Aristotle - The Organon - index for ANALYTICA PRIORIA Book 2 Part 9

... in the second figure

Paragraph 1 In the second figure it is not possible to refute the premiss which concerns the major extreme by establishing something contrary to it, whichever form the conversion of the conclusion may take.
Paragraph 2 If the syllogism is particular, when the conclusion is converted into its contrary neither premiss can be refuted, as also happened in the first figure, if the conclusion is converted into its contradictory, both premisses can be refuted.

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