An Integrated Approach To Reference And Presupposition Resolution

We describe an approach to resolving definite descriptious and pronominal anaphora as subcases of a general strategy for presupposition satisfaction. Generally, a presupposition is satisfied in a context if the context contains a specific type of information and is organized in such a way that this information can be retrieved by the interlocutors. The model of discourse context we develop assumes that discourse structure is organized around a stack of questions under discussion, which plays a crucial role in narrowing the search for referents and other presupposed information. The algeri thmA for maintaining the discourse structures and retrieving presupposed information are presented and illustrated by several example dialogues in which human users interact with an agent tomake hotel reservatious. I I n t r o d u c t i o n "Any theory of referring expressions must take into account the discourse context in which they ot~ur. Indeed, previous research has shown that the hierarchical organization of discourse is fundamentally related to the rderence resolution process. In this paper, we show how a highly structured discourse model, in conjunction with a treatment of referring expressions as presuppmitional, enables us to develop a common strategy for z~olving a number of reference resolution problems, such as pronominal anaphora and definite descriptions. We also outline how this approach extends to a larger group of phenomena which we take to be presuppositional,. including domain restriction, ellipsis, and lexically and syntactically triggered presuppositions. All of these constructions are presuppositional in a broad sense, in that their use assumes that certain informarion can be retrieved from the discourse context. RecoL, nlzing the structure of the discourse will therefore play a crucial role in narrowing the search for referents and other presupposed information. We will illustrate our approach with four example human-computer dialogues, shown below. SYS indicates the utterances spoken by the computer system. Example I illustrates a case of pronominal anaphora resolution (it in (8)), in which recowniT.ing the hierarchical structure of the discourse is crucial for identifying the antecedent, which was introduced many utterances earlier. The overall topic of the conversation is the question of where the user can find a hotel for June 15th in New York, and th|8 snper -ques t ion both facilitates and constrains the intexpretafion of fl in (8). This example is similar to the well-known examples of long-distance anaphora in task-oriented dialogues described by Grosz (1981). Our approach is consistent with previous research that uses the intentional structure of discourse to determine a set of potential antecedents for pronomina l anaphora. The following examples will illustrate how a broader range of reference and presuppositional constructions may also be addressed by using the discourse structure to guide the search for relevant information. Example L 1) USER: rm lookln~ for a hotel for June lSth in New York. 2) SYS: What part of the city would you prefer? 3) USEI~ Manhattan, near Central Park. 4) SYS: How many nights? 5) USF~ Just 1. 6) SYS: Will anyone be traveling with you? 7) USEI~ No. 8) USEP~ Oh, I want it to have a swimming pool too. " Paul C. Davis is the recipient et ~ a Motorola Partnemhipe in Research Grant. Example II shows a definite description, the hotel in (7), whose referent can only be uniquely determined with respect to the indefinite hotel description ( a hotel close to Madison Square Garden) in the question under discussion (1):