Toward a Synthesis of Two Accounts of 
Discourse Structure 
Megan Moser* 
University of Pittsburgh 
Johanna D. Moore t 
University of Pittsburgh 
Among researchers interested in computational models of discourse, there has been a long-standing 
debate between proponents of approaches based on domain-independent rhetorical relations, and 
those who subscribe to approaches based on intentionality. In this paper, we argue that the main 
theories representing these two approaches, RST (Mann and Thompson 1988) and G&S (Grosz 
and Sidner 1986), make similar claims about how speakers' intentions determine a structure of 
their discourse. The similarity occurs because the nucleus-satellite relation among text spans in 
RST corresponds to the dominance relation among intentions in G&S. Building on this similarity, 
we sketch a partial mapping between the two theories to show that the main points of the two 
theories are equivalent. Furthermore, the additional claims found in only RST or only G&S are 
largely consistent. The issue of what structure is determined by semantic (domain) relations in 
the discourse and how this structure might be related to the intentional structure is discussed. We 
suggest the synthesis of the two theories would be useful to researchers in both natural language 
interpretation and generation. 
1. Introduction 
Within the computational discourse community, there is a long-standing debate be- 
tween 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&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 ar- 
gue that G&S and RST are essentially similar in what they say about how speakers' 
intentions determine a structure of their discourse. 
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. 
The relations between intentions indicate whether one intention contributes to the sat- 
isfaction 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. 
Moore and Pollack (1992) argue that both intentional and informational analyses are 
needed simultaneously. 
* 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 there- 
fore 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 inten- 
tional 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 in- 
tentional structure is an important factor in determining when to generate discourse 
cues (e.g., "because," "thus," "although"), which discourse cues to select, and where 
to place those cues (Moser and Moore 1995, in preparation). 
In this paper, we compare what G&S and RST say about intentional structure. 
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. 
The definition of ILS comprises one of the major claims in G&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&S and the nucleus:satellite relation 
between text spans in RST. Roughly speaking, an RST nucleus expresses a G&S in- 
tention In, a satellite expresses another intention Is and, in G&S terms, In dominates 
Is. The correspondence between G&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&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. 
2. Intentional Linguistic Structure in G&S 
G&S is formulated in terms of the interdependence of three distinct structures. Of 
the three structures, it is the effect of intentional structure on linguistic structure that 
concerns us in this paper. This effect is an explicit claim about ILS. 
In G&S, the intentional structure consists of the set of the speaker's communicative 
intentions throughout the discourse, and the relations of dominance and satisfaction- 
precedence among these intentions. The speaker tries to realize each intention by say- 
ing something; i.e., each intention is the purpose behind one or more of the speaker's 
utterances. Intentions are thus an extension of the intentions in Grice's (1957) theory 
of utterance meaning. Speakers intend for the intentions behind their utterances to be 
recognized and for that recognition to be part of what makes their utterances effective. 
A purpose I,, dominates another purpose In when satisfying I, is part of satisfying 
Ira. A purpose In satisfaction-precedes another purpose Im when In must be satisfied 
first. The dominance and satisfaction-precedence relations impose a structure on the 
set of the speaker's intentions, the intentional structure of the discourse, and this in 
turn determines the linguistic structure. 
The linguistic structure of a particular discourse is made up of segments, which 
are sets of utterances, related by embeddedness and sequential order. A segment DS~ 
originates with the speaker's intention: it is exactly those utterances that the speaker 
produces in order to satisfy a communicative intention In in the intentional structure. 
In other words, I,, is the discourse segment purpose (DSP) of DS,. DSH is embed- 
ded in another segment DS,, just when the purposes of the two segments are in the 
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Moser and Moore Discourse Structure 
Intentional Structure 
I0: Intends(IntendH a) I 
I1: Intends(Believe, b) I 
I2: Intends(BelieveH c) 
DS0 
Linguistic Structure 
~S ome and see the LA Chamber Ballet's concert. 
1 ~The show should be very entertaining. 
DS2 ~ It presents all new choreography. 
Figure 1 
For G&S, dominance in intentional structure determines embedding in linguistic structure. 
dominance relation, i.e., Im dominates In. The dominance relation among intentions 
fully determines the embeddedness relations of the discourse segments that realize 
them. For example, consider the discourse shown in Figure 1, adapted from Mann 
and Thompson (1988). The whole discourse is a segment, DS0, that attempts to realize 
I0, the speaker's intention for the hearer to adopt the intention of attending the ballet. 1 
As part of her plan to achieve I0, the speaker generates I1, the intention for the hearer 
to adopt the belief that the ballet will be very entertaining. Then, as part of her plan 
to achieve I1, the speaker generates I2, the intention that the hearer believe that the 
show is made up of all new choreography. As shown on the left in Figure 1, I0 domi- 
nates I1, which in turn dominates I2, Due to these dominance relations, the discourse 
segment that realizes I2 is embedded in the discourse segment for I1, which is in turn 
embedded within the discourse segment for I0, as shown on the right in the figure. 
The dominance of intentions directly determines embedding of segments. 
When one DSP In satisfaction-precedes another Ira, then DSn precedes DS,I in the 
discourse. The satisfaction-precedes relation among intentions constrains the order of 
segments in the discourse, but it does not fully determine it. In the example in Figure 1, 
none of the intentions satisfaction-precedes the others. 
Here we introduce a concept which is not part of the G&S theory, but which will 
be important to our discussion below. We coin the term core to refer to that part of the 
segment that expresses the segment purpose. A segment may contain individual utter- 
ances as well as embedded segments. Most likely, the core of the segment is found in 
these unembedded utterances. In the example, (a) is the core of DS0, (b) the core of DS1 
and (c) the core of DS2. As will be discussed, a core functions to manifest the purpose 
of the segment, while the embedded segments serve to help achieve that purpose. The 
defining feature of the core is its function of expressing the purpose of the segment. 
While the core's position in the G&S linguistic structure is most likely an unembedded 
utterance, it is also possible that the core could be an embedded segment. This could 
occur when the expression of the segment purpose is more elaborate than simply stat- 
ing what the hearer should do or believe. To simplify our discussion, however, we 
assume the core of a segment is an utterance not embedded in any subsegment. 
It should be clear that the theory-independent notion of ILS as it was character- 
ized above is exactly the linguistic structure in G&S. ILS is something G&S makes 
explicit claims about. By choosing to modify the terminology from simply "linguistic 
1 In subsequent work, Grosz and Kraus have developed a model of collaborative planning in which it is not necessary for one agent to have intentions-to towards an act of another agent (Grosz and Kraus, in 
press; see also Grosz and Sidner 1990). For the purposes of this paper, however, we will use the notation of G&S. 
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Computational Linguistics Volume 22, Number 3 
structure" to "intentional linguistic structure," we mean to suggest that consideration 
of something other than speaker intentions--for example, semantic relations--could 
determine another kind of structure to the discourse. Clearly, the semantic (or infor- 
mational) relations among discourse entities can in principle be the determinant of a 
separate linguistic structure. Whether or not such an informational structure is use- 
ful or is related in an interesting way to ILS is a question requiring further research. 
We discuss the relationship between ILS and possible approaches to informational 
structure briefly in Section 5. 
3. Intentional Linguistic Structure in RST 
In contrast to its explicitness in G&S, ILS is only implicit in RST. To identify the 
implicit claims about ILS, we must first identify the components of an RST analysis 
that involve a judgement about the relation between intentions underlying text spans. 
The range of possible RST text structures is defined by a set of schemas, which 
describe the structural arrangement of spans, or text constituents. Schemas are basic 
structural units or patterns in the application of RST relations. There are five schema 
patterns, each consisting of two or more spans, a specification of each span as either 
nucleus or satellite, and a specification of the RST relation(s) that exist between these 
spans. In this paper, we focus on the most commonly occurring RST schema, which 
consists of two text spans (a nucleus and a satellite) and a single RST relation that 
holds between them. The nucleus is defined as the element that is "more essential 
to the speaker's purpose," while the satellite is functionally dependent on the nu- 
cleus and could be replaced with a different satellite without changing the function 
of the schema. As we argue below, this functional distinction between nucleus and 
satellite is an implicit claim about ILS, and is a crucial notion in understanding the 
correspondence between RST and G&S. 
A schema application describes the structure of a larger span of text in terms of 
multiple constituent spans. Each of the constituent spans may in turn have a structure 
of subconstituent spans. Thus, the application of RST schemas in the analysis of a 
text is recursive, i.e., one schema application may be embedded in another. To be an 
acceptable RST analysis, there must be one schema application under which the entire 
text is subsumed and which accounts for all minimal units, usually clauses, of the 
text. In addition, each minimal unit can appear in exactly one schema application, and 
the spans constituting each schema application must be adjacent in the text. These 
constraints guarantee that a correct RST analysis will form a tree structure. 
An instantiated schema specifies the RST relation(s) between its constituent spans. 
Each relation is defined in terms of a set of constraints on the nucleus, the satellite, 
and the nucleus-satellite combination, as well as a specification of the effect that the 
speaker is attempting to achieve on the bearer's beliefs or inclinations. An RST analyst 
must judge which schema consists of RST relation definitions whose constraints and 
effects best describe the nucleus and satellite spans in the schema application. Mann 
and Thompson claimed that, for each two consecutive spans in a coherent discourse, a 
single RST relation will be primary. For reasons discussed in Section 5.1, we consider 
only the RST presentational relations, or what Moore and Pollack (1992) call intentional 
relations, in identifying the ILS claims of RST. 
To illustrate how a speaker's intentions determine discourse structure in this the- 
ory, consider the RST analysis of the example discourse from Figure 1. As shown in 
Figure 2, at the top level, the text is broken down into two spans: (a) and (b-c). The 
span (b-c) forms a satellite that stands in a motivation relation to (a). This span can 
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Moser and Moore Discourse Structure 
a-c 
motivation 
l, ~. .... \ 
a b-c 
evidence 
//-• \ 
b c 
Figure 2 
The RST structure assigned to the example discourse in Figure 1. 
be further broken down into the two minimal units (b) and (c), where (c) is a satellite 
that stands in an evidence relation to (b). 2 
While there is no direct representation of intentions in RST, the asymmetry be- 
tween a nucleus and its satellite originates with the speaker's intentions. The nucleus 
expresses a belief or action that the hearer is intended to adopt. The satellite provides 
information that is intended to increase the hearer's belief in or desire to adopt the 
nucleus. Implicitly, this is a claim that the text is structured by the speaker's intentions 
and, more specifically, by the difference between the intention that the hearer adopt a 
belief or desire expressed in a text span and the intention that a span contribute to this 
adoption. In the example, the nucleus (a) expresses an action that the speaker intends 
the hearer to adopt. The satellite (b-c) is intended to facilitate this adoption by pro- 
viding the hearer with a motivation for doing the suggested action. In the embedded 
span, the nucleus (b) expresses a belief that the speaker intends the hearer to adopt 
and the satellite (c) is intended to facilitate this adoption by providing evidence for 
the belief. 
The second implicit RST claim about ILS is a refinement of the first. The intentional 
relations specify the ways in which a speaker can affect the hearer's adoption of a 
nucleus by including a satellite. That is, not only is there a functional distinction 
between nucleus and satellites, there is also a classification of satellites according to 
how they help achieve the hearer's adoption of the nucleus. Translating this into a 
claim about ILS, text is structured by the ways in which some utterances are intended 
to help other utterances achieve their purpose. 
4. Correspondence between Dominance and Nuclearity 
Now we are in a position to compare the explicit claims of G&S about ILS with the 
implicit ones of RST. Both theories agree that a discourse is structured into a hierarchy 
of non-overlapping constituents, segments in G&S and spans in RST. Each subcon- 
stituent may in turn be structured in exactly the same way as the larger constituent. 
Superficially, the similarity ends there because the internal structure of segments and 
spans is different. In G&S, the internal structure of a segment consists of any number 
of embedded segments plus what we are calling the core, the (usually unembedded) 
2 As discussed in Mann and Thompson (1988), a motivation relation occurs when a speaker intends the 
satellite to increase the hearer's desire to perform the action specified in the nucleus. An evidence 
relation occurs when a speaker intends the satellite to increase the bearer's belief in the nucleus• 
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Computational Linguistics Volume 22, Number 3 
utterances that express the discourse segment purpose. In RST, the internal structure 
of a span consists of a nucleus, which we have characterized as expressing a belief or 
action the hearer is intended to adopt, a satellite, which is intended to facilitate that 
adoption, and an intentional relation between the nucleus and satellite. 
If we look more closely at the correspondence between dominance and nuclearity, 
we find that the structure of spans and segments is nearly identical. Specifically, an 
embedded segment corresponds to a satellite, and the core corresponds to the nucleus. 
Or, because G&S do not have the notion of core in their theory, a more accurate char- 
acterization of the correspondence would be that the nucleus manifests a dominating 
intention, while a satellite manifests a dominated intention. That is, dominance in G&S 
corresponds closely to nuclearity in RST. There is a relationship, which we can crudely 
characterize as that of linguistic manifestation, that links the nucleus to a dominating 
intention and a satellite to a dominated intention. Exactly how to derive a commu- 
nicative intention from an utterance, and vice versa, is one of the main research issues 
in computational linguistics. Here we simply assume that an utterance conveys either 
a belief or an action p and thereby makes manifest the speaker's intention that the 
hearer adopt belief in or an intention to perform p. 
The correspondence suggests a mapping between G&S linguistic structure and 
RST text structure. An embedded segment in G&S will be analyzed as a satellite in 
RST, and the segment core will be the nucleus. When there are multiple embedded 
segments in G&S, each subsegment will be analyzed as an RST satellite. In these cases 
of multiple subsegments, the RST structure will depend on whether the RST relations 
are the same or different. The entire segment may be a single RST span with the G&S 
core as nucleus and each subsegment as a satellite of that nucleus. This occurs when 
the multiple satellites bear the same RST relation to the nucleus. Alternatively, the G&S 
core and an adjacent subsegment may be analyzed as an RST nucleus and satellite, 
forming an RST span. This span is then the nucleus of a higher span in which the 
satellite is an additional G&S subsegment from the same segment. This occurs when 
the multiple satellites bear different relations to their nucleus. 
Because cores are a central aspect of the mapping between the two theories, and 
because cores are not part of the G&S proposal, it is natural to ask whether a segment 
necessarily has a core. Given the nature of segment purposes, a coreless segment 
seems intuitively unlikely. Recall that segment purposes, like the utterance intentions 
discussed by Grice, have the property that they are intended to achieve their effect 
in part from being recognized. The core has an important function: it manifests the 
purpose of the segment. Without a core, the segment purpose must be inferred from 
the subsegments alone. In such a case, the speaker intends that the hearer recognize 
a purpose, but does not supply an utterance that manifests that purpose. 
The question of whether or not coreless segments actually occur, however, is best 
answered by corpus analysis rather than theorizing. For our present purposes, we 
wish to consider the possibility of a coreless segment only because such a segment 
would complicate the mapping between the two theories presented above. In G&S, 
the definition of linguistic structure does not require a segment to contain a core. In 
the RST schemas considered thus far, a span always consists of a nucleus and satellite. 
A less common schema pattern, known as the joint schema, contains multiple spans 
with no nucleus-satellite distinction among them joined into a single span. Should a 
coreless segment occur in a G&S analysis, it can be mapped to a joint schema in RST. 
Building on the correspondence between dominance and nuclearity, we raise two 
issues in the following sections. First, how do informational relations fit into the dis- 
course structure? Second, what synthesis of the two theories emerges when we recog- 
nize the correspondence? 
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Moser and Moore Discourse Structure 
The intentional perspective: 
a-b 
motivation 
a b 
The informational perspective: 
a-b 
condition 
/ / :% 
a b 
(a) Come home by 5. 
(b) Then we can go to the store before it closes. 
(a) Come home by 5:00. 
(b) Then we can go to the store before it closes. 
Figure 3 
RST intentional and informational relations may determine incompatible structures. 
5. Informational Structure 
Moore and Pollack (1992) argued that RST defines two types of relations: intentional 
relations, which arise from the ways in which consecutive discourse elements par- 
ticipate in the speaker's plan to affect the hearer's mental state, and informational 
relations, which obtain between the content conveyed in consecutive elements of a co- 
herent discourse. This is consistent with Mann and Thompson's (1988, 256) distinction 
between "presentational" (intentional) and "subject matter" (informational) relations. 
However, while Mann and Thompson maintain that for any two consecutive elements 
of a coherent discourse, one rhetorical relation will be primary (i.e., related by an 
informational or an intentional relation), Moore and Pollack showed that discourse 
interpretation and generation require that intentional and informational analyses ex- 
ist simultaneously. Thus, in addition to the Intentional Linguistic Structure discussed 
so far, a discourse may simultaneously have an informational structure, imposed by 
domain relations among the objects, states, and events being discussed. 
5.1 Can Intentional and Informational Structure Differ in RST? 
In addition to their claim that intentional and informational analyses must co-exist, 
Moore and Pollack presented an example in which the intentional and informational 
relations can impose a different structure on the discourse. It is important to under- 
stand, however, that their example shows that the discourse structure determined by 
informational relations as defined in RST can be incompatible with the one determined 
by intentional relations. Here we argue that the problem is due to the inclusion of nu- 
clearity in the definition of RST subject matter (informational) relations. As shown in 
Figure 3, the incompatibility arises because the nucleus and satellite of the intentional 
relation may be inverted in the RST informational relation. 3 
In Section 4, we argued that nuclearity in an RST analysis is an implicit claim about 
3 The particular example given by Moore and Pollack had an additional relation embedded in (b), which 
had the same incompatibility between the intentional and informational analysis. 
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Computational Linguistics Volume 22, Number 3 
Context I 
Our current president is far more liberal than 
the last one. 
Context II 
Legislation passed this week will not be 
enacted. 
(a) Clinton supports environmental protection. 
(b) He is sure to veto House Bill 1366. 
Figure 4 
Either relatum may be the nucleus when an instance of a domain relation is used. 
speaker intentions, corresponding to the G&S relation of dominance among intentions. 
That is, nuclearity rightly belongs in the definitions of intentional relations. In contrast, 
informational relations, properly construed, should not distinguish between nucleus 
and satellite in their definitions. As an example, consider the pair of RST relations 
volitional-cause and volitional-result. The volitional-cause relation is defined as one 
in which the nucleus presents a volitional action and the satellite presents a situation 
that could have caused the agent to perform the action. The effect of this relation 
is that the reader "recognizes the situation presented in the satellite as a cause of 
the volitional action presented in the nucleus." The volitional-result relation is nearly 
identical except that the cause of the action is the nucleus and the result is the satellite. 
Why does RST need two relations to capture this? The reason is that the same domain 
relation, call it cause-effect, links a cause and effect regardless of which is the nucleus. 
In Figure 4, note that, while (a) causes (b), either (a) or (b) can be the nucleus of the 
relation. For a particular instance of a cause-effect in the domain, it is equally plausible 
for a speaker to mention the effect to facilitate the hearer's adoption of belief in the 
cause, as would be suggested by context I in Figure 4, or to mention the cause to 
facilitate the hearer's adoption of belief in the effect, as suggested by context II. 
Moreover, this is precisely what the intentional relations capture. By incorporat- 
ing the nucleus-satellite distinction into the definitions of RST informational relations, 
these relations include an implicit analysis of intentional structure. As a consequence, 
strict application of the RST informational relations can result in a different structure 
than that imposed by the intentional relations, and this is the source of the problem 
noted by Moore and Pollack. Because nuclearity can only be determined by consid- 
eration of intentions, and intentional and informational analyses of a discourse must 
co-exist, we argue that the solution to the problem is to properly relegate informa- 
tion about nuclearity (intention dominance) to the intentional analysis, and remove 
it from definitions of informational relations. In this way, these two determinants of 
discourse structure cannot conflict. In addition, note that this is preferable to adding 
surplus informational relations to allow either relatum to be the nucleus (as was done 
in the volitional-cause and volitional-result case) because (1) this obscures the fact that 
relations such as volitional-cause and volitional-result appeal to the same underlying 
domain relation and (2) the proliferation of relations weakens the restrictive power of 
the framework. 
5.2 Relationship between ILS and Informational Structure 
Once we recognize that an informational analysis is needed simultaneously with ILS 
and that the informational analysis should be determined by domain relations without 
reference to how the relations are employed by the speaker, exactly how to determine 
informational structure becomes an underconstrained question. Should all domain 
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Moser and Moore Discourse Structure 
relations across utterances be analyzed in the informational structure? What patterns 
of informational relations are employed in realizing various kinds of intentions, and 
what analysis provides a reliable means for identifying such patterns? Final answers to 
these questions require further research. Because constraints may be needed in order 
to make progress on these issues, we point out two approaches to constraining the 
definition of informational structure. In Section 6.2 we suggest that RST informational 
relations provide a version of one of these approaches. 
The most inclusive definition of informational structure would contain all the 
domain relations between the things being talked about. Included would be causal 
relations of various sorts, set relations, relations underlying bridging inferences (Clark 
and Haviland 1977), and the relation of identity between domain objects underly- 
ing coreference of noun phrases across utterances. By this definition, informational 
structure is a complex network of domain relations that is defined independently of 
the intentional structure. Keeping track of all domain relations in a discourse is an 
overwhelming task and is often infeasible. One approach to constraining informa- 
tional structure is to define it as parasitic on intentional structure. The informational 
structure would contain an accompanying informational relation for each intentional 
relation. A second approach to constraining informational structure is to define it as 
a network of domain relations with type restrictions on the relata. The informational 
structure would contain only the relations among situations, events, and actions, that 
is, the types of entities referred to by clauses. 
6. A Partial Synthesis 
The discussion in Section 4 suggests that RST and G&S share a large amount of com- 
mon ground. That is, many of the claims in the two theories, although formulated 
differently, are essentially equivalent. To begin this section, we state the common 
ground that emerges from relating dominance and nuclearity. Then we briefly re- 
view the claims of each theory that are outside this common ground. Each theory has 
some consistent ground, additional claims that concern issues simply not addressed 
by the other theory. The actual contentious ground, claims made by one theory that 
are incompatible with the other, is quite small. 
6.1 Common Ground 
Building on the correspondence between dominance and nuclearity, a partial synthesis 
of G&S and RST would be roughly the following: A segment/span arises because its 
speaker is attempting to achieve a communicative purpose. Such purposes have the 
feature that they are achieved in part by being recognized by hearers. Thus, the plan 
for achieving the purpose typically has two distinct parts: (1) one or more utterances 
that serve to make the purpose manifest by expressing a belief or action for the hearer 
to adopt (the core/nucleus) and (2) a set of subparts that contribute to achieving 
the purpose by manifesting subpurposes dominated by that purpose (the embedded 
segments / satellites). 
Note that this synthesis encompasses the ILS claims of both theories regarding 
the example discourse in Figure 1. DS0 is a segment/span designed to achieve the 
purpose I0. The plan for achieving I0 is to first manifest I0 by expressing the action in 
(a), the core/nucleus, and then to contribute to the achievement of I0 by providing the 
motivation in (b-c), the embedded segment/satellite. In turn, DS1 is a segment/span 
designed to achieve the purpose I1 by first manifesting I1 in the expression of the 
core/nucleus (b) and then providing evidence in the embedded segment/satellite (c). 
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Computational Linguistics Volume 22, Number 3 
Finally, I2 is made manifest by (c), though no additional contribution to achieving this 
intention is provided. 
6.2 Consistent Ground 
RST and G&S each makes claims about issues not addressed by the other theory. We 
review these claims briefly in order to establish that they are consistent. 
First, the two theories offer different but consistent perspectives on the ordering 
of segments/spans. In G&S, intentions may be related by satisfaction-precedence in 
addition to dominance. One intention satisfaction-precedes another when it must be 
realized before the other. This relation between intentions partially constrains the order 
of what is said and thus introduces a distinction between necessary order, originat- 
ing with a satisfaction-precedence relation of the underlying intentions, and artifac- 
tual order, additional ordering that must be imposed to produce linearized text. G&S 
makes no claim about the relative ordering between a core and embedded segments. 
In RST, because the underlying intentions are not analyzed explicitly, the distinction 
between necessary and artifactual order is not available. Instead, the relative order- 
ing of core/nucleus and embedded segment/satellite is highlighted. RST's authors 
claim that many relations have a typical ordering of their nucleus and satellite. The 
two theories address different aspects of ordering without suggesting any points of 
contention. 
Second, in addition to intentional and linguistic structure, G&S posits an atten- 
tional structure. This component determines which discourse entities will be most 
salient and thereby imposes constraints on available referents for pronouns and re- 
duced definite NPs. This is an important issue, but one that RST simply does not 
make claims about. As noted earlier, the recognition of intentional structure is crucial 
for anaphora resolution, among other discourse-processing tasks. By synthesizing RST 
and G&S, work done using both approaches can be applied to accomplishing these 
tasks during interpretation and generation. 
Finally, while G&S recognize that informational structure is a cue to recognition of 
intentional structure, the theory does not provide detail. As discussed in Sections 5.1 
and 5.2, the analysis of informational relations provided by RST is inadequate and 
incomplete. In either theory, more research is needed to understand how informational 
relations are used to achieve discourse intentions. 
6.3 Contentious Ground 
The claims of G&S and RST discussed so far have been, we argued, either equiva- 
lent or compatible. We now turn to a point of contention between the two theories. 
There are distinctions among the RST intentional relations that, in G&S, would be 
subtypes of the dominance relation among intentions. However, G&S specifies that 
the only relations among intentions affecting discourse structure are dominance and 
satisfaction-precedence. Should the various RST intentional relations be incorporated 
into a synthesized theory? 
The question may be approached from either an empirical or a practical perspec- 
tive, and the two perspectives may lead to different answers. To answer the question 
empirically, one could code a corpus for its intentional relations and attempt to identify 
linguistic cues that correlate with distinctions among the relations. To answer the ques- 
tion practically, one would consider whether distinct intentional relations are useful 
for computational systems that generate and/or interpret natural language. In fact, the 
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Moser and Moore Discourse Structure 
practical application of these intentional relations may be quite different in generation 
and interpretation systems. Further research is required to resolve this question. 
7. Conclusion 
In this paper, we have argued that two of the most important theories of discourse 
structure in computational linguistics, G&S and RST, are not incompatible, but in fact 
have considerable common ground. The key to the basic similarity between these two 
theories is understanding the correspondence between the notions of dominance in 
G&S and nuclearity in RST. Understanding this correspondence between the theories 
will enable computational models that effectively synthesize the contributions of the 
theories, and thereby are useful both for interpretation and generation of discourse. 
Acknowledgments 
The research described in this paper was 
supported by the Office of Naval Research, 
Cognitive and Neural Sciences Division 
(Grant Number: N00014-91-J-1694), and a 
grant from the DoD FY93 Augmentation of 
Awards for Science and Engineering 
Research Training (ASSERT) Program 
(Grant Number: N00014-93-I-0812). The 
authors wish to thank Robert Dale, Barbara 
Di Eugenio, Donia Scott, Lynn Walker and 
two anonymous reviews for helpful 
comments on an earlier draft of this squib. 
References 
Asher, Nicholas and Alex Lascarides. 1994. 
Intentions and information in discourse. 
In Proceedings of the 32nd Annual Meeting of 
the Association for Computational Linguistics. 
Clark, H. H. and S. E. Haviland. 1977. 
Comprehension and the given-new 
contract. In R. O. Freedle, editor, Discourse 
Production and Comprehension. Lawrence 
Erlbaum Associates, Hillsdale, NJ. 
Grice, H. Paul. 1957. Meaning. Philosophical 
Review 68:377-388. 
Grosz, Barbara and Sarit Kraus. In press. 
Collaborative plans for complex group 
action. ArtiJicical Intelligence. 
Grosz, Barbara J. and Candace L. Sidner. 
1986. Attention, intention, and the 
structure of discourse. Computational 
Linguistics 12(3):175-204. 
Grosz, Barbara J. and Candace L. Sidner. 
1990. Plans for discourse. In Philip R. 
Cohen, Jerry Morgan, and Martha E. 
Pollack, editor, Intentions in 
Communication. MIT Press, Cambridge, 
MA, pages 417-444. 
Hirschberg, Julia B., Diane J. Litman, Janet 
Pierrehumbert, and Gregory L. Ward. 
1987. Intonation and the intentional 
structure of discourse. In Proceedings of the 
Tenth International Joint Conference on 
Artificial Intellligence, pages 636-639, 
Milan, Italy. 
Hobbs, Jerry R. 1985. On the coherence and 
structure of discourse. Technical Report 
CSLI-85-37, Center for the Study of 
Language and Information, Leland 
Stanford Junior University, Stanford, 
California. 
Hobbs, Jerry R. 1993. Intention, information, 
and structure in discourse: A first draft. 
In Burning Issues in Discourse, NATO 
Advanced Research Workshop, pages 41--66, 
Maratea, Italy. 
Mann, William C. and Sandra A. 
Thompson. 1988. Rhetorical Structure 
Theory: Towards a functional theory of 
text organization. Text 8(3):243-281. 
Moore, Johanna D. 1995. Participating in 
Explanatory Dialogues: Interpreting and 
Responding to Questions in Context. MIT 
Press, Cambridge, MA. 
Moore, Johanna D. In press. The role of 
plans in discourse generation. In Daniel 
Everett and Sarah G. Thomason, editors, 
Discourse: Linguistic, Computational, and 
Philosophical Perspectives. 
Moore, Johanna D. and C4cile L. Paris. 1993. 
Planning text for advisory dialogues: 
Capturing intentional and rhetorical 
information. Computational Linguistics 
19(4): 651-695. 
Moore, Johanna D. and Martha E. Pollack. 
1992. A problem for RST: The need for 
multi-level discourse analysis. 
Computational Linguistics 18(4):537-544. 
Moser, Megan and Johanna D. Moore. 1995. 
Investigating cue selection and placement 
in tutorial discourse. In Proceedings of the 
33rd Annual Meeting of the Association for 
Computational Linguistics. 130-135. 
Moser, Megan and Johanna D. Moore. In 
preparation. On the correlation of cue 
placement with discourse structure: 
results from a corpus study. 
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