This essay reexamines the nineteenth-century origins of medical specialization. It suggests that by the 1880s, specialization had become perceived as a necessity of medical science as a result of the realization of two preconditions: First, a new collective desire to expand medical knowledge prompted clinical researchers to specialize; only specialization, it was believed, permitted the rigorous observation of many cases. Second, administrative rationality suggested that one could best manage large populations through proper classification, gathering together individuals belonging to the same class and separating those belonging to different categories. Both of these conditions emerged first and most powerfully in early nineteenth-century Paris. They were, in contrast, uniquely underdeveloped in the fragmented medical community of London during this period.