Aristotle - The Organon - index for ANALYTICA PRIORIA Book 1 Part 16

One premiss necessary, the other problematic

Paragraph 1 Whenever one premiss is necessary, the other problematic, there will be a syllogism when the terms are related as before;
Paragraph 2 If the premisses are affirmative, clearly the conclusion which follows is not necessary.
Paragraph 3 But if the premisses are not similar in quality, suppose first that the negative premiss is necessary, and let necessarily A not be possible for any B, but let B be possible for all C.
Paragraph 4 The same relation will obtain in particular syllogisms.
Paragraph 5 Clearly then from what has been said a syllogism results or not from similar relations of the terms whether we are dealing with simple existence or necessity, with this exception, that if the negative premiss is assertoric the conclusion is problematic, but if the negative premiss is necessary the conclusion is both problematic and negative assertoric.

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