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