The United Nations has given Iran a deadline to prove that it isn't trying to develop nuclear weapons. (See this BBC story, for example).

This strikes me as being somewhat unfair.

If you accused me of being a paedophile, and challenged me to prove that I wasn't, I think I'd have great difficulty in findind 'proof' of my innocence. In fact, I don't see how I could.

An extreme example I know, but seriously, can you prove a negative?