“This very distinguished philosophy professor came out on the platform in front of this gang of students and took a bit of chalk and scrawled up a proposition in symbolic logic on the board. He turned to the audience and said, ‘Well now, ladies and gentlemen, I think you’ll agree that that’s obvious?’
“Then he looked at it a bit more and started to scratch his head and after a while he said, ‘Excuse me!’ And he disappeared.
“About half an hour later he came back beaming all over his face and said triumphantly, ‘Yes, I was right — it is obvious!'”