QUOTE : It may well be the case that one cannot prove the existence of a nine-eyed, multitentacled pink monster named Herbert, but that does not mean that, somewhere in the universe, there is not a nine-eyed multitentacled pink monster named Herbert wondering why nobody writes to him. Just because he hasn't been seen doesn't mean that he isn't out there. This is known as an inductive argument. But the argument is probable, not definite. If there's actually a pretty good chance he exists, there's at least as good a chance that he doesn't exist. So you can prove a negative, at least as much as you can prove anything at all.