Some Facts about Centers, Indexicals, and Demonstratives

Certain pronoun contexts are argued to establish a local center (LC), i.e., a conventionalized indexical similar to l s t /2nd pers. pronouns. Demonstrat ive pronouns, also indexicals, are shown to access entities that are not LCs because they lack discourse relevance or because they are not yet in the universe of discourse. 1 I n t r o d u c t i o n Referring expressions in discourse are multifunctional and dual-faced. Besides functioning to specify 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 at tentional status of being 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 relative to specific types of discourse contexts. The discourse reference functions of a few types of pronouns are examined, taking into account their multifunctionality and their dual nature, in order to clarify their processing requirements in dialogic natural language systems. In particular, a comparison 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 Kaplan [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 established as part of the current universe of discourse 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 indexicals such as I and you (i.e., the speaker and addressee) are necessary components of the discourse circumstances. 2 Indexical pronouns can be further classified into pure indexicals versus demonstratives, 8 depending on how the current discourse 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 ent i ty via the same type of referring function. Pure indexicals do not add entities to a context, or change the at tent ional status of their referents. In contrast, the referent of a demonstrat ive pronoun is not completely determined by the context plus the semantic rules. An accompanying demonstration is required, such as a physical or vocal gesture to something in the immediate discourse circumstances. Further, demonstratives can refer to anything in the context tha t can be demonstrated. In the cases of discourse deixis discussed by Webber [Web90], e.g., demonstrat ive pronouns are used to refer to discourse segments. Webber notes that in these cases, the demonstrat ion consists in the intention to refer signalled by the use of the demonstrative, plus the semantics of the containing clause, plus at tent ional constraints on which discourse segments can be demonstrated. 4 Thus, 3d pronouns, pure indexical pronouns, and demonstratives all differ with respect to the set of contextual elements that are available referents, and the manner in which the referent is related to the referring expression. Investigating their distinct discourse functions leads to extensions to the tri2The term indexical includes devices whose meaning pertalns to the time, the place and the perceived environment of a discourse context, e.g., tense, deictic adverbs (here, there) and deictic pronouns (this, that) [Pei35]. 3The view of indexicals presented here is largely drawn from Kaplan [Kap89]. 4Webber [Weh90] argues that only segments on the right frontier are available referents.