  Some authors introduce aspectual considerations into the taxonomy, drawing on Aristotle’s distinction between Energeia and Kinêsis (Ackrill 1965). The idea is that different verbs describe different types of events: verbs with no continuous form (‘know’) correspond to states; verbs with continuous form for which the present continuous entails the past perfect (‘John is walking uphill’ entails ‘John walked uphill’) correspond to activities; and verbs for which the present continuous entails the negation of the past perfect (‘John is climbing the mountain’ entails ‘John has not (yet) climbed the mountain’, at least in the relevant context) correspond to performances (Mourelatos 1978). Several authors have followed in these footsteps to develop linguistically sophisticated theories (Taylor 1977; Dowty 1979; Freed 1979; Roberts 1979; Bach 1981; Galton 1984; Verkuyl 1989; Smith 1991; Kühl 2008), but the legitimacy of drawing ontological categorizations from such linguistic distinctions has been questioned (Gill 1993).