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<Paper uid="J96-3006">
  <Title>Toward a Synthesis of Two Accounts of Discourse Structure</Title>
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1. Introduction
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    <Paragraph position="0"> Within the computational discourse community, there is a long-standing debate between proponents of theories based on domain-independent rhetorical relations (most notably Rhetorical Structure Theory, Mann and Thompson 1988, henceforth RST; see also Hobbs 1985) and those who subscribe to theories based on intentionality (most notably that of Grosz and Sidner 1986, henceforth G&amp;S). While some researchers have tried to integrate the two approaches (Moore and Paris 1993; Asher and Lascarides 1994; Hobbs 1993), the two are usually viewed as competing theories. Here we argue that G&amp;S and RST are essentially similar in what they say about how speakers' intentions determine a structure of their discourse.</Paragraph>
    <Paragraph position="1"> Intentional structure describes the roles that discourse actions play in the speaker's communicative plan to achieve desired effects on the hearer's mental state. Intentions encode what the speaker was trying to accomplish with a given portion of discourse.</Paragraph>
    <Paragraph position="2"> The relations between intentions indicate whether one intention contributes to the satisfaction of another (dominance) or whether one intention must be satisfied before another (satisfaction-precedence) (Grosz and Sidner 1986). In contrast, informational structure is concerned with domain relations among the things being talked about.</Paragraph>
    <Paragraph position="3"> Moore and Pollack (1992) argue that both intentional and informational analyses are  needed simultaneously.</Paragraph>
    <Paragraph position="4"> * Learning Research and Development Center, and Department of Linguistics, University of Pittsburgh, Pittsburgh, PA 15260. E-mail: moser@isp.pitt.edu t Department of Computer Science, and Learning Research and Development Center, University of Pittsburgh, Pittsburgh, PA 15260. E-mail: jmoore@cs.pitt.edu (~) 1996 Association for Computational Linguistics  Computational Linguistics Volume 22, Number 3 Prior research has established that recognition of intentional structure, and therefore appropriate generation of cues to such structure, is crucial for many discourse-processing tasks. Grosz and Sidner (1986) argued that intentional structure is crucial for anaphora resolution and plan recognition. Hirschberg et al. (1987) show that intentional structure plays a role in intonation. In addition, Moore (1995, in press) shows that intentional structure is crucial for responding effectively to questions that refer to prior discourse and to communication failures. Further research indicates that intentional structure is an important factor in determining when to generate discourse cues (e.g., &amp;quot;because,&amp;quot; &amp;quot;thus,&amp;quot; &amp;quot;although&amp;quot;), which discourse cues to select, and where to place those cues (Moser and Moore 1995, in preparation).</Paragraph>
    <Paragraph position="5"> In this paper, we compare what G&amp;S and RST say about intentional structure.</Paragraph>
    <Paragraph position="6"> We use the term Intentional Linguistic Structure, or ILS, as a theory-neutral way of referring to the structure of a discourse determined by the speaker's intentions.</Paragraph>
    <Paragraph position="7"> The definition of ILS comprises one of the major claims in G&amp;S. ILS is not directly addressed in RST, but is implicit in the RST concept of nuclearity. We argue that the key to reconciling ILS in the two theories lies in the correspondence between the dominance relation between intentions in G&amp;S and the nucleus:satellite relation between text spans in RST. Roughly speaking, an RST nucleus expresses a G&amp;S intention In, a satellite expresses another intention Is and, in G&amp;S terms, In dominates Is. The correspondence between G&amp;S dominance and RST nuclearity helps to clarify the relationship between ILS and informational structure, the structure determined by underlying semantic (domain) relations. In particular, the undesirable possibility that informational structure could be incompatible with intentional structure (Moore and Pollack 1992) is explained. Further, we argue that a synthesis of G&amp;S and RST is possible because the correspondence between dominance and nuclearity forms a great deal of common ground, and because the remaining claims in the two theories are consistent.</Paragraph>
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