Mathematical
Curry's Paradox
If this sentence is true, then Santa Claus exists
Overview
A self-referential paradox that can prove any statement without using negation.
Consider: 'If this sentence is true, then Santa Claus exists.' If the sentence is true, then its conditional holds, so Santa exists. If we assume it's true, we can derive Santa exists. So by conditional proof, the sentence is true - therefore Santa exists!