I always thought that the statement "You can never know that this statement is true" illustrates the principle most clearly.
@: Hal Finney:
Certainly the box is either full or empty. But the only way to get the money in the hidden box is to precommit to taking only that one box. Not pretend to precommit, really precommit. If you try to take the $1,000, well then I guess you really hadn't precommitted after all. I might vascillate, I might even be unable to make such a rigid precommitment with myself (though I suspect I am), but it seems hard to argue that taking only one box is not the correct choice.
I'm not entirely certain that acting rationally in this situation doesn't require an element of doublethink, but thats a topic for another post.
This really reminds me of some of what Kuhn wrote about in The Structure of Scientific Revolutions. One important additional way to identify living traditions that Kuhn notes would be that the texts used to instruct students are not the original works where ideas were first outlined but have been re-written to be clearer. The ability to say the same thing in different words is almost central to a tradition's life.