This is an important book. The author entitles it an ‘essay’, an apt term for a sharply focused but extended examination of the question. It is not a review, but an argument; but the argument analyses much that has previously been said about the subject.

Bolton is ideally placed to write such a piece, being a philosopher, clinical psychologist and researcher. He also co-directs a Masters programme on the ‘Philosophy of mental disorder’.

Bolton notes that the question has a curious status: ‘barely visible yet of widespread importance’. Clinicians may pay little regard to it in day-to-day practice, but the implications for social exclusion are major. Particularly troubling is the role of ‘values’, as opposed to facts, in determining what mental disorder is.

The essay starts by examining the assumptions underlying the diagnostic manuals, including some major recent critiques, such as that of Horwitz & Wakefield. ^{Reference Horwitz and Wakefield1} Bolton then asks what the bio-behavioural sciences now have to tell us about the phenomena. This is a valuable discussion, particularly the claim that Jaspers' celebrated dichotomy between ‘understanding’ and ‘explanation’ should be superseded by a more inclusive concept of ‘intentional causality’. This encompasses biological and psychological processes construed within the context of evolutionary design, and can lead to coherent ‘pluralistic’ accounts of causes.

Then on to the claim that mental disorders could be ‘natural facts’. The strongest case is Wakefield's, who argues that mental disorders are harmful disruptions of psychological functions designed by evolution. This receives a sympathetic hearing, but is not endorsed because many proposed ‘functions’ are hypotheses, not facts, and are hugely pervaded by social meaning.

So we cannot escape a critical role for ‘values’ in defining mental disorder. The social aspects of mental disorder are then examined, much influenced by Foucault. Bolton's view of the implications of ‘post-modernism’ is that uncertainty about ‘boundaries’ presents the necessity for a range of ‘stakeholder’ voices to be heard and to be reconciled. Bolton finally settles on a pragmatic view of mental disorder – complex, often messy agreements based on judgements of ‘distress or disability’ that lead to a perceived need for treatment. This does not help in relation to interventions to protect the public, which he argues should be regulated by human rights protections, not definitions of mental disorder. Sadly, this is unlikely to work in practice.

The book is clearly organised and is written in an engaging style. The reader need not fear abstruse philosophical analysis. Anyone with an interest in the subject would do well to read the book – and that should include all clinicians.