In its absolute barest form, Gödel's discovery involves the translation of an ancient paradox in philosophy into mathematical terms. That paradox is the so-called Epimenides paradox, or liar paradox. Epimenides was a Cretan who made one immortal statement: “All Cretans are liars.”

Whereas the Epimenides statement creates a paradox since it is neither true nor false, the Gödel sentence G is unprovable (inside P.M.) but true. The grand conclusion? That the system of Principia Mathematica is “incomplete”—there are true statements of number theory which its methods of proof are too weak to demonstrate.