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Brain in a vat
In philosophy, the brain in a vat (alternately known as brain in a jar) is a scenario used in a variety of thought experiments intended to draw out certain features of our ideas of knowledge, reality, truth, mind, consciousness and meaning. It is an updated version of René Descartes' Evil Demon thought experiment originated by Hilary Putnam. Common to many science fiction stories, it outlines a scenario in which a mad scientist, machine, or other entity 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 reality (including appropriate responses to the brain's own output) and the "disembodied" brain would continue to have perfectly normal conscious experiences, such as those of a person with an embodied brain, without these being related to objects or events in the real world.
==Uses==
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 exactly the 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 they believe, say, that they are walking down the street, or eating ice-cream); in the latter case their beliefs are false. Since the argument says one cannot know whether one is a brain in a vat, then one cannot know whether most of one's beliefs might be completely false. Since, in principle, it is impossible to rule out oneself being a brain in a vat, there cannot be good grounds for believing any of the things one believes; a skeptical argument would contend that one certainly cannot ''know'' them, raising issues with the definition of knowledge.
However, if one accepts a utilitarian or some logical positivist ethical philosophy, then one should behave as though the external world is real. If one believes the external world is not real, it follows that other humans and beings do not exist, and therefore cannot be helped or harmed by one's actions. Thus, if one behaves as though the world is real and it is not, one has not seriously harmed one's own happiness (particularly because cause and effect may or may not operate the way one believes) or at least has only made one's own existence miserable. However, if one behaves as though the world is not real and it is, one's selfish decisions can cause serious harm to the happiness of large numbers of people. Therefore, for the cost–benefit analysis of belief in the existence of the external world to support disbelief, one must estimate the probability that the world does not exist as very high. Though the world does not appear wholly logical or consistent, enough of it is logical and consistent to suggest that there is a substantial probability that I am (or the reader, assuming he or she exists, is) part of a real universe. So, the cost–benefit analysis favors belief in the external world's existence.
The brain in a vat is a contemporary version of the argument given in Hindu Maya illusion, Plato's Allegory of the Cave, Zhuangzi's "Zhuangzi dreamed he was a butterfly", and the evil demon in René Descartes' ''Meditations on First Philosophy''.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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