Aristotle - The Organon - index for ANALYTICA PRIORIA Book 1 Part 17

Second figure, both premisses problematic

Paragraph 1 In the second figure whenever both premisses are problematic, no syllogism is possible, whether the premisses are affirmative or negative, universal or particular.
Paragraph 2 First we must point out that the negative problematic proposition is not convertible, e.g. if A may belong to no B, it does not follow that B may belong to no A.
Paragraph 3 Moreover it is not possible to prove the convertibility of these propositions by a reductio ad absurdum, i.e. by claiming assent to the following argument:
Paragraph 4 This being proved, suppose it possible that A may belong to no B and to all C.

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