Terry Pratchett > Quotes > Quote > Creeper liked it

“In fact he was incurably insane and hallucinated more or less continuously, but by a remarkable stroke of lateral thinking his fellow wizards had reasoned that, in that case, the whole business could be sorted out if only they could find a formula that caused him to hallucinate that he was completely sane.*

*This is a very common hallucination, shared by most people.”
― Terry Pratchett, The Truth: Stage Adaptation