A Compositional Semantics of Temporal Expressions in English

A COMPOSITIONAL SEMANTICS OF TEMPORAL EXPRESSIONS IN ENGLISH Erhard W. Hinrichs BBN Laboratories Inc. 10 Moulton St. Cambridge, MA 02238 Abstract This paper describes a compositional semantics for temporal expressions as part of the meaning representation language (MRL) of the JANUS system, a natural language understanding and generation sys- tem under joint development by BBN Laboratoires and the Information Sciences Institute. 1 The analysis is based on a higher order intansional logic described in detail in Hinrichs, Ayuso and Scha (1987). Tem- poral expressions of English are translated into this language as quantifiers over times which bind tem- poral indices on predicates. The semantic evaluation of time-dependent predicates is defined relative to a set of discourse contexts, which, following Reichen- bach (1947), include the parameters of speech time and reference time. The resulting context-dependent and multi-indexed interpretation of temporal expres- sions solves a set of well-known problems that arise when traditional systems of tense logic are applied to natural language semantics. Based on the principle of rule-to-rule translation, the compositional nature of the analysis provides a straightforward and well- defined interface between the parsing component and the semantic interpretation component of JANUS. 1 Introduction JANUS is a natural language understanding and generation system which allows the user to interface with several knowledge bases maintained by the US NAVY. The knowledge bases contain, among other things, information about the deployment schedules, locations and readiness conditions of the ships in the Pacific Reet. (1) a. Did the admiral deploy the ship? b. Which C3 ships are now C4? c. When will Vincent arrive in Hawaii? d. Who was Frederick's previous commander? As the sample queries in (1) demonstrate, much of IThe work presented here was supported under DARPA contract #N00014-85-C-0016. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the Defense Advanced Research Projects Agency or of the United States Government. this information is highly time-dependant: Ships change locations in accordance with their deployment schedules, incur equipment failures or undergo per- sonnel changes which can lead to changes in the ship's readiness rating. It is, therefore, imperative that at the level of semantic representation of the natural language input an adequate analysis can be provided for those linguistic expressions that carry time infor- mation, for example, tenses, temporal adverbials and temporal adjectives. 2 Applying Classical Tense Logic To Natural Language Semantics My own treatment of temporal expressions is very much a response to the kinds of analyses that have been provided in classical tense logic. When I refer to classical tense logic I mean the kinds of logics that orginate in the work of the logician Arthur Prior (Prior 1967) and that have been applied by Montague (Montague 1973) and others to natural language semantics. In classical tense logic time-dependency of infor- mation enters into the definition of the notion of a proposition. Propositions are defined as functions from a set of …

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