An important use of [the modal words] is connected with the notions of an ability and of a disposition and with the verb ‘can’. For example : Jones can speak German (=it is possible for Jones to make himself understood in German) ; Jones cannot speak German (=it is impossible for Jones to make himself understood in German). We shall call the modal concepts which refer to abilities and dispositions dynamic modalities (I am indebted for the term to Mr. Geach)… The question whether the dynamic modalities, i.e. the logic of abilities and dispositions, is subject to exactly the same formal rules as the alethic modalities will have to be investigated separately. (An Essay in Modal Logic, Amsterdam, 1951, p. 54.)