Jaspers' textbook General Psychopathology has in the past been described as the ‘most important single book on the aims and logic of psychological medicine’ (Shepherd, 1990). In recent years it seems to have rather fallen off the psychiatric syllabus. Its diminished importance is exemplified by changing opinions about delusions. Nowadays, delusions are considered by many to lie on a continuum with normal beliefs, and this model underpins modern cognitive therapy for psychosis (Kingdon & Turkington, 1994). Jaspers, in contrast, considered that delusions were distinct from normal beliefs. This required him to formulate a distinct mechanism of delusions that resembles, to large extent, the concepts of ‘modularity’ proposed by Fodor (Fodor, 1983) (who has not yet entered the psychiatric syllabus!). The continuum model suggests that delusions ought to be treatable. However, trials of cognitive-behavioural therapy for psychosis suggest significant benefit in only about half of patients treated (see Turkington & McKenna, 2003). What prevents therapeutic change along the dimensions of belief in non-responsive patients? One possibility is that Jaspers was right after all. In this month's debate the modular view of delusions is taken by Dr Hugh Jones and the case for a continuum model is argued by Professors Philippe Delespaul and Jim van Os.