From “Which Way Madness Lies” by Rachel Aviv in the December issue of Harper’s magazine (subscription):

[Doctors] have to identify delusions before the patient really believes in them.  When does a strong idea take on a pathological flavor?  How does a metaphysical crisis morph into a medical one?  At what point does our interpretation of the world become so fixed that it no longer matters “what almost everyone else believes”?  Even William James admitted that he struggled to distinguish a schizophrenic break from a mystical experience.