This article examines the historical construction of depression over about a hundred years, employing the social life of methods as an explanatory framework. Specifically, it considers how emerging methodologies in the measurement of psychological constructs contributed to changes in epistemological approaches to mental illness and created the conditions of possibility for major shifts in the construction of depression. While depression was once seen as a feature of psychotic personality, measurement technologies made it possible for it to be reconstructed as changeable and treatable. Different types of scaling techniques (Likert versus dichotomous scales) enabled the separation of depressive personality from reactive depression, paving the way for measuring the severity and intensity of emotions. Techniques to test sensitivity to change provided a means of demonstrating the efficacy of new psychoactive drug treatments. Later, more advanced techniques of precision scaling enabled the management of a new measurement problem, clinician unreliability, associated with the growing number of professionals involved in mental health care. Through statistical management of unreliability, the construct of depression has dramatically reduced over this period from hundreds of questionnaire items to potentially just two. Exploring the history of depression through this lens produces an alternative narrative to those that have emerged as a result of medicalisation and the actions of individuals and pressure groups.