Linguistic Coherence: A Plan-Based Alternative

LINGUISTIC COHERENCE: A PLAN-BASED ALTERNATIVE Diane J. Litman AT&T Bell Laboratories 3C-408A 600 Mountain Avenue Murray Hill, NJ 079741 ABSTRACT To fully understand a sequence of utterances, one must be able to infer implicit relationships between the utterances. Although the identification of sets of utterance relationships forms the basis for many theories of discourse, the formalization and recogni- tion of such relationships has proven to be an extremely difficult computational task. This paper presents a plan-based approach to the representation and recognition of implicit relation- ships between utterances. Relationships are formu- lated as discourse plans, which allows their representa- tion in terms of planning operators and their computa- tion via a plan recognition process. By incorporating complex inferential processes relating utterances into a plan-based framework, a formalization and computa- bility not available in the earlier works is provided. INTRODUCTION In order to interpret a sequence of utterances fully, one must know how the utterances cohere; that is, one must be able to infer implicit relationships as well as non-relationships between the utterances. Con- sider the following fragment, taken from a terminal transcript between a user and a computer operator (Mann [12]): Could you mount a magtape for me? It's tape 1. Such a fragment appears coherent because it is easy to infer how the second utterance is related to the first. Contrast this with the following fragment: Could you mount a magtape for me? It's snowing like crazy. This sequence appears much less coherent since now there is no obvious connection between the two utter- ances. While one could postulate some connection (e.g., the speaker's magtape contains a database of places to go skiing), more likely one would say that there is no relationship between the utterances. Furth- IThis work was done at the Department of Computer Sci- ence. University of Rochester. Rochester NY 14627. and support- ed in part by DARPA under Grant N00014-82-K-0193. NSF under Grant DCR8351665. and ONR under Grant N0014-80-C-0197. ermore, because the second utterance violates an expectation of discourse coherence (Reichman [16]. Hobbs [8], Grosz, Joshi, and Weinstein [6]), the utter- ance seems inappropriate since there are no linguistic clues (for example, prefacing the utterance with "incidentally") marking it as a topic change. The identification and specification of sets of linguistic relationships between utterances 2 forms the basis for many computational models of discourse (Reichman [17], McKeown [14], Mann [13], Hobbs [8], Cohen [3]). By limiting the relationships allowed in a system and the ways in which relationships coherently interact, efficient mechanisms for understanding and generating well organized discourse can be developed. Furthermore, the approach provides a framework for explaining the use of surface linguistic phenomena such as clue words, words like "incidentally" that often correspond to particular relationships between utter- ances. Unfortunately. while these theories propose relationships that seem intuitive (e.g. "elaboration," as might be used in the first fragment above), there has been little agreement on what the set of possible rela- tionships should be, or even if such a set can be …

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