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Negative conclusion from affirmative premises is a syllogistic fallacy committed when a categorical syllogism has a negative conclusion yet both premises are affirmative. The inability of affirmative premises to reach a negative conclusion is usually cited as one of the basic rules of constructing a valid categorical syllogism. Statements in syllogisms can be identified as the following forms: * a: All A is B. (affirmative) * e: No A is B. (negative) * i: Some A is B. (affirmative) * o: Some A is not B. (negative) The rule states that a syllogism in which both premises are of form ''a'' or ''i'' (affirmative) cannot reach a conclusion of form ''e'' or ''o'' (negative). Exactly one of the premises must be negative to construct a valid syllogism with a negative conclusion. (A syllogism with two negative premises commits the related fallacy of exclusive premises.) Example (invalid aae form): :Premise: All colonels are officers. :Premise: All officers are soldiers. :Conclusion: Therefore, no colonels are soldiers. The aao-4 form is perhaps more subtle as it follows many of the rules governing valid syllogisms, except it reaches a negative conclusion from affirmative premises. Invalid aao-4 form: :All A is B. :All B is C. :Therefore, some C is not A. This is valid only if A is a proper subset of B and/or B is a proper subset of C. However, this argument reaches a faulty conclusion if A, B, and C are equivalent. In the case that A = B = C, the conclusion of the following simple aaa-1 syllogism would contradict the aao-4 argument above: :All B is A. :All C is B. :Therefore, all C is A. ==See also== * Affirmative conclusion from a negative premise, in which a syllogism is invalid because an affirmative conclusion is reached from a negative premise * Fallacy of exclusive premises, in which a syllogism is invalid because both premises are negative 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「Negative conclusion from affirmative premises」の詳細全文を読む スポンサード リンク
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