The origins of delusional beliefs have been a question of great interest for centuries. For many years, the widely held view was that there was a fundamental defect in reasoning in the patient that gave rise to the false belief. Much effort was directed at establishing the kind of defect and the circumstances that triggered it. The search for the basic reasoning defect has met with limited success, mainly because the flaws found in the reasoning of deluded persons are also found with substantial frequency in the general population. The co-occurrence of hallucinations and delusions is consistent with findings that repeated anomalous experiences of various kinds are followed by the development of delusions. In this case, it would be reasonable to regard the hallucinations as exercising a causative role in the development of delusions.