PROVIDING A UNIFIED ACCOUNT OF 
DEFINITE NOUN PHRASES IN DISCOURSE 
Barbara J. Grosz 
,M'tificial Intelligence Center 
SRI International 
Menlo Park. CA 
Aravind K. Joshi 
Dept. of Computer and Information Science 
University of Pennsylvania 
Philadelphia, PA 
Scott Wcinstein 
Dept. of Philosophy 
University of Pennsylvania 
Philadelphia, PA 
1. Overview 
Linguistic theories typically assign various linguistic 
phenomena to one of the categories, syntactic, semantic, 
or pragmatic, as if the phenomena in each category were 
relatively independent of those in the others. However, 
various phenomena in discourse do not seem to yield 
comfortably to any account that is strictly a syntactic or 
semantic or pragmatic one. This paper focuses on 
particular phenomena of this sort-the use of various 
referring expressions such as definite noun phrases and 
pronouns-and examines their interaction with 
mechanisms used to maintain discourse coherence. 
Even a casual survey of the literature on definite 
descriptions and referring expressions reveals not only 
defects in the individual accounts provided by theorists 
(from several different disciplines), but also deep 
confusions about the roles that syntactic, semantic, and 
pragmatic factors play in accounting for these 
phenomena. The research we have undertaken is an 
attempt to sort out some of these confusions and to create 
the basis for a theoretical framework that can account for 
a variety of discourse phenomena in which all three 
factors of language use interact. The major premise on 
which our research depends is that the concepts necessary 
for an adequate understanding of the phenomena in 
question are not exclusively either syntactic or semantic 
or pragmatic. 
The next section of this paper defines two levels of 
discourse coherence and describes their roles in accounting 
for the use of singular definite noun phrases. To illustrate 
the integration of factors in explaining the uses of 
referring expressions, their use on one of these levels, i.e., 
the local one, is discussed in Sections 3 and 4. This 
account requires introducing the notion of the centers of a 
sentence in a discourse, a notion that cannot be defined in 
terms of factors that are exclusively syntactic or semantic 
or pragmatic. In Section 5, the interactions of the two 
levels with these factors and their effects on the uses of 
referring expressions in discourse are discussed. 
2. The Effects of Different Levels of Discourse 
Coherence 
A discourse comprises utterances that combine into 
subconstituents of the discourse, namely, units of 
discourse that are typically larger than a single sentence, 
but smaller than the complete discourse. However, the 
constituent structure is not determined solely by the 
linear sequence of utterances. It is common for two 
contiguous utterances to be members of different 
subconstituents of the discourse (as with breaks between 
phrases in the syntactic analysis of a sentence); likewise, it 
is common for two utterances that are not contiguous to 
be members of the same subconstituent. 
An individual subcoastituent of a discourse exhibits both 
internal coherence and coherence with the other 
subconstituents. That is, discourses have been shown to 
have two levels of coherence. Global coherence refers to 
the ways in which the larger segments of discourse relate 
to one another. It depends on such things as the function 
of a discourse, its subject matter, and rhetorical schema 
\[Grosz, 1977, 1981; Reichman, 1981 I. Local coherence 
refers to the ways in which individual sentences bind 
together to form larger discourse segments. It depends on 
such things as the syntactic structure of an utterance, 
ellipsis, and the use of pronominal referring expressions 
\[Sidner, 1981 I. 
The two levels of discourse coherence correspond to two 
levels of focusing--global focusing and centering. 
Participants are said to be globally focused on a set of 
entitie.~ relevant to the overall discourse. These entities 
may either have been explicitly introduced into the 
discourse or be sufficiently closely related to such entities 
to be considered implicitly in focus \[Grosz, 19811 . In 
contrast, centering refers to a more local focusing process, 
one relates to identifying the single entity that an 
individual utterance most centrally concerns \[Sidner, 
1979; Joshi and Weinstein, 1981\]. 
IThis research was supported in part by the National Science 
Foundation under Grant MCS-8115105 to SRI International, and Grant MCS81-07290 to the University of Pennsylvania. 
44 
The two levels of focusing/coherence have different 
effects on the processing of pronominal and 
nonpronominal definite noun phrases. Global coherence 
and focusing are major factors in the generation and 
interpretation of nonpronominal def'lnite referring 
expressions. 2 Local coherence and centering have greater 
effect on the processing of pronominal expressions. In 
Section 5 we shall describe the rules governing the use of 
these kinds of expressions and shall explain why 
additional processing by the hearer (needed for drawing 
additional inferences} is involved when pronominal 
expressions are used to refer to globally focused entities or 
nonpronominal expressions are used to refer to centered 
entities. 
Many approaches to language interpretation have 
ignored these differences, depending instead on powerful 
inference mechanisms to identify the referents of referring 
expressions. Although such approaches may suffice, 
especially for well-formed texts, they are insufficient in 
general. In particular, such approaches will not work for 
generation. Here the relationships among focusing, 
coherence, and referring expressions are essential and 
must be explicitly provided for. Theories-and systems 
based on them--will generate unacceptable uses of 
referring expressions if they do not take these 
relationships into account. 3 
3. Centering and Anaphora 
In our theory, the centers of a sentence in a discourse 
serve to integrate that sentence into the discourse. Each 
sentence, S, has a single backward-looking center, Cb(S), 
and a set of forward-looking centers, Cf(S). Cb(S) serves 
to link S to the preceding discourse, while Cf(S) provides 
a set of entities to which the succeeding discourse may be 
linked. To avoid confusion, the phrase =the center" will 
be used to refer only to Cb(S). 
To clarify the notion of center, we will consider a 
number of discourses illustrating the various factors that 
are combined in its definition (abstractly) and in its 
identification in a discourse. In Section 5 we define center 
more precisely, show how it relates to Sidner's \[1981\] 
immediate focus and potential loci, and discuss how the 
linkages established by the centers of a sentence help to 
determine the degree of intelligibility of a discourse. We 
begin by showing that the center cannot be defined in 
syntactic terms alone. The interaction of semantics and 
centering is more complex and is discussed in Section 4. 
The following examples, drawn from Reinhart \[1982\], 
illustrate the point that the notion of center is not 
syntactically definable, 4 i.e., the syntax of a sentence S 
does not determine which of its NPs realizes Cb(S). (The 
2They differ in other respects also. Reichman \[19811 a~d Grosz \[19811 discuss 
some of these. 
3Initial attempts to incorporate focusing mechanisms in generation 
systems are described in \[Appelt, 1981 and MeKeown, 1982\]. 
41ntonation can obviously affect the interpretation; for the purposes of this paper, it may be regarded a~ part of a syntax. 
reasons for the use of this terminology axe discussed in 
Section 4.) 
(t&) Who did Max see yesterday? 
(lb) Max saw Rosa. 
(2a) Did anyone see Ros& yesterday? 
(2b) Max s~w Rosa. 
Although (lb) and (2b) are identical, Cb(lb) is Max and 
Cb(2b) is Rosa. This can be seen in part by noticing that 
=He saw Rosa" seems more natural than (lb) and =Max 
saw her" than (2b) (a fact consistent with the centering 
rule introduced in Section 5.) The subject NP is the 
center in one context, the object NP in the other. 
Even when the NP used to realize Cb(S) can be 
syntactically determined, the Cb(S) itself is not yet fully 
determined, for Cb(S) is typically not a linguistic entity 
(i.e., it is not a particular linguistic expression). Rosa, not 
°Rosa ° is the Cb(2b). Consider. the discourse: 
(3z) How is Rosa? 
(3b) Did anyone see her yesterday? 
(3e) Max saw her. 
Here, Cb(3c) is Rosa, but clearly would not be in other 
contexts where the expression "her" still realized the 
backward-looking center of "Max saw her." This is seen 
most simply by considering the discourse that would 
result if "How is Joan?" replaced (3a). In the discourse 
that resulted, Joan, not Rosa, would be the center of (3c). 
4. Centering and Realization 
The interactions of semantic and pragmatic factors with 
centering and their effects on referring expressions are 
more complex than the preceding discussion suggests. In 
the examples given above, the NPs that realize Cb(S) also 
denote it., but this is not always the case: we used the 
term "realize" in the above discussion advisedly. In this 
section, we consider two kinds of examples in which the 
center of a sentence is not simply the denotation of some 
noun phrase occurring in the sentence. First, we will 
examine several examples in which the choice of and 
interaction among different kinds of interpretations of 
definite noun phrases are affected by the local discourse 
context (i.e., centering}. Second, the role of pragmatic 
factors in some problematic cases of referential uses of 
definite descriptions \[Donnellan 1966\] is discussed. 
4.1. Realization and Value-Free and Value-Loaded 
Interpretations 
The distinction between realization and semantic 
denotation is necessary to treat the interaction between 
value-free and value-loaded interpretations \[Barwise and 
Perry, 1982\] of definite descriptions, as they occur in 
extended discourse. Consider, for example, the following 
sequence: 
45 
(4a) The vice president of the United States 
is also president of the Senate. 
(4b) Historically, he is the president's key 
man in negotiations with Congress. 
(4b') As Ambassador to China, he handled 
many tricky negotiations, so he is 
well prepared for this Job. 
Cb(4b) and Cb(4b') are each realized by the anaphoric 
element "he. = But (4b) expresses the same thing as 
"Historically, the vice president of the United States is 
the president's key man in negotiations with Congress" 
(in which it is clear that no single individual vice 
president is being referred to) whereas (4b') expresses the 
same thing as, "As ambassador to China, the \[person who 
is now\] vice president of the United States handled many 
tricky negotiations,..." This can be accounted for by 
observing that "the vice president of the United States" 
contributes both its value-free interpretation and its 
value-loading at the world type to Cf(4a). Cb(4b) is then 
the value-free interpretation and Cb(4b') is the value- 
loading, i.e., George Bush. 
In this example, both value-free and value-loaded 
interpretations are showu to stem from the same full 
definite noun phrase. It is also possible for the movement 
of the center from a value-free interpretation (for Cb(S)) 
to a value-loaded interpretation (for Cb of the next 
sentence)-or vice versa-to be accomplished solely with 
pronouns. That is, although (4b)-(4b') is (at least for 
some readers) not a natural dialogue, similar sequences 
are possible. 
There appear to be strong constraints on the kinds of 
transitions that are allowed. In particular, if a given 
sentence forces either the value-free or value-loaded 
interpretation, then only that interpretation becomes 
possible in a subsequent sentence. However, if some 
sentence in a given context merely prefers one 
interpretation while allowing the other, then either one is 
possible in a subsequent sentence. 
For example, the sequence. 
(Sa) The vice president of the United States 
is also president of the Senate. (Sb) He's the president's key a~ in ne~otiatione 
with Congress. 
in which "he" may be interpreted a~ either value-free 
(iT') or value-loaded (VL}, may be followed by either of 
the following two sentences: 
(5c) As ambassador to China. he 
handled many tricky negotiations. (VL) 
(5c') He is required to he at least 35 years old. (V'F') 
tlowever, if we change (Sb) to force the value-loaded 
interpretation, as in (5b'), then only (5c) is possible. 
($b') Right non he is the president's 
key man £n negotiations sith Congress. 
Similarly, if {5b) is changed to force the value-free 
interpretation, as in {4b), then only (5c') is possible. 
If an intermediate sentence allows both interpretations 
but prefers one in a given context, then either is possible 
in the third sentence. A use with preference for a value- 
loaded interpretation followed by a use indicating the 
value-free interpretation is illustrated in the sequence: 
John thinks that the telephone £s a toy. 
He plays with it every day. (V~ preferred; V~ok) 
He doesn't realize that £t is tn £nventlon 
that changed the world. (V~ 
The preference for a value-free interpretation that is 
followed bv a value-loaded one is easiest to see in a 
dialogue situation: 
st: The vice president of the United States 
is also president of the Senate. 
s2: I thought he played some 
important role in the House. (VFpreferred; VL ok) 
st: He did. but that van before he vu VP. (V~) 
4.2. Realization and Referential Use 
From these examples, it might appear that the concepts 
of value-free and value-loaded interpretation are identical 
to Donnellan's I19661 attributive and referential uses of 
noun phrases. However, there is an important difference 
between these two distinctions. The importance to our 
theory is that the referential use of definite noun phrases 
introduces the need to take pragmatic factors (in 
particular speaker intention) into account, not just 
seman| ic factors. 
DonnelIan \[1966\[ describes the referential and 
attributive uses of definite descriptions in the following 
way: 
"A speaker who uses a definite description 
attributively in an assertion states something 
about whoever or whatever is the so-and-so. A 
speaker who uses a definite description 
referentially in an a~sertion, on the other hand , 
uses the description to enable his audience to 
pick out whom or what he is talking about and 
states something about that person or thing. In 
the first case the definite description might be 
said to occur essentially, for the speaker wishes 
to assert something about whatever or whoever 
fits that description; but in the referential use 
the definite description is merely one tool for 
doing a certain job--calling attention to a person 
or thing--and in gefieral any other device for 
doing the same job, another description or a 
name. would do as well. In the attributive use, 
the attribute of being the so-and-so is all 
imp~,rtant, while it is not in the referential use.* 
The distinction Donnellan suggests can be formulated in 
terms of the different propositions a sentence S containing 
a definite description D may be used to express on 
differcn! occasions of use. When D is used referentially, it 
contributes its denotation to the proposition expressed by 
~6 
S; when it is used attributively, it contributes to the 
proposition expressed by S a semantic interpretation 
related to the descriptive content of D. The identity of 
this semantic interpretation is not something about which 
Donnellan is explicit. Distinct formal treatments of the 
semantics of definite descriptions in natural language 
would construe the appropriate interpretation differently. 
In semantic treatments based on possible worlds, the 
appropriate interpretation would be a (partial} function 
from possible worlds to objects; in the situation semantics 
expounded by Barwise and Perry, the appropriate 
interpretation is a (partial} function from resource 
situations 5 to objects. 
.As just described, the referential-attributive distinction 
appears to be exactly the distinction that Barwise and 
Perry formulate in terms of the value-loaded and value- 
free interpretations of definite noun phrases. But this 
gloss omits an essential aspect of the referential- 
attributive distinction as elaborated by Donnellan. In 
Donnellan's view, a speaker may use a description 
referentially to refer to an object distinct from the 
semantic denotation of the description, and, moreover, to 
refer to an object even when the description has no 
semantic denotation. 
In one sense, this phenomenon arises within the 
framework of Barwise and Perry's treatment of 
descriptions. If we understand the semantic denotation of 
a description to be the unique object that satisfies the 
content of the description, if there is one, then Barwise 
and Perry would allow that there are referential uses of a 
description D that contribute objects other than the 
semantic denotation of D to the propositions expressed by 
uses of sentences in which D occurs. But this is only 
because Barwise and Perry allow that a description may 
be evaluated at ~ resource situation other than the 
complete situation in order to arrive at its denotation on 
a given occasion of use. Still, the denotation of the 
description relative to a given resource situation is the 
unique object in the situation that satisfies the description 
relative to that situation. 
The referential uses of descriptions that Donnellan gives 
examples of do not seem to arise by evaluation of 
descriptions at alternative resource situations, but rather 
through the "referential intentions" of the speaker in his 
use of the description. This aspect of referential use is a 
pragmatic rather than a semantic phenomenon and is best 
analyzed in terms of the distinction between semantic 
reference and speaker's reference elaborated in Kripke \[10vv\]. 
Con~idcr the following discourses drawn from Kripke {lov~'l: 
(6a) Her husband is kind to her. 
(6b) No. he isn't. The usa you're 
referring to isn't her husband. 
(7a) Her husband is kind to her. 
(7b) He is kind to her but be isn't her husband. 
With (6a) and (7a), Kripke has in mind a case like the 
one discussed in Donnellan \[1066\], in which a speaker uses 
a description to refer to something other than the 
semantic referent of that description, i.e., the unique thing 
that satisfies the description (if there is one). Kripke 
analyzes this case as an instance of the general 
phenomenon of a clash of intentions in language use. In 
the case at hand, the speaker has a general intention to 
use the description to refer to its semantic referent; his 
specific intention, distinct from his general semantic 
intention, is to use it to refer to a particular individual. 
He incorrectly believes that these two intentions coincide 
and this gives rise to a use of the referring expression "her 
husband" in which the speaker's reference and the 
semantic reference are distinct. "8 (The speaker's referent 
is presumably the woman's \]over). 
From our point of view, the importance of the case 
resides in its showing that Cf(S) may include more than 
one entity, that is realized by a single NP in S. In this 
case, "her husband" contributes both the husband and 
the lover to Cf{6a} and Cf(Ta). This can be seen by 
observing that both discourses seem equally appropriate 
and that the backward-looking centers of (6b) and /7b) 
are the husband and the lover, respectively, realized by 
their anaphoric elements. Hence, the forward-looking 
centers of a sentence may be related not semantically but 
pragmatically to the NPs that realize them. 
Hence, the importance of the referential/attributive 
distinction from our point of view is that it leads to cases 
in which the centers of a sentence may be pragmatically 
rather than semantically related to the noun phrases that 
realize them. 
5. Center Movement and Center Realization-- 
Constraints 
In the foregoing sections we have discussed a number of 
examples to illustrate two essential points. First, the noun 
phrase that realizes the backward-looking center of an 
utterance in a discourse cannot be determined from the 
syntax of the utterance alone. Second, the relation N 
realizes c between noun phrases N and centers c is 
neither solely a semantic nor solely a pragmatic relation. 
This discussion has proceeded at a rather intuitive level, 
without explicit elaboration of the framework we regard 
as appropriate for dealing with centering and its role in 
explaining disco,trse phenomena. Before going on to 
describe constraints on the realization relation that 
5Roughly, *any situation on which the speaker can focus 
attention ° is a potential candidate for a resource situation with respect to which the 
speaker may value load his uses of definite descriptions. Such resource situations must contain a unique object 
which satisfies the description. 
6There are, of course, several alternative explanations; e.g., the 
speaker may believe that the description is more likely than an accurate one to be interpreted correctly by the hearer. Ferreting out 
exactly what the case is in a given situation requires accounts of 
mutual belief and the like. A discussion of these issues is beyond the 
scope of this paper. 
h7 
explain certain phenomena in discourse, we should be 
somewhat more explicit about the notions of center and 
realization. 
We have said that each utterance S in a discourse has 
associated with it a backward-looking center, Cb(S), and 
a set of forward-looking centers, Cf(S). What manner of 
objects are these centers? They are the sort of objects 
that can serve as the semantic interpretations of singular 
noun phrases. 7 That is, either they are objects in the 
world (e.g., planets, people, numbers} or they are 
functions from possible worlds (situations, etc.} to objects 
in the world that can be used to interpret definite 
descriptions. That is, whatever serves to interpret a 
definite noun phrase can be a center. 
For the sake of concreteness in many of the examples in 
the preceding discussion, we have relied on the situation 
semantics of Barwise and Perry. The theory we are 
developing does not depend on this particular semantical 
treatment of definite noun phrases, but it does require 
several of the distinctions that treatment provides. In 
particular, our theory requires a semantical treatment 
that accommodates the distinction between 
interpretations of definite noun phrases that contribute 
their content to the propositions expressed by sentences in 
which they occur and interpretations that contribute only 
their denotation-in other words, the distinction between 
value-free and value-loaded interpretations. As noted, a 
distinction of this sort can be effected within the 
framework of "possible-worlds" approaches to the 
semantics of natural language. In addition, we see the 
need for interpretations of definite noun phrases to be 
dependent on their discourse context. Once again, this is a 
feature of interpretations that is accommodated in the 
relational approach to semantics advocated by Barwise 
and Perry, but it might be accommodated within other 
approaches as well. 8 
Given that Cb(S), the center of sentence S in a 
discourse, is the interpretation of a definite noun phrase, 
how does it become related to S? In a typical example, S 
will contain a full definite noun phrase or pronoun that 
realizes the center. The realization relation is neither 
semantic nor pragmatic. For example, N realizes c may 
hold in cages where N is a definite description and c is its 
denotation, its value-free interpretation, or an object 
related to it by a "speaker's reference." More 
importantly, when N is a pronoun, the principles that 
govern which c are such that N realizes c derive from 
neither semantics nor pragmatics exclusively. They are 
principles that must be elicited from the study of 
discourse itself. A tentative formulation of some such 
principles is given below. 
Though it is typical that, when c is a center of S, S 
contains an N such that N realizes c, it is by no means 
necessary. In particular, for sentences containing noun 
7In a fuller treatment of our theory we will consider centers that 
are realized by constituents in other syntactic categories. 
81srael \[1983\] discusses some of these issues and compares several properties of situation semantics with Montague semantics. 
phrases that express functional relations (e.g., "the door," 
• the owner'} whose arguments are not exhibited 
explicitly (e.g., a house is the current center, but so far 
neither its door nor its owner has been mentioned), 9 it is 
sometimes the case that such an argument can be the 
backward-looking center of the sentence. We are 
currently studying such cases and expect to integrate that 
study into our theory of discourse phenomena. 
The basic rule that constrains the realization of the 
backward-looking center of an utterance is a constraint on 
the speaker, namely: 
\[f the Cb of the current utterance is the same as the 
Cb of the previous utterance, a pronoun should be used. 
There are two things to note about this rule. First, it 
does not preclude using pronouns for other entities as long 
as one is used for the center. Second, it is not a hard 
rule, but rather a principle, like a Gricean maxim, that 
can be violated. However, such violations lead at best to 
conditions in which the hearer is forced to draw 
additional inferences. 
As a simple example, consider the following sequence, 
assuming at the outset that John is the center of the 
discourse: 
(Sa) He called up Mike yesterday. (he=John) 
(Sb) He ,as annoyed by John's call. 
(8b) is unacceptable, unless it is possible to consider the 
introduction of a second person named "John." However, 
intervening sentences that provide for a shift in center 
from John to Mike (e.g., "He was studying for his driver's 
test') suffice to make (8b) completely acceptable. 
Sidner's discourse focus corresponds roughly to Cb(S), 
while her potential foci correspond approximately to 
Cf(S). However, she also introduces an actor focus to 
handle multiple pronouns in a single utterance. The basic 
centering rule not only aLlows us to handle the same 
examples more simply, but also appears to avoid one of 
the complications in Sidner's account. Example D4 from 
Sidner \[1081} illustrates this problem: 
(9-1)I haven't seen Jeff for several days, 
(9-2)Carl thinks he's studying for his exams. 
(9-3)But I think he Tent bo the Cape with Llnda. 
On Sidner's account, Carl is the actor focus after (0-2) 
and Jeff is the discourse focus (Cb(9-2)). Because the actor 
focus is preferred as the referrent of pronominal 
expressions, Carl is the leading candidate for the entity 
referred to by he in {9-3}. It is difficult to rule this case 
out without invoking fairly special rules. On our account, 
Jeff is Cb(0-2) and there is no problem. The addition of 
actor focus was made to handle multiple pronouns--for 
example, if (9-3) were replaced by 
He thinks he studies too much. 
The center rule allows such uses, without introducing a 
9Grosz \[1977\] refers to this a~ "implicit focusing'; other examples are presented in Joshi and Weinstein \[1981\] 
~,8 
second kind of focus (or center), by permitting entities 
other than Cb(S) to be pronominalized as long as Cb(S) 
is.l° 
Two aspects of centering affect the kinds of inferences a 
hearer must draw in interpreting a definite description. 
First, the shifting of center from one entity to another 
requires recognition of this change. Most often such 
changes are affected by the use of full definite noun 
phrases, but in some instances a pronoun may be used. 
For example, Grosz \[1977\] presents several examples of 
pronouns being used to refer to objects mentioned many 
utterances back. Second, the hearer must process 
(interpret) the particular linguistic expression that realizes 
the center. 
Most previous attempts to account for the interaction of 
different kinds of referring expressions with centering and 
focusing (or "topic') have conflated these two. For 
example, Joshi and Weinstein \[1981\] present a preliminary 
report on their research regarding the connection between 
the computational complexity of the inferences required 
to process a discourse and the coherence of that discourse 
as assessed by measures that invoke the centering 
phenomenon. However, several of the examples combine 
changes of expression and shifts in centering. 
Violations of the basic centering rule require the hearer 
to draw two different kinds of inferences. The kind 
required depends on whether a full definite noun phrase is 
used to express the center or whether a pronoun is used 
for a noncentered entity. We will consider each case 
separately. 
Several different functions may be served by the use of 
a full definite noun phrase to realize the currently 
centered entity. For instance, the full noun phrase may 
include some new and unshared information about the 
entity. In such cases, additional inferences arise from the 
need to determine that the center has not shifted and that 
the properties expressed hold for the centered entity. For 
example, in the following sequences 
(I0) I toole i 7 clog to the vet the other day. 
The mangy old beast... 
(11) I'm reading The French Lieutenant's 
Woman. The book, which In 
Fowles best .... 
the full definite noun phrases that are in boldface do 
more than merely refer. 
When the current center is not pronominalized (it may 
not be present in the sentence), the use of a pronoun to 
express an entity other than the current center, is 
strongly constrained. The particular cases that have been 
identified involve instances in which attention is being 
shifted back to a previously centered entity (e.g., Grosz, 
1977; Reichman, 1978) or to one element of a set that is 
currently centered. In such cases, additional inferences 
10Obviously, if Cb(S) is not expressed'in the next sentence then this issue does not arise. 
are required to determine that the pronoun does not refer 
to the current center, as well as to identify the context 
back to which attention is shifting. These shifts, though 
indicated by linguistic expressions typically used for 
centering (pronouns), correspond to a shift in global focus. 
8. Summary 
The main purpose of the paper was to sort out the 
confusion about the roles of syntactic, semantic, and 
pragmatic factors in the interpretation and generation of 
definite noun phrases in discourse. Specific mechanisms 
that account for the interactions among these factors were 
presented. Discourses were shown to be coherent at two 
different levels, i.e., with referring expressions used to 
identify entities that are centered locally and those 
focused upon more globally. The differences between 
references at the global and local levels were discussed, 
and the interaction of the syntactic role of a given noun 
phrase and its semantic interpretation with centering was 
described. 
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