Gerhart Zeller’s prize-winning 1961 study of the history of the concept of unitary psychosis has never before been published in German or English. To date, the manuscript remains by far the best historical account of the early development of the concept in Germany. The editors’ introduction analyzes Zeller’s monograph on three temporal levels. First, as a rediscovered original contribution to historical scholarship on nineteenth century psychiatric classification. Second, as an historical document in its own right, situated on the threshold between the murderous atrocities of the Holocaust and the psychiatric reforms of the 1970s and ’80s (Psychiatrie-Enquête). Third, as a unique contribution to post-Neo-Kraepelinian discourse and to the critiques of contemporary psychiatric nosologies (DSM) and research agendas (RDoC). In helping to elucidate the historical evolution of dimensional approaches to psychiatric classification, the monograph is likely to resonate within wider current biomedical trends toward personalized medicine and molecular diagnostics, and to find a readership among psychiatric practitioners and historians alike.