Belief systems are supposed to be governed by norms of rationality. Yet some people seem to believe quite extraordinary things: for example, that they are dead, or that their closest relatives have been replaced by impostors, or that the person they see in the mirror is not really them, or that someone else's thoughts are being inserted into their mind. Do people really believe such things? Could beliefs like these simply be rational interpretations of unusual experiences? Why are these beliefs maintained despite their utter implausibility and the uniform skepticism with which others greet them? In this book, psychologists and philosophers describe and discuss a range of case studies of delusional beliefs, drawing out general lessons both for the cognitive architecture of the mind and for the notion of rationality, and exploring connections between the delusional beliefs that occur in schizophrenia and the flawed understanding of beliefs that is characteristic of autism.