Aristotle - The Organon - index for ANALYTICA POSTERIORA Book 1 Part 21

That the regress of middles terminates

Paragraph 1 Further, if in affirmative demonstration the series terminates in both directions, clearly it will terminate too in negative demonstration.
Paragraph 2 The third figure shows it as follows:
Paragraph 3 Thus it is plain that the regress of middles terminates in the case of negative demonstration, if it does so also in the case of affirmative demonstration.

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