Understanding after the fact confers no special perspicacity. The real test for any diviner can only lie in grasping the outcome at the outset. Correct predictions, in themselves, offer no proof of true wisdom, for how can we distinguish dumb luck from horse sense? The only good experiment is, alas, the most undoable of all intriguing thoughts in a world of irrevocable history-to run back the tape and play it again, Sam.