One can either debate the possibility of the historical sociology of scientific knowledge or one can do it. Ludwik Fleck took the latter course of action. In Entstehung und Entwicklung einer wissenschaftlichen Tatsache Fleck's overriding concern was with the interpretation of a particular episode in the history of science, and his focus never strayed from the empirical materials pertinent to that task. His more general theoretical statements always arose out of and referred to the historical particulars and circumstances of that episode. Thus, one way of characterizing Fleck's book is to regard it as the work of a practising scientist, intimately familiar with the genesis and career of the Wassermann test: and this would not be an incorrect characterization. Another way of appreciating his accomplishment would be to see it as a piece of empirical history, providing a concrete exemplification of the sociology of scientific knowledge. The only wholly misguided approach to Fleck's work would be to distill his theorizing out of the empirical concerns in which it was grounded.