Psychiatric classification has been the subject of tremendous scrutiny in recent decades. Critics have often applied stringent standards to assess their legitimacy, requiring that they represent ‘natural kinds’ with underlying essences in the form of neurobiological dysfunctions or biomarker associations. They also demand that psychiatric categories must ‘carve nature at its joints’ if they are to be considered valid and that their practical, value-laden nature is an imperfection and an impediment. Gregory L. Murphy's book, Categories We Live By, offers a thought-provoking exploration of the ubiquitous human practice of categorisation, and serves as a useful antidote to naivete about the nature of scientific categories. For psychiatrists, it is a powerful reminder that psychiatric categories are not unique in the conceptual and scientific difficulties encountered. The debate about the definition of a ‘planet’ in astronomy that led to Pluto's demotion is familiar to many, but such problems are common across the natural and social sciences. Even well-established scientific concepts can be ‘surprisingly fuzzy’. I chuckled at this amusing remark by the metallurgist Robert Pond: ‘There's a big group of people who don't know what a metal is. Do you know what we call them? Metallurgists!’

The book explores how categories shape our understanding of ourselves, others and the world throughout our lives, and is structured into two main parts. The first addresses the foundational aspects of categories – how philosophers and psychologists understand them, how they are human constructs yet reflective of reality and how they are represented through language and culture. The second part explores specific case studies, including legal categories, psychodiagnostic categories, species, food items like peanut butter and potato chips as well as racial categories and the concept of ‘death’.

Murphy recounts his initial belief that biologists must have definitive definitions for each species, only to discover that not only is this not the case but biology lacks a universally accepted definition of ‘species’ to begin with. Psychiatrists will find such examples reassuring. In fact, the book includes a chapter devoted to psychodiagnostic categories, providing an informative overview of conceptual developments in this area. Murphy views psychiatric categories as a heterogeneous collection: some are extremes of variations on psychological dimensions (categorisable for practical purposes), others are fuzzy prototypes consisting of a cluster of related features that co-occur but lack a clear essence and some psychiatric categories (neuropsychiatric diseases) are defined by neurobiological essences, such as Huntington disease. Proposed alternatives such the Research Domain Criteria are also discussed, but Murphy correctly notes that even dimensional frameworks inevitably generate their own categories. Categories play a crucial and indispensable role in how we engage with the world, and in the psy-sciences, how we engage with psychiatric suffering. However, it is important to scrutinise these categories, and to recognise the reality they possess and the reality they do not. Categories We Live By is a valuable guide in this undertaking.