Dynamic Semantics for Tense and Aspect

A semantics for tense, modality, and aspect in natural language must capture causal and contingent relations between events and states as welt as merely temporal ones The paper investigates a non-reified dynamic logic based, formulation of the situation calculus is a formalism for a computational semantics for a. number of temporal categories in English and suggests that some recent claims that dynamic logics are inherently unsuitable for this purpose have taken too narrow a view of the situation calculi 1 T e m p o r a l O n t o l o g y The most important thing to observe about the temporal ontology implicit in natural language tense and aspect is that it IR not purely temporal To take a simple example the English perfect when predicated of an event like losing a watch says that some contextually retrievable consequences of the event in question hold at the time under discussion (Such consequences have sometimes been described under the heading of ""present relevance of the perfect ) As a result, conjoining such a perfect with a further clause denying those consequences is infelicitous (1) I have lost my watch (# but 1 have found it again) In this respect the perfect stands in contrast to the more purely temporal tenses, such as the past, which make no comparable claim about the consequences of the core event (2) Yesterday I lost my watrh (but 1 (have) found it again) It is because categories like the perfect are not purely temporal that it is usual to distinguish them from the tenses proper as ' aspects Another aspect whose meaning is not purelv temporal is the progressive or lmperfective The predication that it makes concerning the core event is a subtle one While the progressive clearly states that some event is ongoing at the time under discussion it is not necessarily the event that is actuallv mentioned Thus in a helow there seems to be a factive entailment about an event of wri t ing But in b, there 18 no such entailment concerning an event of writ ing a sonnet, for b is true even if the author was interrupted before he could complete the action Dowty [1979] named this rather surprising property of the progressive the imperfective paradox The imper fective paradox is a sign that we must distinguish various types or sorts of core event in natural language tempo ral ontology This system, which is described at greater length in [Steedman in press.], is briefly summarised as follows There are two key insights into this system which most theories either build upon or are forced to reinvent I he first concerns the temporal ontolog) itself and is usually attributed to Vendler [1967] though there are precedents in work by Jespersen Kenny and many earlier author itjes including Aristotle Vendler s taxonom) was importantly refined bv \erkuyl and Dowty, and ha.s been further extended by manv others Such taxonomies typically distinguish 'states from events' and divide the latter into a number of wr ts or types \endler dislin guished 'activities , (events which have duration but don t change state like heat* writing), achievements (events which have no duration but do chang* stat< like h eats amvmg), and accomplishments (which hav* duration and change state like heats writing a sonnet) Many authors have proposed recursive sort hierarchies Moens [1987, 1988] explained the aspectual sort hierarchy and possible coercions among Akiionsaricn m terms of a structure of the kind represented schematically in figure 1, representing an association in memory or the knowledge representation of all events with characteristic preparations and consequents, an idea that has 6ince been adopted in DR. Theory (Kamp and Reyle, [1993, p 557-570] Moens claimed Lhat the preparation is in Vendler's terms an activity, the consequent is a (perfect) state, and that the core event is an achievement Ihere is a great deal more to say about the status of these categories, but we wil l take it as read here, noting merely that we Bhall follow these authors m assuming that accomplishments like writing a sonnet are composites of an activity of writing and a culminating achievement of 1292 NATURAL LANGUAGE