“Well, how can that be real?”
“Prove that it is not,” said Xenophilius.
Hermione looked outraged.
“But that’s — 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!”
“Yes, you could,” said Xenophilius. “I’m glad to see that you are opening your mind a little.”
Harry Potter and the Deathly Hallows – J.K. Rowling