Aristotle - The Organon - index for ANALYTICA PRIORIA Book 1 Part 14

Reasoning about possibilities

Paragraph 1 Whenever A may possibly belong to all B, and B to all C, there will be a perfect syllogism to prove that A may possibly belong to all C.
Paragraph 2 But if one of the premisses is universal, the other particular, when the major premiss is universal there will be a perfect syllogism.
Paragraph 3 But if the major premiss is the minor universal, whether both are affirmative, or negative, or different in quality, or if both are indefinite or particular, in no way will a syllogism be possible.
Paragraph 4 It is clear that if the terms are universal in possible premisses a syllogism always results in the first figure, whether they are affirmative or negative, only a perfect syllogism results in the first case, an imperfect in the second.

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