Subsequent Reference: Syntactic and Rhetorical Constraints

Subsequent Reference: Syntactic and Rhetorical Constraints David D. McDonald MIT Artificial Intelligence Laboratory Cambridge, Massachusetts 0Z139 Abstract Once a_.nn ~ is introduced into a discourse, the form of subsequent references to it are strongly governed by convention. This paper discusses how those conventions can be represented for use by a generation facility. A multistage representation is used, allowing decisions to be made when and where the information is available. It is suggested that a specification of rhetorical structure of the intended message should be included with the present syntactic one, and the conventions eventually reformulated in terms of it. Introduction Whenever a speaker wants fo refer in text or speech to some object, action, state, etc., she must find phrase which will both provide an adequate description and fit the context. What governs her choice? One way to find out might be to look at the selected phrase afier the fact and try to develop a static characterization of the relation between it and its context. This is what most non-computational linguisfs do. However, relations derived fron~ finished texts are at best incomplete. They will not tell us how the choice was made or even guarentee tllat the relation(s) was apparent w.hen the choice had to be made. To get a dear picture of what people know about making references, we have to focus our attention of the process that they 8_,o through. It must involve making decisions on the basis of some contextual evidence. What is the evidence? How and when is it computed? How is it described? Is the decision of what phrase to use made all at once or as a gradual refinement? How is this process interleaved with the larger process of constructing the rest of the utterance? This report describes research done at the Artificial Intelligence Laboralory of the Massachusetts Institute of Technology. Support for the laboratory's artificial intellience research is provided in part by the Advanced Researc'h Projects Agenc:y of the Department of Defence under Office of Naval Research contract N00014-75-C-0643. We can narrow tile research problem by. distinguishing two kinds of references: initial and subsequent. This classification divides instances of reference by their position in a discourse. "Initial" references introduce new entities into the discourse, while "subsequent" references are another mention of one already introduced. An inilial reference must be an encompassing enough description of the new entity that the audence will be able to recognize it, This requires matchin 8 goals with evidence from a model of what the audience is likely to already know and how likely ttley are to understand various choices of wording (e.8. which of its properties should be emphasized? - why is it being introduced?). This is not easy. People talking or writin 8 about unfamiliar things or to unfamiliar audiences are not particularly good at it. Subsequent references are another matter. They are very highly grammalisized. Willie an initial reference may take almost any form: noun phrases with unrestricted numbers of adjectives and qualifying phrases, nominalized …

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