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<Paper uid="C00-1076">
  <Title>Extending a Formal and Computational Model of Rhetorical Structure Theory with Intentional Structures h la Grosz and Sidner</Title>
  <Section position="1" start_page="0" end_page="523" type="abstr">
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Abstract
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    <Paragraph position="0"> In the last decade, members of the computational lingt, istics community have adopted a perspective on discourse based primarily on either Rhetorical Structure Theory or Grosz and Sidner's Theory. However, only recently, re.+ searchers have started to investigate the relationship between the two perspectives. In this paper, we use Moscr and Moore's (1996) work as a departure point for extending Marcu's formalization of RST (1996). The result is a tirst-order axiomatization of the mathematical prol+erties o1' text structures and of the rehttionship between the strttcture of text and intentions. The axiomatization enables one lo use intentions for reducing the ambiguity o1' discourse and the structure of discourse for deriving intentional inferences.</Paragraph>
    <Paragraph position="1"> I Motivation I n the last decade, members of the computational l inguislies cotnnmnity have adopted a perspective on discourse based prinlarily on either l{hetorical Structure Theory (P, ST) (Matin and Thompson, 1988) or Grosz and Sid her's Theory (GST) (Grosz and Sidnet. 1986).</Paragraph>
    <Paragraph position="2"> In GSq, the linguistic constituents are called discom'xe segments (DS) and the lingt, istic discourse slructure is explicitly stipulated to be a tree o1' recursively embedded discourse segments. Each discourse segment is charactel+ized by a prinmry intention, which is called discomwe segment lmrpose (DSP). GST identilies only two kinds o1' intention-based relations that hold between the DSPs of two discourse segments: domittance and sati.@tction precedence. When a discourse segment purpose DSPI that characterizes discourse segment DS1 provides part of the satisfaction of a discourse segment purpose DSP., that characterizes discourse segment DS..,, with DS1 being embedded in DS2, it is said that there exists a dominance relation between DSP~ and DSlq, i.e., DSP.e dominates DSpI. 1t' the salislhction of DSP, is a condition of the satisfaction oI'DSP2, it is said that DSP1 sati,@tctionprecedes DSP.,.</Paragraph>
    <Paragraph position="3"> RST has a richer ontology of relations than GST: intentional and semantic rhetorical relations are considered to hold between non-overlaplfing textual spans. Most of these relations are asymmetric, i.e., they distinguish between their associated nuclei, which express what is most essential to the writer's purpose, and their satellites, which support tile nuclei. In RS'I, the linguisticdiscourse structure is modeled recursively as a tree of related segments. Hence, unlike GSq, where relations are considered to hold between the DSPs associated with embedded segments, relations in RST hold between adjacent, non-overlapping segments.</Paragraph>
    <Paragraph position="4"> Because RST has traditionally been applied to build discourse trees of liner granularity than GST, we will use it here as the starting point of our discussion. Assume, for example, that we are given tim following text (in which the elementary textual units arc labelled lbr reference).</Paragraph>
    <Paragraph position="5"> (I) INo matlcr how much one wants to stay a non-smoker, ^~ \] \[the truth is that the pressure to smoke in junior high is greater than it will be any other time of one's life) q \] IWe know tim\[ 3,000 teens start smoking each day, q \] lalthough it is a fact that 90% of them once thoughl thai smoking was something that Ihey'd never do. D~ I Assume for the moment that we do not analyze this text as a whole, but rather, we dctcrlnine what rhetorical relations could hold between every pair of elementary units. When we apply, for example, the definitions proposed by Mann and Thompson (1988), we obtain the set given below, l</Paragraph>
    <Paragraph position="7"> These relations hold because the tmdcrstanding of both A1 (teens want to stay non-smokers) and I:h (90% o1' the teens think that smoking is something that they wotdd never do) will increase the reader's readiness to accept the writer's right to present Ih (the pressure on teens to start smoking is greater than it will be any other time of their lives); the understanding of c1 (3000 teens start smoking each day) will increase the reader's belief of 1~1; the recognition of Ih as something compatible with IThroughoul this paper, we use the convention lhat rhelorical relations are represented as stated, lirst-order predicales having lhe fornl rhct_rel(,act.me, mLzellite, ?~mlcus). Mullint|clear relalions are represented as predicales having Ihe \['orlll rhct_rcl( ~m',~c, n~tcle'usl , ~uclcus~ ).</Paragraph>
    <Paragraph position="9"> the situation presented in c1 will increase the reader's negative regard for the situation presented in cl ; and the situation presented in D, is a restatement o1' the situation presented in &amp;.</Paragraph>
    <Paragraph position="10"> Marcu (1996) has shown that on the basis of only the rhetorical judgments in (2) and without considering intentions, there are five valid RS-trces that one can build for text ( I ) (see figure l ). What happens though when we consider intentions as well? Moore and Pollack (1992) have already shown that different high-level intentions yield different RS-trces. But how do we formalize tile relationship between intentions and rhetorical structures? For example, how can we use the discourse trees in figure 1 in order to determine the primary intention associated with each analysis? And how can we determine what would be the corresponding dominance relations in a GST account of tile same text'? Consider also a slightly difl'erent problem: assume that besides rhetorical judgments, such as those shown in (2), one can also make intentional judgments. For example, assume that one is interested in an interpretation in which one knows that the DSP of seg,nent \[&amp;, D1\], which contains all units from A1 tO 1)1, dominates the DSP of segment \[c1, l)~\]. Then what is the primary intention of the text in that case'? And how many discourse trees are both valid and consistent with that intentional judgment? Neither RST nor GST can answer these questions on their own. However, a unified theory can. Ill this paper, we provide such a theory.</Paragraph>
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