Published texts, unpublished documents and, to a lesser extent, artefacts are the stuff from which historians of science fashion their interpretations of the past. From these residues we attempt to reconstruct the lost fabric of personalities, activities and institutions that constituted the practice of science, and to comprehend the flow of thought that was its substance. Like the sensory data of the empirical sciences, these raw materials are not pure chunks of reality. They must be interpreted in the light of contemporary discourse and practice of which they were part. Over time, and in the hands of different commentators they acquire new significance, sometimes of mythic proportions; moreover, they are inevitably viewed in the light of present suppositions and fashions, both scientific and historiographical.