[jungjedi]

i ran across this thought experiment on another forum and thought to juxtosopose it here with a few other thought experiments as well as popular movies along the same lines.here's what wiki has to say about the mysterious 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 "Zhuangzi dreamed he was a butterfly".). In this latter argument the worry about active deception is removed
Note that these are all references to the "brain in a vat" thought experiment, as described above. For a list of media that simply feature brains in jars, see Media featuring brains in jars.
The use of similar ideas in movies is not infrequent, as in 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).
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.
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.
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".
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 "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." Reasoning that all stimuli Bomb #20 is experiencing is not true, the bomb assumes itself to be God, declares, "Let there be light", and explodes.
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 a Mayan boy following his traffic accident. In the end, it is revealed that the Mayan boy has been dreaming of the motorcyclist in his state of fear and delusion.
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 children's cartoon show The Adventures of Batman 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 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.
one of my favorites is the movie eXistenZ with jennifer jason liegh,and wiki has something on that
The story is based on the confusion between reality and virtual reality as the characters move in and out of a quasi-organic role-playing computer game called eXistenZ, the aim of which is unknown. The players are linked to the virtual world of the game by a console that resembles a living lump of animal tissue which is connected to the player's nervous system through a 'bio-port' drilled in the player's lower back; however, near the end of the film the gamers seem to be connected to the virtual world by electronic devices connected to their heads and wrists. The ending, immediately following that switch into what appears to be a real world in which gamers were merely playing virtual reality, with electronic devices on their hands and wrists, leaves open the question of how many layers of virtual reality are still left between characters and the real world.
The virtual world of the game features many aspects of traditional video games, particularly graphical adventure games of the 1980s and early 1990s. Some of these are explicit, such as the repetitive "loops" of actions that minor characters perform, or the need to provide certain trigger phrases to make progress possible. There are many other references that are more subtle, for example the sparsely populated nature of the game world and the physical proximity of certain locations for no sensible reason (e.g., a Chinese restaurant next to a fish processing factory in thick forest). Another trait repeatedly used is the tendency of characters within the game to perform certain actions to quickly establish their personality which, presented in a more real world, would be nonsensical, such as the soldier firing a weapon, apparently randomly, in the middle of a conversation. Clothing in the film is primarily blank colours without patterns, as with games of the time.
There is a tension throughout the film between what appear to be rival game companies (Antenna Research and Cortical Systematics) that want to gain the services of a famous game designer, or kill the game designer if they cannot gain cooperation, while a third party known as the Realist Underground tries to subvert both game companies.
The plot involves existentialist themes, similar to The Matrix. Both films were released in the same year, but The Matrix received far more attention.

[edit] Philosophical issues

The film calls into question the nature of reality and how to discern between reality and illusion.
It also raises ethical issues surrounding the distortion of reality and how it might equate to psychosis for those who become psychologically absorbed into their virtual roles as characters within a game.
The film portrays the emotional reasons for the popularity of video games, and explores the theoretical issues of self-reflexivity and absorption of a game player for the sake of entertainment. Playing the game is compared to psychosis, following implanted "game urge" to advance the plot: a meek marketing trainee acts increasingly like a macho sociopath, while the game's designer becomes narcissistic and grandiose. Free will is also discussed; several times the characters are compelled by "game urges" to commit acts in order to advance the game. When Pikul asks Geller if there is any free will in the program, she replies that, as in real life, there's "just enough to make it interesting".
Other films that depict similar existential problems that emerge from virtual reality are Abre Los Ojos, The Lawnmower Man, The Matrix, The Thirteenth Floor, Total Recall, TRON, The Truman Show, Vanilla Sky, Avalon and Cronenberg's own earlier Videodrome and Naked Lunch. The film also mirrors parts of Plato's "Allegory of the Cave". The same themes are often explored in many stories by Philip K Dick.
Ive often felt that life was some sort of lucid dream in which some essential part of me was moved to an artificial plane of existence.how can i click my heels three times to return to my natural habitat?