Negative conclusion from affirmative premises information
In logic, a type of syllogistic fallacy
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.[1][2] 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.
^Alfred Sidgwick (1901). The use of words in reasoning. A. & C. Black. pp. 297–300.
^Fred Richman (July 26, 2003). "Equivalence of syllogisms" (PDF). Florida Atlantic University. p. 16. Archived from the original (PDF) on June 19, 2010.
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