My dissertation examines the relationship between processes of rationalization and knowledge about the human being that emerged primarily in the last half of the nineteenth century in Germany. During this time, Germany experienced radical changes in social organization as a result of industrial growth. Developing industrial technologies, and the scientific knowledge that made these technologies possible, transformed the relation of large populations to work, urban space, health, and lifestyle. Large populations also became an object of knowledge and regulation directed towards improving social conditions and optimizing labor, particularly in fields such as psychiatry, eugenics, and public health. The social and technological changes around this time led to intense reflection on human nature in science, philosophy, and literature. How was the human being to engage with the technologies and institutions organizing social life? What possibilities and dangers arose in the processes of rationalization represented by these technologies and institutions? My claim is that efforts to manage populations according to rational principles thus created new sources for constructing and managing the self. Rationalization provided not only a concrete means of engineering social change on a large scale, but also an experimental field for individual conduct. In this sense, my emphasis is on the creative potential of rationalization—a potential often overlooked in discussions on the growing power of state, industry, and technology. To elaborate my argument, I will focus on texts by the doctor Rudolf Virchow, the eugenicist Alfred Ploetz, the biologist Ernst Haeckel, the writers Gustav Freytag, Wilhelm Raabe, and Gerhart Hauptmann, as well as the philosopher of technology Ernst Kapp. These philosophers, writers, and scientists shared an interest in negotiating the place of the human being—on an individual and collective scale—in an increasingly complex social and technological environment.