This is probably the best treatise in recent times on the subject of delusions. Bortolotti brings her professional expertise to bear on a subject that is central to our understanding of what it means to be severely afflicted with schizophrenia or any psychosis. What is remarkable is that Bortolotti has mastered the literature on delusions as a psychopathological phenomenon. She does not treat delusions merely as an excuse for high-flown philosophical arguments, but takes the reality of delusions seriously, recognising that the empirical facts about delusions are important and that philosophical enquiry ought to ‘make sure that the theory to be developed is compatible with the relevant empirical data’ (p. 7). This is a refreshing approach from a philosopher.

Bortolotti's thesis is that delusions are beliefs and that they cannot be denied belief status by virtue of being irrational. For most psychiatrists, it is a given that delusions are false beliefs. This distinguishes them from hallucinations, which are defined as false perceptions. In this model the terms ‘beliefs’ and ‘perceptions’ are operating as markers of difference that point at the putative domains of cognitive functions where impairments may lie. But in Bortolotti's thesis, for something to count as a belief it must be rational in at least one of three senses: (1) it must be procedurally rational by at least being well-integrated in a system with other beliefs; (2) it must be epistemically rational in that it must be well supported and responsive to available evidence; and (3) the person who is the believer must be agentially rational by being able to offer good reasons in support of the content of their belief and by acting in such a way that is consistent with and explicable by the content of their belief.

Bortolotti goes on to show that delusions do indeed breach these features of what she refers to as the ‘rationality constraint of beliefs’. Her aim, though, is to argue that ordinary beliefs also breach these constraints and that there is much continuity between beliefs and delusions. Her analysis is of great importance to psychiatrists, particularly her examination of Berrios’ ^{Reference Berrios1} contention that delusions are ‘empty speech acts’ and by implication that they have no intentional content. Bortolotti argues against this position but in the end she misses the point that apparent thought content (i.e. belief), even when that content has symbolic meaning, is not evidence of content-full speech. In other words, it is perfectly possible for a person to utter a speech that has apparent meaning yet for that speech to be empty. An indirect example is an eyewink which is a tic; superficially, it appears to communicate but in reality it is empty of any symbolic communication.

Bortolotti accepts that delusions are not unitary in nature but in her analysis she continues to fall into the trap of treating them as such. There are delusions that occur as sudden irruptions into consciousness; the so-called autochthonous delusions that resemble inspired beliefs; delusions that follow from veridical experiences, usually abnormal perceptions such as hallucinations. There are other delusions that follow from subtle abnormalities of perception that are only apparent under psychometric testing, such as delusional misidentification syndromes that seem at least to require abnormalities of facial or visual perception. And finally, there are other delusions that derive from altered mood, altered atmosphere, or weak reasoning ability. What this means for normal psychology and for philosophy is that the class of cognition termed belief is not unitary or homogeneous, even though, superficially, all beliefs seem to behave similarly.

Bortolotti's book is an important contribution to our understanding of the nature of beliefs and hence of our understanding of delusions. It shows that psychiatry has a lot to learn from philosophy and no doubt philosophy too can only benefit from dialogue with psychiatrists. There are many original insights in this book.