"But that's-- I'm sorry, but that's completely ridiculous! How can I possibly prove it doesn't exist?... 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 am glad to see that you are opening your mind a little."
[image by Remo Savisaar, quote from Harry Potter and the Deathly Hallows]