Logical
Epimenides Paradox
A Cretan says 'All Cretans are liars'
Overview
An ancient version of the liar paradox attributed to Epimenides the Cretan.
Epimenides, a Cretan, states: 'All Cretans are liars.' If he's telling the truth, then as a Cretan, he must be a liar, making the statement false. If he's lying, then not all Cretans are liars, which could include him telling the truth.