 In a number of influential papers Brendan Maher (1974, 1988, 1999, 2003) argues that delusions are not ill-formed beliefs, and that there is nothing irrational in the relationship between the evidence supporting the delusional hypothesis and the formation of such a hypothesis. According to Maher, the unusual nature of the delusion is due to the unusual nature of the experiences on the basis of which the delusion is formed. By reference to Maher’s model, Blaney (1999) describes delusions as ‘false but reasonable’. Some difficulties have been identified with this strategy. A first difficulty is that there seem to be people who suffer from the same type of brain damage, and plausibly have the same experience, as the people who develop the delusion, but do not accept any delusional hypotheses. How can these people avoid forming a delusion? One possible answer is that those who have unusual experiences and do not form the delusion have hypothesis-evaluation mechanisms that work efficiently, and thus end up rejecting hypotheses with low probability and plausibility. But those who have unusual experiences and do form the delusion are instead affected by an additional problem, maybe a deficit at the level of hypothesis evaluation, which can be conceived as a failure of rationality.