Presuppositions as Beliefs

Presuppositions as Beliefs Diane HORTON and Graeme HII:tST Department of Computer Science University of "ibronto Toronto, Canada M5S 1A4 dianeh@ai.toronto.edu (CSNET) Abstract Most theori~s of presupposition implicitly assume that pre- supposition,; are facts, and that all agents involved in a discourse share belief in the presuppositions that it gen- erates. These unrealistic assumptions can be eliminated if each presupposition is treated as the belief of an agent. However, it ~s not enough to consider only the beliefs of the speaker; we show that the beliefs of other agents are often involved. We describe a new model, including an improved definition of presupposition, that treats presuppositions as beliefs and considers the beliefs of all agents involved in the discourse. We show that treating presuppositions as beliefs makes it possible to explain phenomena that cannot be cxplainecl otherwise. 1 Introduction In addition 'I;o its literM meaning, a sentence or utterance conveys a host of indirect information that can be prag- matically in~rred. Presuppositions, whidl we mark ">>", are one pa~'t of that information. Table 1 gives several ex- amples of presupposition with their traditional analyses 1. Roughly, a presupposition is a proposition that is con- veyed by a sentence or utterance 2 but is not part of the main point, and must be consistent with the established context in order for that sentence or utterance to be fe- licitous. For example, the following is infelicitous because the second .,~entence presupposes that Angle quit, which contradicts the first sentence: (1) *Angle didn't quit. It's surprising that she quit. Other types of pragmatic inference include entailment, conversational implicature, and conventional implicature (see Levinson (1983) for detailed descriptions). Presuppo- sitions can be distinguistmd from other sorts of pragmatic inference by their unique behavior when the sentence from which they originate is negated. These basic ideas are generally agreed upon; however, their formalization into a theory of presupposition has been difficult. We will now introduce two problems and our approach to solving them. 1 Throughout this paper, we use the sentence itself a.q short form for its semantic representation, in order to avoid addressing the or- thogonal issu*., of semantic representation. 2The sentence/utterance distinction will be made clear in the pre- sentation of ouc approach. Horton (1987) reviews several theories, including those of Karttunen (1973, 1974), Karttunen and Pe- ters (1979), Wcischedcl (1975, 1979), Gazdar (1979a, 1979b), Wilson and Sperber (1979), and Atlas and Levin- son (1981). One problem is that many theories of presupposition iiaplicitly make the' following unrealistic assumptions s: • Truth Assumption: If sentence S (or its utterance) pre- supposes proposition P, then P is true. • Shared Belief Assumption: If sentence S (or its ut- terance) presupposes proposition P, then all agents involved share the prior belief that P is true. Weischedel and Gazdar are exceptions; e~tch of them at- tributes presuppositions to the speaker as either knowl-. edge or belief. However, we will show th~tt the beliefs of agents other than the speaker must be considered in order to correctly express …

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