Planning Natural Language Referring Expressions

PLANNING NATURAL LANGUAGE REFERRING EXPRESSIONS Douglas E. Appelt SRI International Menlo Park, California ABSTRACT This paper describes how a language-planning system can produce natural-language referring expressions that satisfy multiple goals. It describes a formal representation for reasoning about several agents' mutual knowledge us- ing possible-worlds semantics and the general organization of a system that uses the formalism to reason about plans combining physical and linguistic actions at different levels of abstraction. It discusses the planning of concept ac- tivation actions that are realized by definite referring ex- pressions in the planned utterances, and shows how it is possible to integrate physical actions for communicating intentions with linguistic actions, resulting in plans that include pointing as one of the communicative actions avail- able to the speaker. I. INTRODUCTION One of the mo~t important constituent processes of natural-language generation is the production of referring expressions, which occur in almost every utterance. Refer- ring expressions often carry the burden of informing the hearer of propositions as well as referring to objects. There- fore, many phenomena that are observed in dialogues can- et.¥_w~eet../- J "-°'~ ""' "-~ Figure 1 Satisfying Multiple Goals with a Referring Expression The author gratefully acknowledges the support for this research provided in part by the Office of Naval Research under contract N0014-80-C-0296 and in part by the National Science Foundation under grant MCS-8115105. not be explained by the simple view that referring expres- sions are descriptions of the intended referent sufficient to distinguish the referent from other objects in the domain or in focus. Consider the situation (depicted in Figure 1) in which two agents, an apprentice and an expert, are cooperating on a common task, such as disassembling an air compres- sor. Several tools are lying on the workbench, and al- though the apprentice knows that the objects are there, he may not necessarily know where they are. The expert might say: Use the wheelpuller to remove the flywheel. (1) while pointing at the wheelpuller. The apprentice may think to himself at this point, "Ah, ha, so that's a wheel- puller," and then proceed to remove the flywheel. What the expert is accomplishing through the utterance of (1) by using the noun phrase "the wheelpuller" cannot be fully explained by treating definite referring expressions simply as descriptions that are uniquely true of some ob- ject, even taking focusing [71[11] into account. The expert uses "the wheelpuller" to refer to an object that in fact uniquely fits the description predicated of it, so this simple analysis is incapable of accounting for the effects the expert intends his utterance to have. If one takes the knowledge and intentions of the speaker and hearer into account, a more accurate account of the speaker's use of the referring expression can be developed. The apprentice does not know what the object is that fits the description "the wheelpuller". The expert knows that the apprentice doesn't know this, and performs the pointing action to guarantee that his intentions will be recognized correctly. The apprentice …

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