I recently watched Sydney Coleman's 'Quantum Mechanics In Your Face,' and very much enjoyed it. (Highly recommended.) In it, Professor Coleman quotes a very clever comment by a philosopher called Ludwig Wittgenstein. Wittgenstein found it strange that people used to believe in geocentrism (Sun goes around the Earth.) When asked why he felt it strange, he responded: "well, how would it have have looked if it were the other way around." Of course it would look exactly the same. Personally, I loved Wittgenstein's response and thought it showed impressive insight although, clearly, things are always simpler in hindsight than in foresight.
This got me thinking. There must be hundreds of equally elegant / beautiful quotes of famous physicists out there. I'm sure almost everyone has a favourite. So, the question, quite simply, is this: what is your favourite famous-physicist quote, and why??