Aristotle - The Organon - index for ANALYTICA POSTERIORA Book 1 Part 6

Demonstrative knowledge rests on necessary truths

Paragraph 1 Demonstrative knowledge must rest on necessary basic truths;
Paragraph 2 We must either state the case thus, or else premise that the conclusion of demonstration is necessary and that a demonstrated conclusion cannot be other than it is, and then infer that the conclusion must be developed from necessary premisses.
Paragraph 3 A further proof that the conclusion must be the development of necessary premisses is as follows.
Paragraph 4 When the conclusion is necessary, the middle through which it was proved may yet quite easily be non-necessary.
Paragraph 5 To sum up, then:
Paragraph 6 Of accidents that are not essential according to our definition of essential there is no demonstrative knowledge;
Paragraph 7 Since it is just those attributes within every genus which are essential and possessed by their respective subjects as such that are necessary it is clear that both the conclusions and the premisses of demonstrations which produce scientific knowledge are essential.

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