The detection and representation of ambiguities of intension and description

The detection and representation of ambiguities of intension and description Brenda Fawcett and Graeme Hirst Department of Computer Science University of Toronto Toronto, Ontario CANADA M5S 1A4 Abstract Ambiguities related to intension and their consequent inference failures are a diverse group, both syntacti- cally and semantically. One particular kind of ambi- guity that has received little attention so far is a whether it is the speaker or the third party to whom description in an opaque third-party attitude report should be attributed. The different readings lead to different inferences in a system modeling the beliefs of external agents. We propose that a unified approach to the representation of the alternative readings of intension-related ambiguities can be based on the notion of a descriptor that is evaluated with respect to intensionality, the beliefs of agents, and a time of application. We describe such a representation, built on a standard modal logic, and show how it may be used in conjunction with a knowledge base of back- ground assumptions to license restricted substitution of equals in opaque contexts. 1. Introduction Certain problems of ambiguity and inference failure in opaque contexts are well known, opaque contexts being those in which an expression can denote its intension or underlying concept rather than any par- ticular extension or instance. For example, (1) admits two readings: (1) Nadia is advertising for a penguin with whom she could have a long-term meaningful relationship. On the transparent (or extensional or de re) reading, there is some particular penguin that Nadia is after: (2) Nudia is advertising for a penguin with whom she could have a long-term meaningful relationship, whom she met at a singles bar last week and fell madly in love with, but lost the phone number of. On the opaque (or intensional or de dicto) reading, Nadia wants any entity that meets her criteria: (3) Nadia is advertising for any penguin with whom she could have a long-term meaningful relation- ship. On this reading, the rule of existential generalization fails; that is, we cannot infer from (3), as we could from (2), that: (4) There exists a penguin with whom Nudia could have a long-term meaningful relationship Another rule of inference that fails in opaque con- texts is substitution of equals; (5) and (6) do not per- mit the conclusion (7): (5) Nadia believes that the number of penguins cam- paigning for Greenpeace is twenty-two. (6) The number of penguins campaigning for Green- peace is forty-eight. (7) =/~ Therefore, Nadia believes that forty-eight is twenty-two. Although these facts are familiar, little research has been done on how a practical NLU system can detect and resolve intensional ambiguities (which can occur in many constructions besides the 'standard' examples; see Fodor 1980, Fawcett 1985), and con- trol its inference accordingly. The same is true of certain other complications of opaque contexts that are of special relevance to systems that use explicit representations of knowledge and belief. In particu- lar, the interaction between intensional ambiguities 192 and the beliefs …

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