I've heard this joke a few times. But my colleague Adam re-told it to me recently, and he has the best delivery I've heard yet.
Bertrand Russell once taught a class in logic. He was trying to teach his students why inconsistency was important: because you can prove that anything is true if you start from false assumptions.
One of his students interpreted that as a challenge. "Given that one equals two, prove that you're the pope," he shouted.
Russell immediately replied, "Nothing could be simpler. The pope and I are two, and two equals one, so the pope and I are one. Therefore, I am the pope."