sanity or insanity

He was incurably insane and hallucinated more or less consistently, 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