I’m sorry, but that’s completely ridiculous! How can I possibly prove it doesn’t exist? Do you expect me to get hold of - of all the pebbles in the world and test them? I mean, you could claim that anything’s real if the only basis for believing in it is that nobody’s proved it doesn’t exist!
Hermione Granger, “Harry Potter and the Deathly Hallows”, Chapter 21: The Tale of the Three Brothers