A Practical Nonmonotonic Theory for Reasoning about Speech Acts

A Practical Nonmonotonic Theory for Reasoning about Speech Acts Douglas Appelt, Kurt Konolige Artificial Intelligence Center and Center for the Study of Language and Information SRI International Menlo Park, California Abstract A prerequisite to a theory of the way agents un- derstand speech acts is a theory of how their be- liefs and intentions are revised as a consequence of events. This process of attitude revision is an interesting domain for the application of non- monotonic reasoning because speech acts have a conventional aspect that is readily represented by defaults, but that interacts with an agent's be- liefs and intentions in many complex ways that may override the defaults. Perrault has devel- oped a theory of speech acts, based on Rieter's default logic, that captures the conventional as- pect; it does not, however, adequately account for certain easily observed facts about attitude revi- sion resulting from speech acts. A natural the- ory of attitude revision seems to require a method of stating preferences among competing defaults. We present here a speech act theory, formalized in hierarchic autoepistemic logic (a refinement of Moore's autoepistemic logic), in which revision of both the speaker's and hearer's attitudes can be adequately described. As a collateral benefit, effi- cient automatic reasoning methods for the formal- ism exist. The theory has been implemented and is now being employed by an utterance-planning system. 1 Introduction The general idea of utterance planning has been at the focus of much NL processing research for the last ten years. The central thesis of this 170 approach is that utterances are actions that are planned to satisfy particular speaker goals. This has led researchers to formalize speech acts in a way that would permit them to be used as op- erators in a planning system [1,2]. The central problem in formalizing speech acts is to correctly capture the pertinent facts about the revision of the speaker's and hearer's attitudes that ensues as a consequence of the act. This turns out to be quite difficult bemuse the results of the attitude revision are highly conditional upon the context of the utterance. To consider just a small number of the contin- gencies that may arise, consider a speaker S utter- ing a declarative sentence with propositional con- tent P to hearer H. One is inclined to say that, if H believes S is sincere, H will believe P. How- ever, if H believes -~P initially, he may not be convinced, even if he thinks S is sincere. On the other hand, he may change his beliefs, or he may suspend belief as to whether P is true. H may not believe --P, but simply believe that S is neiter competent nor sincere, and so may not come to believe P. The problem one is then faced with is this: How does one describe the effect of ut- tering the declarative sentence so that given the appropriate contextual elements, any one of these possibilities can follow from the description? One possible approach to this …

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