Uses of nonmonotonic logic in natural language understanding: generalized implicatures

This work discusses two approaches to modeling generalized implicatures using nonmonotonic logics. The first approach seeks to capture the implicature as the entailment in some logic. That is, the semantic content of a sentence would entail, in that logic, the implicature carried by the sentence. The limitations of such an approach are discussed. The second approach is less ambitious; it just attempts to represent (and not infer) the generalized implicatures, and to nonmonotonically add them to the content of the sentence if they do not contradict other information. This approach uses circumscription to achieve the effect of nonmonotonicity, and it correctly accounts for a variety of examples of generalized implicatures. This work also extends the theory of circumscription into a modal domain, so that formulas that contain modal operators can also be circumscribed.