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The fallacy of the undistributed middle is a formal fallacy that is committed when the middle term in a categorical syllogism is not distributed in either the minor premise or the major premise. It is thus a syllogistic fallacy. ==Classical formulation== In classical syllogisms, all statements consist of two terms and are in the form of "A" (all), "E" (none), "I" (some), or "O" (some not). The first term is distributed in A statements; the second is distributed in O statements; both are distributed in E statements; and none are distributed in I statements. The fallacy of the undistributed middle occurs when the term that links the two premises is never distributed. In this example, distribution is marked in boldface: # All Z is B #All y is B #Therefore, all y is Z B is the common term between the two premises (the middle term) but is never distributed, so this syllogism is invalid. Also, a related rule of logic is that anything distributed in the conclusion must be distributed in at least one premise. #All Z is B #Some Y is Z #Therefore, all Y is B The middle term - Z - is distributed, but Y is distributed in the conclusion and not in any premise, so this syllogism is invalid. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「fallacy of the undistributed middle」の詳細全文を読む スポンサード リンク
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