I want to examine the difficult and rather obscure idea of a style of thought (Denkstil) as it occurs in Fleck's book on the Genesis and Development of a Scientific Fact ¹ and his paper "Zur Krise der "Wirklichkeit" ".² I shall try to break down this idea and isolate some of its components. I can see at least some ways of building a bridge between what Fleck says about thought-styles and the detailed work in the history of science that Shapin has outlined for us. If I am right then it should prove possible to define some of the causes of thought-styles and to relate them, in the way that Fleck would have wanted, to facts about the social structure of the Denkkollektiv.