Temporal Ontology in Natural Language

TEMPORAL ONTOLOGY IN NATURAL LANGUAGE s Marc Moen-~ and Mark Steedman t Centre for Cognitive Science *t and Dept. of AT t, Univ. ~ Edinb-rgh, and Dept. of Computer and Information Science, Univ. of Pennsylvania t ABSTRACT A semantics of linguistic categories like tense, aspect, and cer- tain temporal adverbials, and a theory of their use in defining the temporal relations of events, both require a more complex structure on the domain underlying the meaning representa- tions than is commonly assumed. The paper proposes an ontology based on such notions as causation and consequence, rather than on purely temporal primitives. We claim that any manageable logic or other formal system for natural language temporal descriptions will have to embody such an ontology, as will any usable temporal database for knowledge about events which is to be interrogated using natural language. 1. Introduction It has usually been assumed that the semantics of temporal expressions is directly related to the linear dimensional con- caption of time familiar from high-school physics - that is, to a model based on the number-line. However, there are good reasons for suspecting that such a conception is not the one that our linguistic categories are most directly related to. When-clauses provide an example of the mismatch between linguistic temporal categories and a semantics based on such an assumption. Consider the following examples: (1) When they built the 39th Street bridge... (a) ...a local architect drew up the plans. Co) ...they used the best materials. (c) ...they solved most of their traffic problems. To map the temporal relations expressed in these examples onto linear time, and to try to express the semantics of when in terms of points or intervals (poss~ly associated with events), would appear to imply either that when is multiply ambiguous, allowing these points or intervals to be temporally related in at least three different ways, or that the relation expressed between main and when-clauses is one of "approximate coin- cidence". However, neither of these tactics explains the pecu- liarity of utterances like the following: (2) #When my car broke down, the sun set. The oddity of this statement seems to arise because the when- clause predicates something more than mere temporal coin- cidence, that is, some contingent relation such as a causa/link between the two events. Of course, our knowledge of the world does not easily support such a link. This aspect of the sentence's meaning must stem from the sense-meaning of when, because parallel utterances using just after, at approxi. mate/y the same t/me as, and the like, which predicate purely temporal coincidence, are perfectly felicitous. We shall claim that the different temporal relations conveyed in examples (1)do not arise from any sense-ambiguity of when, or from any "fuzziness" in the relation that it expresses between the times refered to in the clauses it conjoins, but from the fact that the meaning of when is not primarily temporal at an. We shall argue that when has single sense-meaning reflecting its role of establishing …

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