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<Paper uid="P88-1021">
  <Title>A Practical Nonmonotonic Theory for Reasoning about Speech Acts</Title>
  <Section position="4" start_page="0" end_page="171" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> 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  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 operators 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.</Paragraph>
    <Paragraph position="1"> To consider just a small number of the contingencies that may arise, consider a speaker S uttering a declarative sentence with propositional content P to hearer H. One is inclined to say that, if H believes S is sincere, H will believe P. However, 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 uttering 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 problem would be to find some fundamental, context-independent effect of informing that is true every time a declarative sentence is uttered. If one's general theory of the world and of rational behavior were sufficiently strong and detailed, any of the consequences of attitude revision would be derivable from the basic effect in combination with the elaborate theory of rationality. The initial efforts made along this path \[3,5\] entailed the axiomatization the effects of speech acts as producing in the hearer the belief that the speaker wants him to recognize the latter's intention to hold some other belief. The effects were characterized by nestings of Goal and Bel operators, as in Bel(H, Goal(S, Bel(H, P))).</Paragraph>
    <Paragraph position="2"> If the right conditions for attitude revision obtained, the conclusion BeI(H,P) would follow from the above assumption.</Paragraph>
    <Paragraph position="3"> This general approach proved inadequate because there is in fact no such statement about b.eliefs about goals about beliefs that is true in every performance of a speech act. It is possible to construct a counterexample contradicting any such effect that might be postulated. In addition, long and complicated chains of reasoning are required to derive the simplest, most basic consequences of an utterance in situations in which all of the &amp;quot;norreal&amp;quot; conditions obtain -- a consequence that runs counter to one's intuitive expectations.</Paragraph>
    <Paragraph position="4"> Cohen and Levesque \[4\] developed a speech act theory in a monotonic modal logic that incorporates context-dependent preconditions in the axioms that state the effects of a speech act. Their approach overcomes the theoretical difficulties of earlier context-independent attempts; however, if one desires to apply their theory in a practical computational system for reasoning about speech acts, one is faced with serious difficulties. Some of the context-dependent conditions that determine the effects of a speech act, according to their theory, involve statements about what an agent does no~ believe, as well as what he does believe.</Paragraph>
    <Paragraph position="5"> This means that for conclusions about the effect of speech acts to follow from the theory, it must include an explicit representation of an agent's ignorance as well as of his knowledge, which in practice is difficult or even impossible to achieve.</Paragraph>
    <Paragraph position="6"> A further complication arises from the type of reasoning necessary for adequate characterization of the attitude revision process. A theory based on monotonic reasoning can only distinguish between belief and lack thereof, whereas one based on non-monotonic reasoning can distinguish between be- null lief (or its absence) as a consequence of known facts, and belief that follows as a default because more specific information is absent. To the extent that such a distinction plays a role in the attitude revision process, it argues for a formalization with a nonmonotonic character.</Paragraph>
    <Paragraph position="7"> Our research is therefore motivated by the following observations: (1) earlier work demonstrates convincingly that any adequate speech-act theory must relate the effects of a speech act to context-dependent preconditions; (2) these preconditions must depend on the ignorance as well as on the knowledge of the relevant agents; (3)any practical system for reasoning about ignorance must be based on nonmonotonic reasoning; (4) existing speech act theories based on nonmonotonic reasoning cannot account for the facts of attitude revision resulting from the performance of speech acts.</Paragraph>
  </Section>
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