Delusions: problems in definition and measurement.

A review is made of definitions of 'delusion'. Problems with these are considered in terms of clarity and consistency with the evidence. Questions concerning the issues of fixity and intensity of delusions are raised and a proposal is made for attempting to operationalize these concepts using a 'personal questionnaire' method. The measurement in a pilot study of the fixity and intensity of the beliefs of two patients using this method is described and the results reported.