deception is removed.

Philosophical responses

Such puzzles have been worked over in many variations by philosophers in recent decades. Some, including Barry Stroud, continue to insist that such puzzles constitute an unanswerable objection to any knowledge claims. Others have argued against them, most notably Hilary Putnam. In the first chapter of his Reason, Truth, and History, Putnam claims that the thought experiment is inconsistent on the grounds that a brain in a vat could not have the sort of history and interaction with the world that would allow its thoughts or words to be about the vat that it is in.

In other words, if a brain in a vat stated "I am a brain in a vat," it would always be stating a falsehood. If the brain making this statement lives in the "real" world, then it is not a brain in a vat. On the other hand, if the brain making this statement is really just a brain in the vat then by stating "I am a brain in a vat" what the brain is really stating is "I am what nerve stimuli have convinced me is a 'brain,' and I reside in an image that I have been convinced is called a 'vat'." That is, a brain in a vat would never be thinking about real brains or real vats, but rather about images sent into it that resemble real brains or real vats. This of course makes our definition of "real" even more muddled. This refutation of the vat theory is a consequence of his endorsement, at that time, of the causal theory of reference. Roughly, in this case: if you've never experienced the real world, then you can't have thoughts about it, whether to deny or affirm them. Putnam contends that by "brain" and "vat" the brain in a vat must be referring not to things in the "outside" world but to elements of its own "virtual world"; and it is clearly not a brain in a vat in that sense.

Many writers, however, have found Putnam's proposed solution unsatisfying, as it appears, in this regard at least, to depend on a shaky theory of meaning: that we cannot meaningfully talk or think about the "external" world because we cannot experience it; sounds like a version of the outmoded verification principle. Consider the following quote: "How can the fact that, in the case of the brains in a vat, the language is connected by the program with sensory inputs which do not intrinsically or extrinsically represent trees (or anything external) possibly bring it about that the whole system of representations, the language in use, does refer to or represent trees or any thing external?" Putnam here argues from the lack of sensory inputs representing (real world) trees to our inability to meaningfully think about trees. But it is not clear why the referents of our terms must be accessible to us in experience. One cannot, for example, have experience of other people's private states of consciousness; does this imply that one cannot meaningfully ascribe mental states to others?

Subsequent writers on the topic, especially among those who agree with Putnam's claim, have been particularly interested in the problems it presents for content: that is, how - if at all - can the brain's thoughts be about a person or place with whom it has never interacted and which perhaps does not exist.

References in popular culture

''Note that these are all references to the "brain in a vat" thought experiment, as described above.

  • In a short story by Julio Cortazar titled "La noche boca arriba" ("The night, face-up"), the reader follows the story of a motorcyclist who has just been involved in an accident and a young native "Moteca" man who is fleeing his own sacrifice. The imagery parallels in the story lead us to question who is dreaming of whom. The reader initially believes that the motorcyclist is dreaming of an Aztec boy following his traffic accident. In the end, it is revealed that the Aztec boy has been dreaming of the motorcyclist in his state of fear and delusion.
  • Simon Wright in Captain Future.
  • The 1996 Dennis Potter miniseries Cold Lazarus explores a semiconscious 20th century head reanimated in a distopian 24th Century.
  • The movie They Saved Hitler's Brain (1966).
  • It appears in The Man With Two Brains, a Warner Brothers movie starring Steve Martin.
  • The use of similar ideas in movies is not infrequent, as in "Abre Los Ojos" (Open Your Eyes) (or its English remake, Vanilla Sky), and The Matrix (a clear reference to both Plato's Allegory of the Cave and the brain-in-a-vat theory, though in that case entire bodies were preserved, rather than just brains). A similar sense of indistinction between virtual reality and reality appears in eXistenZ, a David Cronenberg film.
  • The Tom the Dancing Bug comic strip by Ruben Bolling has a recurring Brain in a Beaker story, detailing the influence of minor events in the real world on the virtual inhabitants of the computer-generated world a disembodied brain.
  • The cartoon series Codename: Kids Next Door features an episode in which Numbuh One is trapped in a body-vat a la The Matrix and is made to believe he is on a dream island. He breaks out by tapping his heels together, which in the "real world" activates his jet boots.
  • In the film Dark Star by John Carpenter, a planet-destroyer bomb has repeatedly been activated and deactivated due to various malfunctions. Dolittle reasons with the bomb's computer, inducing it to question whether its activation signal was real, and Bomb #20 takes up the philosophy of Descartes (I think, therefore I am) and returns to the bomb bay, commenting, "I must think on this further." Later, Pinback attempts to command the bomb to drop from the bomb bay and destroy the nearby planet (as per the mission) but Bomb #20 responds "You are false data." Then, reasoning that all stimuli Bomb #20 is experiencing are not true, the bomb assumes itself to be God, declares, "Let there be light", and explodes.
  • The computer game The Infinite Ocean by Jonas Kyratzes deals with a sentient computer that ponders the same issue. Here the question is rephrased to "how can you know whether you are a human being or just a sentient computer dreaming it is one?" The game A Mind Forever Voyaging by Steve Meretzky of Infocom involves a similar theme, but the sentient computer is informed of his/its nature a short time before the start of play.
  • The idea of a person's brain or even more abstractly, consciousness, being removed from the body appears in some of Stanisław Lem's novels. A related topic, an artificial mind being fed artificial stimuli by its mad-scientist creator, also appears there.
  • The cartoon show Batman: The Animated Series featured an episode, Perchance to Dream, where Batman was incapacitated by the Mad Hatter, and connected to a machine that simulated reality based on existing brain functions (dreams). He realizes the situation, and to escape, throws himself out of a bell tower; the logic being that when dreaming of falling, one always wakes up before hitting the ground.
  • The game Sid Meier's Alpha Centauri involves the base facility "Bioenhancement Center", the construction of which, plays the quote:

"We are all aware that the senses can be deceived, the eyes fooled. But how can we be sure our senses are not being deceived at any particular time, or even all the time? Might I just be a brain in a tank somewhere, tricked all my life into believing in the events of this world by some insane computer? And does my life gain or lose meaning based on my reaction to such solipsism?" This is then ironically revealed to be the thought of an individual in a literal brain in a vat project and is followed by the statement "termination of specimen advised." This vocal sample has been also included in the Prometheus track "O.K. Computer".

  • The comic Green Lantern's Infinite Crisis crossover featured Green Lantern and Green Arrow being attached to the 'Black Mercy', a fictional symbiotic plant that creates an artificial 'perfect world' for the host, who will have no memory of his previous life. The 'Black Mercy' first appeared in Alan Moore's Superman story, For the Man Who Has Everything.
  • In an episode of Stargate SG-1 (The Gamekeeper) the team visits an alien planet where they are imprisoned inside capsules that cause them to experience a simulated world controlled by a "gamekeeper". When they discover what has happened, the gamekeeper causes them to experience leaving the capsules again so that they will think that they have left the artificial world. They then experience an artificial version of their own world nearly indistinguishable from reality, but are eventually able to determine that they are still inside the machines and escape.
  • Motoko Kusanagi, the protagonist of Ghost in the Shell, is something similar to a brain in a vat. Her entire body is cybernetic except for her brain. She muses at several times that maybe she is just a computer program that believes it used to be human. In terms of the series, her ghost is artificial. The movie version opens with a sequence showing the creation of her cybernetic body, in which her brain is shown covered by an electronic shell.
  • In the British SF sitcom Red Dwarf episode "Back to reality", the protagonists are suddenly confronted with the realization that their lives for the last four years - and by inclusion, the series itself - were merely a 'total immersion video game' called Red Dwarf (in which they performed rather poorly). In the end, it is revealed that this realization and the 'real world' were, in fact, a simulated experience - one intended to drive them to suicide.

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