When one person suffers from a delusion it is called insanity. When many people suffer from a delusion it is called Religion.

This is attributed to Pirsig by Richard Dawkins in the Preface to The God Delusion (2006), p. 28, but cannot be found prior to that. It is obviously a paraphrase of the following from Pirsig's Lila - An Inquiry Into Morals (1991): „An insane delusion can't be held by a group at all. A person isn't considered insane if there are a number of people who believe the same way. Insanity isn't supposed to be a communicable disease. If one other person starts to believe him, or maybe two or three, then it's a religion." (books.google; books.google)