 Although Karttunen’s (1974) model turned out to be equivalent to a system which, from a purely technical point of view, is in the Frege-Strawson tradition, Karttunen (1974) was one of the seminal papers of the dynamic zeitgeist that swept through semantics and pragmatics in the last decades of the twentieth century. Also relevant here are Hamblin (1970), Stalnaker (1972; 1974) , Gazdar (1979) and Lewis (1979), all of whom advanced dynamic models of pragmatics in which the (joint) commitments of speakers and hearers evolve as new assertions are made and their content becomes part of the linguistic context available for future utterances. It is against this background that Heim (1982; 1983) offered the first dynamic semantic account of presupposition. Heim’s model utilizes Stalnaker’s notion of a context as a set of all possible worlds compatible with what has been established at that point in a conversation, but involves a crucial twist adapted from Karttunen. In Stalnaker’s model, a single global context is updated each time new information is asserted, but in Heim’s model the context is updated locally in the process of computing the meanings of subparts of a complex  expression.[18]  We can define a simplified version of Heim’s system as follows: