Bertrand Russell was giving a lecture in logic to a group of students. He was explaining why a contradictory statement cannot be true.

"If you accept a contradiction", he said, "it follows that absolutely anything you can think of, no matter how ridiculous, can be proved true".

"All right", said one disbelieving student, "prove that if 1+1=3, you are the Pope".

"Well", said Russell, "subtract 1 from each side of the equation. Then 1=2. Therefore, 2=1. Now, I and the Pope are two people; but 2=1, so I and the Pope are one. Therefore, I am the Pope!"