This article is an attempt to shed some further light on the people and the processes involved in the identification of mental deficiency in children and young people. In order to do this, it turns away from the themes that have been most prominent in the historiography to date: elite and professional ideas, parliamentary and public debates and the formulation of policy. Instead, the paper is concerned with a single instance of diagnosis of imbecility in an 11-year-old schoolboy in a rural village in the English county of Hertfordshire. As far as is possible it reconstructs this diagnosis and charts and explains a remarkable and successful challenge to it in the High Court. In doing so, it draws on a variety of documentary records--educational, legal and medical--as well as the testimony of some of the surviving members of the family concerned. In employing these sources particular attention is paid to the actions of the people involved in diagnosis, and it seeks to explain and understand those actions with explanatory tools taken from cultural history. (Contains 74 footnotes.)