Our original paper (Grosz, Joshi, and Weinstein 1983) on centering claimed that certain entities mentioned in an utterance were more central than others and that this property imposed constraints on a speaker's use of different types of referring expressions. Centering was proposed as a model that accounted for this phenomenon. We argued that the coherence of discourse was affected by the compatibility between centering properties of an utterance and choice of referring expression. Subsequently, we revised and expanded the ideas presented therein. We defined various centering constructs and proposed two centering rules in terms of these constructs. A draft manuscript describing this elaborated centering framework and presenting some initial theoretical claims has been in wide circulation since 1986. This draft (Grosz, Joshi, and Weinstein 1986) has led to a number of papers by others on this topic and has been extensively cited, but has never been published. 1 We have been urged to publish the more detailed description of the centering framework and theory proposed in Grosz, Joshi, and Weinstein (1986) so that an official version would be archivally available. The task of completing and revising this draft became more daunting as time passed and more and more papers appeared on centering. Many of these papers proposed extensions to or revisions of the theory and attempted to answer questions posed in Grosz, Joshi, and Weinstein (1986) . It has become ever more clear that it would be useful to have a "definitive" statement of * Division of Applied Sciences, Harvard University, Cambridge, MA. E-mail: grosz@das.harvard.edu. t Computer and Information Science, University of Pennsylvania, Philadelphia, PA. E-mail: joshi@linc.cis.upenn.edu. :~ Department of Philosophy, University of Pennsylvania, Philadelphia, PA. E-mail: weinstei@linc.cis.upenn.edu. 1 Early drafts of Grosz, Joshi, and Weinstein (1986) were in circulation from 1983. Some citations to other work have dates between 1983 and 1986. This work utilized these earlier drafts. Computational Linguistics Volume 21, Number 2 the original motivations for centering, the basic definitions underlying the centering framework, and the original theoretical claims. This paper attempts to meet that need. To accomplish this goal, we have chosen to remove descriptions of many open research questions posed in Grosz, Joshi, and Weinstein (1986) as well as solutions that were only partially developed. We have also greatly shortened the discussion of criteria for and constraints on a possible semantic theory as a foundation for this work.
In this paper, our method is based on Centering Theory REF , a theory that relates the salience of entities in an utterance with the form of linguistic expressions and the local coherence of discourse.
Centering: A Framework For Modeling The Local Coherence Of Discourse