This paper depicts Ian Hacking’s ‘styles of reasoning’ as conditions of possibility. After distinguishing between possibilities and causes, it articulates the implicit stratigraphical metaphor used to describe the relationship between different conditions of possibility, with ‘lower’ layers being necessary for ‘higher’ ones. It notes the use of this stratigraphical metaphor in the work of multiple scholars in history and in science studies. The paper suggests three ways in which this model can be useful: clarifying the definition and use of ‘context’ (...) in history of science; redefining counterfactuals as ‘possible historical worlds’; and thinking up new forms of ‘big picture’ histories of science. (shrink)