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 ientX; r ef D cdsD &mX erents. \she" 2 A Framework for Reference ref X centr edX; r ef D cdsD &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 eref B speech timeB; 1;Ag 0 1 a man died in a park . i j k 9C : faspectsimpl e; A; C g j 2 he had b een sleeping there . i j k C ; ag ent; r ef D namedD; M ary 1 3awoman loved him . m i l & car dD; 1 / n 4 she had hated him . m i l & sl eepC 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 namedD; M ary&car dD; 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 cdsD is a reference to the current 2 discourse state and the prop erties m and f refer to male this case D namedD; M ary&car dD; 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 namedD; M ary&car dD; 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 e94 ing the prop erty centred are prevented from named94; M ar y b eing dereferenced to salient entities, thus en- w oman94 suring that the constraint of disjoint reference f 94 is met. If we fail to nd the centred en- car d94; 1 tity in the current discourse state, we search adul t94 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 namedD; M ary&car dD; 1 is similar but less constrained in that we con- 9X namedX; M ary &w omanX &car dX; 1 sider entities mentioned in al l no des mentioned 8X w omanX ! in the discourse tree, whether op en or closed, f emal eX &X is human&adul tX in order of recency. This means that, essen- 8X f emal eX ! 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.