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Word: proof by contradiction
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Meaning:
Noun:
- logic A proof of the following sort (1): It is desired to prove ''A'', so assume that ''A'' is not true, and derive a contradiction from this assumption. From this it follows that "not ''A''" implies falsity, in other words "not not ''A''" is true. From the law of double negation, it follows that ''A'' is true. Also, a proof of the following sort (2): It is desired to prove "not ''A''", so assume that ''A'' is true, and derive a contradiction. From this it follows that ''A'' implies falsity, in other words "not ''A''". Both sorts of proof are valid in classical logic but only the second sort is valid in intuitionistic logic.
[http://plato.stanford.edu/entries/logic-intuitionistic/]
Source: Wiktionary
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