Making Sense of Reference to the Unfamiliar
Helen Seville and Allan Ramsay
Centre for Computational Linguistics
UMIST, PO Box 88, Manchester M60 1QD, England
heleng/[email protected] c.uk
Abstract they must nevertheless denote individuals famil-
iar to conversants if they are successfully to re-
Computational approaches to reference resolu-
fer. However, there is another class of referring
tion, like Centering Theory, are b est at resolv-
expressions in relation to which we b elieve the
ing referring expressions which denote familiar
concept of uniqueness of meaning do es havean
referents. We demonstrate how, by taking a
essential role to play. These include such def-
pro of-theoretic approach to reference resolution
inite descriptions as \the rst man" and \the
within a Centering-typ e framework, we are able
rst snowdrop of Spring", along with suchvari-
to make sense of referring expressions for un-
ations on these as \the rst three men" and \the
familiar referents. These include, in addition
rst snowdrops of Spring".
to bridging descriptions, de nite descriptions
In implementing a system of reference resolu-
like \the rst man" and \the rst snowdrops of
tion, wehave attempted to reconcile the notions
Spring". We claim that the rst of these denotes
of familiarity and uniqueness. This enables us
a unique subset of a plural discourse antecedent.
to dereference expressions like \the rst snow-
While the second has no discourse antecedent,
drop of Spring" in a uni ed framework alongside
we similarly treat it as denoting a unique subset
1
anaphors , pronouns, referential tenses, names,
of a familiar referent.
and other de nite descriptions like \the man".
1 Intro duction
1 Two men arrived.
How do referring expressions denote? Accord-
2 The rst man sp oke.
ing to Russell, a de nite description such as
\the King of France", denotes a unique individ-
In the case of a referring expression like \the
ual by virtue of its meaning. But, according to
rst man", there maybeanantecedent of sorts
Familiarity Theory Heim, 1983 , referring ex-
in the discourse, but it is not the individual re-
pressions need not denote uniquely by virtue of
ferred to or indeed an individual at all . We
their meaning as they refer to individuals made
will say that the antecedent \two men" intro-
familiar by the discourse or other context. This
duces a set, and that the referring expression
observation plays a key role in Centering The-
\the rst man" denotes, by virtue of the mean-
ory Grosz and Sidner, 1986; Grosz et al., 1995
ing of rst, a unique subset of this familiar set.
and other computational approaches in which
referring expressions are resolved by lo cating
1 Mary saw the rst snowdrop of
their antecedents in the discourse. The refer-
Spring.
ence of pronouns like \he", de nite descriptions
like \the woman", and referential tenses like
In the case of \the rst snowdrop of Spring",
\had" clearly has more to do with salience in
there need b e no explicit antecedent in the dis-
context than with uniqueness of meaning. Sim-
course. We will say that, in the same way
ilarly, while names like \Mary" need not denote
that \Mary" denotes a familiar individual, \the
individuals prominent in the discourse context,
snowdrops of Spring" denotes a familiar set, or
1
Wewould like to thank the anonymous reviewers for We use this term to distinguish re exives like \her-
their detailed and helpful comments. self " from pronouns like \he" and \him".
Given that many referring expressions do not in property. Again, by virtue of the meaning of
themselves denote uniquely, however, we need rst, \the rst snowdrop of Spring" can b e said
a theory of reference resolution to enable us to denote a unique subset of the familiar set. We
to obtain the appropriate i.e., intended ref- will not claim that it denotes a unique individ-
erent for any referring expression. We incorp o- ual, but that rather it denotes a unique subset
rate our theory of reference resolution into the of the sp eci ed cardinality, i.e., 1. This treat-
actual representation of referring expressions; ment has the advantage that it extends to plural
for example, we lab el anaphors with the prop- referring expressions.
erty \salient" and pronouns and also referential Below we outline the approach we have de-
3
tenses with the prop erty \centred" : velop ed to the representation and resolution of
referring expressions, b efore discussing in more
\himself "
detail its extension to deal with unfamiliar ref-
ref X sal ient X; r ef D cds D &m X
erents.
\she"
2 A Framework for Reference
ref X centr ed X; r ef D cds D &f X
Resolution
Reference resolution relies on maintaining, as
Our framework for reference resolution has b een
in Centering Theory, a list of forward-lo oking
implemented in the system of language under-
centres for each discourse state corresp onding
standing describ ed in Ramsay, 1999 . The
to an utterance in the discourse. Furthermore,
starting p oint for reference resolution is the log-
for the purp oses of reference resolution, the dis-
ical form we obtain from parsing. For example,
course states themselves are organized into a
the following is the logical form we get for the
discourse tree, which is constructed automati-
utterance \Mary slept."
4
cally based on referential cues , as describ ed in
Seville, 1999 .
9A : fA is inter v al &
ends bef or e ref B speech time B; 1 ;A g
0 1 a man died in a park .
i j
k
9C : faspect simpl e; A; C g
j 2 he had b een sleeping there .
i j
k
C ; ag ent; r ef D named D; M ary
1 3 awoman loved him .
m i
l
& car d D; 1
/ n 4 she had hated him .
m i
l
& sl eep C
2 3 5 he had hated himself .
i m i
& C is ev ent
/ jn 6 he had loved her .
i m
l
45 6
We use the inference engine describ ed in
Ramsay and Seville, 2000 to up date the dis-
The no des in such a tree corresp ond to dis-
course mo del with a new discourse state con-
course states. Those on the right-hand frontier
taining the information explicitly represented in
are open, which essentially means that the enti-
the logical form together with any further infer-
ties mentioned in them are available to pronom-
ences which are licensed given the existing dis-
inal reference.
course mo del. Reference resolution, which in-
The pro cess of reference resolution for the
volves carrying out a pro of that a referring ex-
various referring expressions can be brie y de-
pression denotes, is implemented as part of the
scrib ed as follows. Anaphors, characterised as
up date step. We anchor a referring expression
salient, are resolved to a less oblique argument
like ref D named D; M ary &car d D; 1 in
of the same verb Pollard and Sag, 1994 within
the discourse mo del by proving the existence of
the current discourse state, which is constructed
an entity in the mo del which satis es the prop-
3
erties sp eci ed by the referring expression, in
Here ref D cds D is a reference to the current
2
discourse state and the prop erties m and f refer to male
this case D named D; M ary &car d D; 1 .
and female gender resp ectively.
2 4
Strictly sp eaking, it is a set which is denoted. For The tree illustrated was constructed using pronomi-
readability, our referring expressions con ate the prop- nal cues. Each discourse state was attached as a daugh-
erties of sets and their memb ers. In this case, the car- ter of the highest no de in the discourse tree to which all
dinality is a prop erty of the set denoted, but the name pronouns and referential tenses like had mentioned in
Mary is a prop erty of its memb er. it could b e anchored.
ref D named D; M ary &car d D; 1 as incrementally. We also start our search for the
the following were amongst the facts contained referents of pronouns and other centred enti-
in Discourse State 0: ties in the current discourse state, which is nec-
essary if we are to resolve such referring ex-
Discourse State 0
pressions as \her" in \Mary to ok John with
|||||{
her." However, referring expressions contain-
f emal e 94
ing the prop erty centred are prevented from
named 94; M ar y
b eing dereferenced to salient entities, thus en-
w oman 94
suring that the constraint of disjoint reference
f 94
is met. If we fail to nd the centred en-
car d 94; 1
tity in the current discourse state, we search
adul t 94
the previous op en no de and, if necessary, fur-
ther op en no des in the discourse tree, in order
These were generated from the lexical meaning
to deal with long-distance pronominalisation.
p ostulates we stipulated for \Mary", \woman",
The dereferencing of other referring expressions
and \female":
like ref D named D; M ary &car d D; 1 is
similar but less constrained in that we con-
9X named X; M ary &w oman X &car d X; 1
sider entities mentioned in al l no des mentioned
8X w oman X !
in the discourse tree, whether op en or closed,
f emal e X & X is human &adul t X
in order of recency. This means that, essen-
8X f emal e X ! f X
tially, names and de nite descriptions are deref-
3 Unfamiliar Referents
erenced to the most recently mentioned refer-
ent whichis appropriate. Unlike in the case of
In this section we showhow, within the frame-
pronouns, we also consider Discourse State 0,
work ab ove, we are able to make sense of a vari-
which do esn't corresp ond to an utterance but,
ety of referring expressions denoting unfamiliar
rather, contains the background knowledge as-
referents. The most straightforward of these are
sumed in the mo del. This is how we are able
bridging descriptions, so we start with these.
to deal with the rst mention of a familiar
3.1 Bridging Descriptions
referent like Mary assuming that the prop er-
1 Mary loves her mother.
ties D named D; M ary &car d D; 1 suce
to distinguish a particular entity in Discourse
In this rst case, \her mother", contains a refer-
State 0 from all the others .
ring expression nested within it. Having deref-
Our approach extends naturally to cases like
erenced this, the knowledge that mother of is a
\the rst snowdrop of Spring" b ecause it is
function enables us to obtain a unique referent.
pro of-theoretic and so able to exploit back-
Our representation of the referring expression
ground knowledge in reference resolution. This
to b e dereferenced is as follows:
can b e illustrated, in the rst instance, by exam-
ining the background knowledge which is used
\her mother"
in up dating the utterance \Mary slept." The
ref B of B;
up date step for this utterance yields Discourse
F mother F ;
State 1, containing amongst others the follow-
ref G sal ient or centr ed G; 1
ing facts:
& f G
& car d B; 1
Discourse State 1
|||||{
The rst step involves anchoring the referring
sl eep 134
expression by dereferencing its nested referring
134; ag ent; 94
5
expression for \her" .
bef or e 4 1 ; 133 ends
aspect simpl e; 133; 134
5
The referent for this is characterised as
sal ient or centr ed as we allow p ossessive pronouns
We were able to prove named 94; M ar y
to be dereferenced as anaphors or, failing that, as
and car d 94; 1 and so dereference pronouns.
8X house X _ car X ! Current Mo del
9Y of Y; Z door Z ;X &car d Y; 1 |||||{
at or af ter 4 1 ; 135 ends
This means that, having used utterance 1
aspect simpl e; 135; 136
ab ove to up date the discourse mo del, we have
136; ag ent; 94
the following amongst the facts in Discourse
love 136
State 1:
The partially constructed current discourse
Discourse State 1
state wehave when we do our dereferencing is as
|||||{
shown. \Mary" has already b een dereferenced
see1 138
to 94 and this has b een entered into the list
138; ag ent; 94
of forward-lo oking centres for the current utter-
138; obj ect; 139
ance. We are able to prove b oth sal ient 94
car d 139; 1
and f 94 , and so our nested referring expres-
house 139
sion is dereferenced to this entity.
bef or e 4 1 ; 137 ends
door 46 139
ref B of B;
entr ance 46 139
F mother F ;
of 46 139 ;A door A ; 139
94
car d 46 139 ; 1
& car d B; 1
aspect simpl e; 137; 138
It is then a straightforward matter to derefer-
In up dating utterance 2 , the bridging descrip-
ence the anchored referring expression, given
tion which needs to b e dereferenced has the fol-
the following facts in Discourse State 0:
lowing representation:
Discourse State 0
ref E door E & car d E; 1
|||||{
Since we cannot guarantee that there will only
mother 60 94
be a single entity in our mo del satisfying the
of 60 94 ;A mother A ; 94
prop erties E door E & car d E; 1 , wewant
f 60 94
to ensure that the referent we obtain is either
car d 60 94 ; 1
the most recently mentioned or that with the
most recently mentioned antecedent, i.e., in this
These derive from our meaning p ostulates for
6
case, the house 139. Our reference resolu-
\mother" and \of ":
tion pro cedure exploits the fact that the house,
8X X is animal &car d X; 1 !
139, is explicitly represented in the forward
9Y of Y; Z mother Z ;X
lo oking centres of Discourse State 1 and that
&car d Y; 1 &f Y
the intended referent, 46 139 , is clearly a
8X 8Y 8Z of X; Y ; Z ! Y:X
function of this its dep endency having b een
preserved bySkolemization . In considering the
Dealing with other bridging descriptions is
p otential referents for our referring expression in
more complicated:
order of recency,we attempt to prove, not sim-
1 Mary saw a house.
ply, for each referent, X , whether door X and
2 She found the door.
car d X; 1 , but door Y and car d Y; 1 where
Y is a function of X . Since 46 139 is
In order to give an analogous treatment to the
a function of the antecedent 139, we obtain
referring expression \the do or", wehave to treat
the appropriate referent in this case by proving
it as elliptical for an expression containing a
door 46 139 and car d 46 139 ; 1 .
nested referring expression, i.e., \the do or of the
house ". In the same way that wehave a mean-
3.2 Sup erlatives
ing p ostulate for the relation mother of,wehave
We are now in a p osition to describ e our treat-
one for the relation door of :
ment of the sup erlatives discussed in the intro-
6
duction. First, we consider a case in which there
Skolemization preserves the dep endency of Y on X,
is a discourse antecedent of sorts: i.e., 94 is present in 60 94 .
In this way, we prove that the referring ex- 1 Two men arrived.
pression makes sense, i.e., denotes. However, 2 The rst man sp oke.
unlike in the previous cases, we do not deref-
Discourse State 1 contains the following facts:
erence to a familiar referent. There are no
existing facts in the database ab out the ref-
Discourse State 1
erent 81 A ear l y A;::: ; 1; 2; 108; 108 .
|||||{
Instead, in this case, we have to add to Dis-
arrive 107
course State 2 the facts wehave proved.
107; ag ent; 108
car d 108; 2
Discourse state 2
man 108
|||||{
mal e 108
speak 112
m 108
theta 112; ag ent;
adul t 108
81 A ear l y A;::: ; 1; 2; 108; 108
bef or e 4 1 ; 106 ends
bef or e 4 2 ; 111 ends
aspect simpl e; 106; 107
speech time 4 2 ; 2
aspect simpl e; 111; 112
Our representation of the referring expression
most 81 A ear l y A;::: ; 1; 2; 108; 108 ;
\the rst man" is as follows:
C ear l y C; D man D ;
ref B most B;
108
C ear l y C; D man D ;
car d 81 A ear l y A;::: ; 1; 2; 108; 108 ;
ref E man E
1
& car d B; 1
ear l y 81 A ear l y A;::: ; 1; 2; 108; 108 ;
The nested referring expression
C man C
ref E man E can be straightforwardly
man 81 A ear l y A;::: ; 1; 2; 108; 108
dereferenced in this case to give the anchored
mal e 81 A ear l y A;::: ; 1; 2; 108; 108
referring expression:
m 81 A ear l y A;::: ; 1; 2; 108; 108
adul t 81 A ear l y A;::: ; 1; 2; 108; 108
ref B most B;
C ear l y C; D man D ;
The further facts we prove, ab out our referent
108
b eing early, male, etc., are required if we are to
& car d B; 1
b e able to subsequently refer to it using referring
Dereferencing this then involves our meaning
expressions such as \he". These are generated
p ostulate for sup erlatives:
from a set of asso ciated meaning p ostulates:
8X 8Z 8C car d Z; C & Z X & :C = 1 &
8X 8Y 8P or der ed P &most Y; P; X ! P:Y
8N 8P :most X; P ; !
8A or der ed B ear l y B; A
9Y most Y; P; X &car d Y; N
8X 8P ear l y X; P ! P:X
8X man X !
This simply says that for any severalton set X ,
X is human &mal e X &adul t X
any prop erty P and any N , there is some set Y
8X mal e X ! m X
containing the N \most P " memb ers of X. This
meaning p ostulate do es not translate into any
In addition to these, wehavetwo further mean-
facts in Discourse State 0, but remains as a rule.
ing p ostulates for sup erlatives:
When wehave a particular referring expression
to dereference, this rule enables us to prove that:
8X 8Y 8P 8C 8Z most Y; P; X &car d Y; C
&most Z; P ; X &car d Z; C most 81 A ear l y A;::: ; 1; 2; 108; 108 ;
! Z = Y C ear l y C; D man D ;
8X 8P 8Y 8N 8C most X; P ; Y & 108
car d X; N &car d Y; C car d 81 A ear l y A;::: ; 1; 2; 108; 108 ;
!:mor e N; C 1
anchor our referring expression by dereferenc- The rst of these, the uniqueness meaning p os-
ing the referring expression nested within it, we tulate, states that if there are two subsets of of
need to intro duce a meaning p ostulate for the a set which share the same cardinality and the
nested referent and one for its nested referent, same sup erlative prop erty, such as rst, then
7
Spring : they must be regarded as identical . The sec-
ond simply ensures that any unfamiliar refer-
9X season X &named X ; S pr ing &car d X; 1
ent which we obtain via our meaning p ostu-
9X of X;
lates can sensibly regarded as a prop er subset
Y snow dr op Y ;
of its antecedent; that is, it prevents us regard-
ref Z named Z ; S pr ing
ing \two men" as a p otential antecedent of \the
&car d X; pl
rst men":
These meaning p ostulates simply intro duce into
1 Two men arrived.
i
Discourse State 0 the fact that there are snow-
2 The rst men sp oke.
f i
drops of Spring, in the same way that the mean-
ing p ostulate for \Mary" intro duced the fact
Our treatment of sup erlatives without dis-
that there is a singleton set containing an in-
course antecedents is similar to that ab ove.
dividual so named.
1 Mary saw the rst snowdrops of
Discourse state 0
Spring.
|||||{
season 98
There is just one ma jor di erence.
named 98; S pr ing
ref E most E;
car d 98; 1
F ear l y F;
extended 98
G of G;
H snow dr op H ;
snow dr op 101
ref I named I;
of 101; A; snow dr op A ; 98
S pr ing
n 101
&car d I; 1 ;
car d 101;pl
ref J of J;
Given the ab ove facts in Discourse State 0, an-
K snow dr op K ;
choring our referring expression is straightfor-
ref L named L; S pr ing
ward.
&car d L; 1
&car d E; pl
ref E most E;
F ear l y F;
The representation we obtain for the referring
G of G;
expression \the rst snowdrops of Spring" is
H snow dr op H ;
shown ab ove. Like that for \the rst man", this
98 ;
contains a nested referring expression:
101
&car d E; pl
ref J of J;
K snow dr op K ;
From this p oint onwards, the pro of that this
ref L named L; S pr ing
referring expression denotes pro ceeeds in the
&car d L; 1
same way as in the previous example. Given
the meaning p ostulates for sup erlatives, we are
The di erence is that, in this case, there is
able to prove:
no discourse antecedent for the nested refer-
most 81 A ear l y A;::: ; pl ; pl ; 101; 101 ;
ring expression. This means that, in order to
D ear l y D;
7
Practically, this meaning p ostulate seems to b e re-
E of E; F snow dr op F ; 98 ;
dundant. Our meaning p ostulates generate for us only
101
one such subset and it is imp ossible for another to be
car d 81 A ear l y A;::: ; pl ; pl ; 101; 101 ;
intro duced through the discourse as \a rst man" is un-
pl grammatical.
ries of reference resolution, likeFamiliarity The- Again, as in the example ab ove, the facts we
ory, and semantic theories, like Russell's, may have proved concern an unfamiliar referent, and
be reconciled. However, it is fair to say that so have to be added to the current discourse
the success of the approach is not yet proven. state.
This is b ecause we have yet to show that we
Discourse state 1
can deal with a set of related referring expres-
|||||{
sions within a single framework. The following
see1 107
example illustrates the kinds of cases we have
theta 107; ag ent; 94
in mind:
theta 107;
1 Three men ate.
obj ect;
i
2 Two men slept.
81 A;::: ; 98 ; pl ; pl ; 101; 101
j
3 The rst men died.
bef or e 4 1 ; 106 ends
i
aspect simpl e; 106; 107
Here, \ rst" in \the rst men" is clearly p er-
most 81 A ::: ; 98 ; pl ; pl ; 101; 101 ;
forming a di erent, discourse-related function
D ear l y D;
from that it plays in the cases we have b een
E of E; F snow dr op F ; 98 ;
considering. Wehaveyet to tackle such dicult
101
cases but, since they seem to require reasoning
car d 81 A ::: ; 98 ; pl ; pl ; 101; 101 ;
ab out sets, we b elieve that our inference-based
pl
approach to reference resolution is a go o d place
ear l y 81 A ::: ; 98 ; pl ; pl ; 101; 101 ;
to start.
D of D; E snow dr op E ;
98
References
of 81 A ::: ; 98 ; pl ; pl ; 101; 101 ;
B. J. Grosz and C. L. Sidner. 1986. Attention,
D snow dr op D ;
intentions, and the structure of discourse.
98
Computational Linguistics, 12 3 :175{204.
snow dr op 81 A ::: ; 98 ;:::
B. J. Grosz, A. K. Joshi, and S. Weinstein.
n 81 A ::: ; 98 ; pl ; pl ; 101; 101
1995. Centering: A framework for mo deling
the lo cal coherence of discourse. Computa-
4 Conclusion
tional Linguistics, 21 2 :203{225.
Wehave shown how, by taking a pro of-theoretic
I. Heim. 1983. File change semantics and
approach to reference resolution, we can extend
the familiarity theory of de niteness. In
a Centering-typ e framework to make sense of
R. Bauerle, C. Schwarze, and A. von Stechow,
referring expressions for a variety of unfamiliar
editors, Meaning, Use, and Interpretation of
referents. Having made sense of such referring
Language, pages 164{189. de Gruyter, Berlin.
expressions, we add their referents to our dis-
C. Pollard and I. A. Sag. 1994. Head-Driven
course mo del. This is how we would normally
Phrase Structure Grammar. University of
deal with inde nites rather than de nites. How-
Chicago Press, London.
ever, this approach makes p erfect sense, given
A. Ramsay and Helen Seville. 2000. Mo dels and
our treatment of such referring expressions as
discourse mo dels. Journal of Language and
denoting unfamiliar subsets of familiar referents
Computation, 1 2 :159{174. forthcoming.
regarded as sets . We claim that we are able
A. Ramsay. 1999. Do es it makeany sense? up-
to use de nite descriptions to refer to the ref-
dating = consistency checking. In K. Turner,
erents in question, despite their unfamiliarity,
editor, The Semantics/Pragmatics Interface
so long as we can prove that, by virtue of their
from Di erent Points of View. Elsevier Sci-
meaning, they denote uniquely.
ence B.V.
Having implemented our approach in a sys-
H. Seville. 1999. Exp eriments with discourse
tem of language understanding which already
structure. In Third International Workshop
deals with a wide variety of referring expres-
on Computational Semantics, pages 233{246,
sions, we have demonstrated its practicality.
Tilburg.
It also has interesting theoretical implications,
since it suggests a way in which pragmatic theo-