 Linguists distinguish four or five lexical aspectual classes or Aktionsarten. The following four were introduced by the philosopher Zeno Vendler (1967: chapter four) on the basis of earlier work by philosophers such as Kenny, Ryle and Aristotle (see also the  entry on events). For a formal definition of Aktionsart the reader is referred to van Lambalgen and Hamm (2005: 85 ff). 