"When you believe it, then you will see it." -– The Rav
"Tell me one last thing," said Harry. "Is this real? Or has this been happening inside my head?"
"Of course it is happening inside your head, Harry, but why on earth should that mean that it is not real?"
"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? ... 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."