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fallacy, in logic, a term used to characterize an invalid argument. Strictly speaking, it refers only to the transition from a set of premises to a conclusion, and is distinguished from falsity, a value attributed to a single statement. The laws of syllogisms were systematically elaborated by Aristotle, and for an argument to be valid, it must adhere to all the laws; to be fallacious, it need only break one (see syllogism). The term fallacy has come to be used in a somewhat wider sense than the purely formal one. Informal fallacies are said to occur when statements are ambiguous or vague as to the logical form they represent, or when a multiplicity of meaning is present and the validity of the argument depends on switching meanings of a word or a phrase in midstream.
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Logic an error in reasoning that renders an argument logically invalid
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005