Aristotle - The Organon - index for ANALYTICA PRIORIA Book 2 Part 13

... in the third figure

Paragraph 1 Similarly they can all be formed in the last figure.
Paragraph 2 But to prove that A belongs to some B, this hypothesis must be made.
Paragraph 3 When A belongs to no B, suppose A belongs to some B, and let it have been assumed that C belongs to all B.
Paragraph 4 But this hypothesis must be made if we are prove that A belongs not to all B.
Paragraph 5 It is clear then that in all the syllogisms which proceed per impossibile the contradictory must be assumed.

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