Presuppositions and Default Reasoning: A Study in Lexical Pragmatics

Explaining how the meaning of words relate to the meaning of the utterance in which they are used is of utmost importance. The most common approaches view the meaning of an utterance as a composition of the meanings of it parts, which of course include the words used to construct the utterance. This approach is successful for entailments. However, similar approaches to explain the presuppositionai behaviour of utterances have for the most part failed. In this paper we describe the application of Default Logic to the representation and the generation of natural language presuppositions. The view is taken that the presuppositions of an utterance are conjectures made by the hearer based upon the assumption that the speaker is following Grice's maxims of cooperative conversation. These conjectures represent information implicitly contained in the utterance which cannot be generated by classicM techniques. The compositional framework is maintained. The difference is that functional units rather than predetermined semantic units axe inherited by the meaning structure. The function's meaning changes depending on the contents of the meaning structure. Hence, we view the study of these functional units as iexicai pragmatics rather than lexical semantics. Default Logic is one formal method for performing default reasoning in the area of Artificial Intelligence called Knowledge Representation. Default reasoning attempts to fill with conjectures the gaps left by classical forms of reasoning. We suggest that the use of non-classical inferencing techniques such as default reasoning will prove fruitful in the realm of lexical reasoning.