Situations and Intervals

The PUNDIT system processes natural language descriptions of situations and the intervais over which they hold using an algorithm that integrates ~peet and ~en~e [ogle. It analyzes the tense and aspect of the realverb to generate representations of three types of situations-states, processes and events-and to locate the situations with respect to the time at which the text was produced. Each situation type has a distinct temporal structure, represented in terms of one or more intervals. Further, every interval has two features whose different values capture the aspectual differences between the three different situation types. Capturing these differences makes it possible to represent very precisely the times for which predications are asserted to hold. 1. I n t r o d u e t l o n This paper describes a semantics of situations and the intervals over which they hold that is neither situation semantics (Barwise and Perry, 1983) nor interval semantics (Dowty, 1979, 1982, 1986; Taylor, 1977). It is unfortunately d~f~cult to avoid the overlap in terminology because what will be described here shares certain goals and assumptions with each. The concerns addressed here, however, arise from the computational task of processing references to situations in natural language text in order to represent what predicates are asserted to hold over what entities and when. S i tua t ion as used here pertains to the linguistic means for referring to things in the world, i.e., to sentences or predications. More specifically, s i t u a t i o n is the superordinate category in Mourelatos' typology of a s p e c t u a l classes of predications, schematised in Fig. 1. SITUATIONS I I I STATES OCCURRENCES P r e u . r e . low. I I I PROCESSES EVENTS Alarm iJ Jounding. Engine/ailed. Fig. 1: Mourelatos' typology of situations The PUNDIT text-processing system s processes references to situations using an algorithm that integrates tense logic (Reichenbach, 1947) with aspect, or what Talmy (1985) calls the~pa~e~w-~ ~ ~ d~tribution o~ ac~io, throug~ time. This paper describes how PUNDIT represents the temporal structure of three types of situations, namely states, processes and events, and how these situations are located in time. 2. P r o b l e m s in C o m p u t i n g A p p r o p r i a t e R e p r e s e n t a t i o n s The critical problems in the semantic analysis of references to situations and their associated times are: I) language encodes several different kinds of temporal information, 2) this information is distributed in many distinct linguistic elements, and finany, 3) the semantic contribution of many of these elements is context-dependent and cannot be computed without looking at co-occurring elements. ZThis work was supported by DARPA under contract N00014-85-C-0012, administered by the Office of Naval Research. APPROVED FOR PUBLIC RELEASE, DISTR/BUTION UNLIMITED. ZFormerly SDC--A Burroughs Company. zPUNDIT is an acronym for Prolog UNderstands and InteErates Text. It is a modular system, implemented in Quintus Prolog, with distinct syntactic, semantic and pragmatic components (cf. Dahl et al., 1987).