The consequentia mirabilis

If a proposition follows from its own negation, then it's true.
(¬A→A)→A
Some instances in which this deductive method is used are:

Descartes:
  • Cogito: I think
  • <> all that thinks exist
  • ergo sum: therefore I exist.
Augustine:
  • Suppose that all my knowledge is false
  • but at least I know that I know something, although it's wrong
  • Therefore in knowing that I know something, I don't deceive myself
References
Angelelli, I. () Here
Mirabell, I. () Here
Kneale, W. () Here

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