, O @ O O An Integrated Approach to Reference and Presupposition O Resolution O Robert T. Kasper, Paul C. Davis~ and Craige Roberts O Department of Linguistics @ Ohio State University 222 Oxley Hall O 1712 Neil Avenue O Columbus, OH 43210" O e-mail: (kasper~ pcdavis, croberts}@ling.ohio-state.edu O RecoL,nlzing the structure of the discourse will there- O Abstract fore play a crucial role in narrowing the search O We describe an approach to resolving definite de- for referents and other presupposed information. scriptious and pronominal anaphora as subcases of We will illustrate our approach with four example O a general strategy for presupposition satisfaction. human-computer dialogues, shown below. SYS in- O Generally, a presupposition is satisfied in a context dicates the utterances spoken by the computer sys- if the context contains a specific type of information tem. @ and is organized in such a way that this information Example I illustrates a case of pronominal can be retrieved by the interlocutors. The model anaphora resolution (it in (8)), in which recowniT.ing O of discourse context we develop assumes that dis- the hierarchical structure of the discourse is crucial O course structure is organized around a stack of ques- for identifying the antecedent, which was introduced tions under discussion, which plays a crucial role in many utterances earlier. The overall topic of the O narrowing the search for referents and other presup- conversation is the question of where the user @ posed information. The algerithmA for maintaining can find a hotel for June 15th in New York, and the discourse structures and retrieving presupposed th|8 snper-question both facilitates and constrains O information are presented and illustrated by several the intexpretafion of fl in (8). This example is example dialogues in which human users interact similar to the well-known examples of long-distance O with an agent tomake hotel reservatious. anaphora in task-oriented dialogues described by O Grosz (1981). Our approach is consistent with pre- I Introduction vious research that uses the intentional structure of O "Any theory of referring expressions must take into discourse to determine a set of potential antecedents O account the discourse context in which they ot~ur. for pronominal anaphora. The following examples Indeed, previous research has shown that the hier- will illustrate how a broader range of reference O archical organization of discourse is fundamentally and presuppositional constructions may also be O related to the rderence resolution process. In this addressed by using the discourse structure to guide paper, we show how a highly structured discourse the search for relevant information. @ model, in conjunction with a treatment of referring expressions as presuppmitional, enables us to de- Example L O velop a common strategy for z~olving a number of 1) USER: rm lookln~ for a hotel for June lSth in O reference resolution problems, such as pronominal New York. anaphora and definite descriptions. We also out- 2) SYS: What part of the city would you prefer? O line how this approach extends to a larger group of 3) USEI~ Manhattan, near Central Park. O phenomena which we take to be presuppositional,. 4) SYS: How many nights? including domain restriction, ellipsis, and lexically 5) USF~ Just 1. O and syntactically triggered presuppositions. All of 6) SYS: Will anyone be traveling with you? these constructions are presuppositional in a broad 7) USEI~ No. O sense, in that their use assumes that certain infor- 8) USEP~ Oh, I want it to have a swimming pool O marion can be retrieved from the discourse context. too. O O " Paul C. Davis is the recipient et~ a Motorola Partnemhipe O in Research Grant. Example II shows a definite description, the structure Of discourse and the related background hotel in (7), whose referent can only be uniquely literature. In section 3, we present algorithms which determined with respect to the indefinite hotel de- we have developed in a partially completed imple- scription ( a hotel close to Madison Square Garden) mentation of a natural language dialogue system in the question under discussion (1): where users interact with an automated hotel reser- vation booking system. In section 4, we discuss the Example II. use of the algorithms and discourse structures to 1) USER: I want to make a reservation at a hotel resolve the reference and presupposition problems close to Madison Square Garden. shown in the above examples. In the final section, @ 2) SYS: What dates will the reservation be for? we highlight the contributions of our approach and 3) USER: March 3rd and 4th. discuss future plans related to this research. 0 4) SYS: Wouldyou like a single room? 0 5) USER: Yes. 2 Background: Discourse Structure 6) USER: Also, I'll need a conference room on We assume the general theoretical framework of 0 the 4th. Roberts (1996), where discourse is formally charac- O 7) USER: I'd prefer it if the hotel had one. terized as a game of intentional inquiry. As in Grosz & Sidner (1986), discourse is organized by the in- 0 Example HI involves a contextually determined terlocutors' goals and intentions and the plans, or O domain restriction, with a quantificationai deter- strategies, which conversational participants develop miner every, innstrating that domain restriction to achieve them. Following Stalnaker (1979), the pri- 0 must be handled in a ~imilar way for a broader mary goal of the language game is communal inquiry, class of expressions than those which are normally i.e., interlocutors attempting to share information 0 regarded as rderring expr_~-_qjons or presupposition about their world, with the repository of that shared 0 triggers. information characterized as the interlocutors' com- Example HI." mon groun~ CG. The set of acceptable moves in the 0 I) USEI~ Does the Holiday Inn have any vacan- game are defined by the (conventional and conver- cies for sational) rules of the game, and are classified on the 0 a) Tuesday, 12/4- Friday 12/7? basis of their relatio_n~blp to the goals. Ignoring im- @ b) Thursday, 12/6 - Saturday 12/87 peratives, there are two main types of moves (see 2) SYS: Yes, several. also Carlson 1983): questions and assertions. If a 0 3) USER: Do they have a breakfast buffet question is accepted by the interlocutors, this com- @ every mor~;=g? mits them to a common dis(x)ur~ goal, l~lding a sat- 4) SYS: isfactory (asserted) answer; like the commitment to 0 a) Yes, Monday through Friday. a goal in Planning Theory, thiA strong commitment @ b) No. There's a breakfast buffet Monday persists until the goal is satJ'~ed or else shown to be through unsatisfi~le. The accepted question becomes the 0 Friday, but none on Saturday. immediate topic of discussion, the quest/on under discussion. An assertion is a move which proposes 0 Finally, in example IV we give a glimpse into an addition of information to the CG. 0 our larger research program, where an elliptical Roberts defines the structure of a discourse at @ question (3) must be resolved with respect to the a given point, its Information Structure, as a tu- question under discussion, in addition to estab- pie which includes (among other things) the ordered 0 lishing the reference of the deflm'te description the set of moves in the discourse (M), CG, and the set Marrio~ where the context might contain more of the questions currently under discussion at that 0 than one hotel with that name~ point (QUD). The QUD is ordered by order of ute • terance and is updated in a stack-!i!~ f~b!on, I with 0 E~-mple IV. questions popped when they are answered (or de- 0 I) USER: Which hotels near the airport have f.-mined to be practically unanswerable). The or- vacancies? dered set of questions under disc~-_~f~on corresponds O 2) SYS: The Holiday Inn and Sheraton have to the hierarchical intentional structure of the dis- 0 vacancies. course. The QUD in this structure constitutes the 3) USER: How about the Marriott7 set of d/sco,rse goa/s of the interlocutors; the dis. 0 4) SYS: No, the airport Marriott doesn't have any course goals are only a subset of the set of common vacancies. .goals of the interlocutors, their domain goals, and 0 1However, all elements of the QUD list are accessibledur. 0 The remainder of the paper is organized as follows. ing the interpretation of an utterance. Only the top element In section 2 we discuss our assumptions about the is wr/table, but any entry is readable. 0 0 2 0 0 0 the discourse goals ar e subordinate to, and subserve (2) Weak Familiarity: A discourse referent i is the domain goals. Hence, the requirement that in- weakly familiar in a context C (i E Domain(C) terlocutors stick to the question under discussion is and C encodes the information that i has prop- just an instance of the more general commitment erties Pi .... , Pk) iff the Common Ground of C to plans; and in turn, in a fully integrated theory entails the existence of an entity with properties we would expect that domain goals and plans would P~, . , Pk. influence interpretation as directly as the discourse goals represented by the questions under discussion. Informational uniqueness only requires that the Any move in a discourse game is interpreted with discourse referent which satisfies the defmite's famil- respect to the Information Structure of the discourse iarity presupposition be unique among the discourse at that point. There are two main aspects to the referents in the context in satisfying the definite's interpretation of any given move: its presupposed descriptive content.