Delusions are held with absolute certainty, a level of certainty that is rarely experienced with nondelusional ideas. For example, most people are confident that their parents really are their parents, but if you ask them if they are absolutely certain, they will confess that they are not. On the other hand, when a schizophrenic patient was asked if he was sure that the FBI was using radio waves to implant messages in his head, he said there could be no doubt. No amount of evidence could convince him otherwise.
