speaking of psychology...

FAB just alerted me to this gem about the ol' DSM IV.

The Diagnostic and Statistical Manual of Mental Disorders defines a delusion, considered still to be little understood in psychiatry, as, essentially, a false belief that is not grounded in reality and that is held with absolute conviction despite proof to the contrary. The manual lists a caveat that a belief is not delusional if it is something widely accepted by other members of a person's culture or subculture--for example, religious faith.

Yep, pretty much.