Aristotle - The Organon - index for ANALYTICA PRIORIA Book 2 Part 11

The syllogism per impossibile

Paragraph 1 It is clear then what conversion is, how it is effected in each figure, and what syllogism results.
Paragraph 2 All the problems can be proved per impossibile in all the figures, excepting the universal affirmative, which is proved in the middle and third figures, but not in the first.
Paragraph 3 But the particular affirmative and the universal and particular negatives can all be proved.
Paragraph 4 Again suppose that A belongs to some B, and let it have been assumed that C belongs to all A.
Paragraph 5 To prove that A does not belong to all B, we must suppose that it belongs to all B:
Paragraph 6 It is clear then that not the contrary but the contradictory ought to be supposed in all the syllogisms.

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