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<Paper uid="W91-0220">
  <Title>Presuppositions and Default Reasoning: A Study in Lexical Pragmatics</Title>
  <Section position="5" start_page="235" end_page="235" type="concl">
    <SectionTitle>
5 Conclusion
</SectionTitle>
    <Paragraph position="0"> We have investigated a formal approach to the definition of natural language presuppositions. It has involved defining the presuppositions of lexical items as default rules together with the use of Default Logic proof theory in appropriate contexts to generate the presuppositions of utterances containing the lexical items that generate presuppositions.</Paragraph>
    <Paragraph position="1"> This is but one use of defaults in the lexicon. Other uses are fairly obvious -- and they are obvious in a sense that makes the boundary between the lexicon and classical knowledge representation very fuzzy. For example, the notion of canonical traits is the classical example given to support the use of default reasoning in knowledge representation: Tweety is a bird, therefore it (probably) flies. As well, what happens when a standard concept is modified, for example, dog vs toy dog. The issue of what gets negated is very similar in nature to the problem of presuppositions: one item in the concept dog is negated everything else (by default) stays the same. So, in the toy dog situation the alive property is negated but toy dogs still have four legs, two ears, and a tail, it barks, and so on. These features may need to be appropriately modified (for example the barking is imaginary). However, the uses in the lexicon of techniques from knowledge representation seem open-ended.</Paragraph>
  </Section>
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