Principles of reasoning in historical epidemiology

The case of John Snow has long been important to epidemiologists and public health officials. However, despite the fact that there have been many discussions about the various aspects of Snow's case, there has been virtually no discussion about what guided Snow's reasoning in his coming to believe his various conclusions about cholera. Here, I want to take up this question in some detail and show that there are a number of specific principles of reasoning that played a crucial role for Snow. Moreover, these principles were epistemologically important to Snow, a fact about which Snow is explicit in many places. An analysis of Snow's case suggests that, because of the epistemic role such principles of reasoning can play, health care practitioners ought to understand their practices to be theoretically informed in these ways, and not just data driven.