Lexical Operations in a Unification-Based Framework

In this paper we consider the nature and extent of lexical operations, arguing for a declarative and eomputat ional ly tractable definition of a lexical rule as a component of a unification-based lexicon employing (default) inheritance and typed feature structures. We claim that this notion of lexical rule is capable of capturing the linguistic element of derivational morphological processes as well as metonymic and metaphoric sense extensions, but is not adequate for the s ta tement of certain types of logical metonymy (e.g. Pustejovsky, 1989a). We argue that such operations must be treated as, in part , 'linguistic' because they have morphological and syntactic consequences, undergo 'blocking', are triggered by grammatically-defined type mismatches and involve default interpretations based on lexical organisation. However, we also argue that such default interpretations can be overridden by contextual information on the basis of more general and open-ended 'pragmatic ' inference. Our account contrasts with that of, say, Hobbs et al. (1987) who posit an underspecified and impoverished lexical semantic representation which is enriched through an open-ended process of abductive or deductive reasoning with world (or domain) knowledge. Our lexical knowledge representation language does not support general inference, although our lexical semantic representations are often 'richer' than those standardly assumed. A standard example of metonymic sense extension is the use of a word denoting a place to refer to (some of) the people inhabiting that place (e.g. "village", "palace"). This process seems to involve the foregrounding of one component of the meaning of the place denoting word we follow Pustejovsky (1989a,b,c) in assuming that the lexical semantic representation of nouns includes information about typical relations which the objects they denote enter into; in particular, the "qualia structure" for such nouns will contain (telic) information which allows direct access to the information that they are inhabited. A well-known example of metaphoric sense extension is that involving use of a word denoting an animal to refer to humans ("John is a pig", "John is a wombat" etc). Although the sense extension from animals into metaphorical senses denoting humans with some particular characteristic is apparently productive, the actual characteristics involved, and even whether the word can be applied to men or to women or both, cannot be predicted from knowledge of the animal sense. Thus, the properties ascribed to a