Reductio ad absurdum

Definition

Reductio ad absurdum proves a claim by assuming its opposite and reasoning from that assumption until a contradiction or manifest absurdity follows — which shows the assumption must be false, and therefore the original claim true. Three moves: assume NOT-P, reason from it consistently, arrive at something impossible; since NOT-P cannot hold, P must.

Memory hook

“Assume the opposite; follow the logic; let it break itself.”

What it sounds like

• Suppose, for argument's sake, that the opposite were true...
• Follow that to its conclusion and you get a contradiction.
• If that were right, then X and not-X would both have to hold.
• The position defeats itself: assume it's true and it becomes false.