Early in his third month of office, President Reagan was on his way to address a conference when John Hinckley fired six gun shots at point blank range, wounding the president and three of his entourage. In the controversial trial that followed, three defence psychiatrists successfully argued that Hinckley was not guilty, on the grounds that he was suffering from the delusion that the assassination would cause Jodie Foster, the actress from Taxi Driver (a film which Hinckley was obsessed with), to fall in love with him. In the same year the award-winning author Philip K. Dick, whose books have been turned into major Hollywood films, such as Blade Runner, Total Recall and Minority Report, published one of his last books. The sprawling and eccentric VALIS is a novel based on delusions resulting from his own psychotic breakdown, which he drew on for much of his prolific career (see box). From these and many other examples, it would appear that unusual or unlikely beliefs have significant consequences and continue to captivate the interest of many of us. But to examine such claims we need to know what is meant by a delusion. How do delusions differ from other abnormal beliefs? Does the study of delusions provide a productive way of understanding beliefs?