Reductio ad absurdum

Tool of logic, also known as "Proof by Contradiction."

Say we wish to prove hypothesis A. The procedure is to show that to assume "not A" (i.e. that A is false) leads to a logical contradiction. Thus A cannot be false, and must therefore be true. For an example, see Cantors Diagonal argument.

The name comes from the Latin for "Reduced to an absurdity."