 The main dividing line is between states and the others. The types (b)–(e) all refer to change. This division has been heavily influential in linguistic theory; mostly, however, research concentrated on its relation to aspect. It is to be noted, for example, that verbs of type (c) can be used with the progressive while verbs of type (d) cannot. In an attempt to explain this, Krifka 1986 and Krifka 1992 have introduced the notion of an incremental theme. The idea is that any eventuality has an underlying activity whose progress can be measured using some underlying participant of the event. If, for example, I write a letter then the progress is measured in amounts of words. The letter is therefore the incremental theme in “I write a letter” since it defines the progress. One implementation of the idea is the theory of aspect by Verkuyl (1993). Another way to implement the idea of change is constituted by a translation into propositional dynamic logic (see Naumann 2001). Van Lambalgen and Hamm (2005) have applied the event calculus by Shanahan (1990) to the description of events.