Although a common clinical phenomenon, delusions are difficult to explain and have a problematic conceptual status. Advances in understanding delusions have come from studies which involve detailed investigation of particular types of delusion. Some of this work is summarised, with the Capgras and Cotard delusions as specific examples. These are used to high-highlight questions for which there is the potential for fruitful dialogue with philosophers. Such questions include the criteria for deciding that a statement represents a belief, the extent to which we integrate our beliefs into a coherent web, and the nature and limits of human rationality.