Some Facts About Centers, Indexicals, and Demonstratives

SOME FACTS ABOUT CENTERS, INDEXICALS, AND DEMONSTRATIVES Rebecca J. Passonneau Columbia University 450 Computer Science Bldg. New York, New York 10027, U.S.A becky@cs.columbia.edu ABSTRACT 1 Certain pronoun contexts are argued to establish a local center (LC), i.e., a conventionalized indexical similar to lst/2nd pers. pronouns. Demonstrative pronouns, also indexicals, are shown to access en- tities that are not LCs because they lack discourse relevance or because they are not yet in the uni- verse of discourse. 1 Introduction Referring expressions in discourse are multifunc- tional and dual-faced. Besides functioning to spec- ify referents, they also indicate the status of their referents in the evolving discourse model, such as the informational status of being given or new [Pri81], or maintain the attentional status of be- ing in focus [Sid83] [Gro77]. They are dual-faced in that the surface form of a referring expression is constrained by the prior discourse context, and then increments the context, serving to constrain subsequent utterances [Isa75]. As a consequence of this latter property, the communicative effect of many referring forms, especially pronouns, is rel- ative to specific types of discourse contexts. The discourse reference functions of a few types of pro- nouns are examined, taking into account their mul- tifunctionality and their dual nature, in order to clarify their processing requirements in dialogic natural language systems. In particular, a compar- ison of the conversational usage of it with two types of indexical pronouns indicates that certain uses of it, referred to as local centering, resemble what Ka- plan [Kap89] refers to as pure indexicals. Several functions of lhat are also identified and shown to contrast with local centering with respect to their preconditions and effects. Third person, definite (3d) pronouns contrast with indexical pronouns because the referents of the former are arbitrary, and must be actively es- tablished as part of the current universe of dis- course in order for the intended referent to be 1 This paper was written under the support of DARPA grant N000039-84-C-0165 and NSF grant IRT-84-51438. I am grateful to Kathy McKeown for her generous support. identified. In contrast, the referents of index- icals such as I and you (i.e., the speaker and addressee) are necessary components of the dis- course circumstances. 2 Indexical pronouns can be further classified into pure indexicals versus demonstratives, 8 depending on how the current dis- course circumstances provide their referents. The referent of a pure indexical is fully determined by the semantic rules and a context, which together pick out a unique referent for each use. Thus I refers to the person who utters it (assuming that I is used to refer). A pure indexical cannot refer to alternative entities, nor can any other expression pick out the relevant entity via the same type of referring function. Pure indexicals do not add en- tities to a context, or change the attentional status of their referents. In contrast, the referent of a demonstrative pro- noun is not completely determined by the context plus the semantic rules. An …

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