Delusions are usually taken to be mistaken beliefs, which are inferred from experience and are highly resistant to change, despite overwhelming evidence against them (e.g. DSM-IV-TR, p. 821). However, there is some controversy as to whether the term "delusion' refers to a distinctive kind of mental state or to a range of different psychological predicaments, and no definition of delusion is uncontroversial. As David (1999, p. 17) remarks, "despite the façade created by psychiatric textbooks, there is no acceptable (rather than accepted) definition of a delusion".¹ Furthermore, it is not always clear what is meant by the claim that delusions are beliefs (see, for example, Bayne and Pacherie 2005; Bortolotti 2005).Despite such concerns, much the same strategy has been employed to explain a range of delusions. Delusions, it is maintained, are the product of two different impairments. First of all, there is an anomalous perception. This is then fed into defective reasoning processes, which generate a delusional belief. It is generally assumed that the "belief' in question is a propositional attitude and that the delusion takes the form "X believes that p' where p is a false proposition that any rational being with access to the same information ought to reject. Approaches like this tend to presuppose a rather impoverished conception of experience. It is construed as a kind of input system that presents the subject with assorted perceptual contents, which are then fed into belief-forming processes.