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Full Idea
Demonstrations by reduction to the impossible assume that everything is asserted or denied.
Gist of Idea
Demonstrations by reductio assume excluded middle
Source
Aristotle (Posterior Analytics [c.327 BCE], 77a23)
Book Ref
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.17
A Reaction
This sounds like the lynchpin of classical logic.