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 their body to suspension 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. 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 head 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 or not 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 little more than a contemporary revision of the argument given by Descartes in Meditations on First Philosophy that he could not trust his perceptions on the grounds that an evil demon might, conceivably, be controlling his every experience. It is also (though more distantly) related to Descartes' argument that he cannot trust his perceptions because he may be dreaming. In this latter argument the worry about active deception is removed.

Such puzzles have been worked over in many variations by philosophers in recent decades. Some, including Barry Stroud, continue to insist that such puzzles consistute 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 of, at that time, 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 "our" world but to elements of its own "virtual world"; and it is clearly not a brain in a vat in that sense. Likewise, whatever we can mean by "brain" and "vat" must be such that we obviously are not brains in vats (the way to tell is to look in a mirror). Many writers have found Putnam's proposed solution unsatisfying, as it appears to depend on irrelevant facts about meaning: whether or not we can express it, there seems to be a real problem of knowledge here.

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.

Brains in vats and popular culture

The use of similar ideas in movies is not infrequent, as in The Truman Show and The Matrix (a weak reference to the brain-in-a-vat theory, though in that case entire bodies were preserved, rather than just brains). The Man with Two Brains was a film that gave a comedy treatment to the subject.

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 a disembodied brain.

See also

Simulated reality -- Solipsism