Exploiting Lexical Regularities in Designing Natural Language Systems

Exploiting Lexical Regularities in Designing Natural Language Systems Boris Katz Artificial Intelligence Laboratory Massachusetts Institute of Technology Cambridge, MA 02139 Beth Levin Department of Linguistics Northwestern Unlversigy Evanston, IL 60208 They've a temper, some of them--particularly verbs: they're the proudest--adjectives you can do anything with, but not verbs-- however, I can manage the whole lot of them[ -- Lewis Carroll, Through the Looking-Glass Abstract This paper presents the lexical component of the STAttT Ques- tion Answering system developed at the MIT Artificial Intelli- gence Laboratory. START is able to interpret correctly a wide range of semantic relationships associated with alternate expres- sions of the arguments of verbs. The design of the system takes advantage of the results of recent linguistic research into the structure of the lexicon, allowing START to attain a broader range of coverage than many existing systems while maintain- ing modular organization. 1. Introduction If asked "Did Sally eat?" after having been told that Sally ate a pear, speakers of English would not hesitate to answer "Yes". But we would not expect English speakers to answer "Yes" if asked "Did David dress?" after being told that David dressed the baby. Here the appropriate answer would be "I don't know". Computational linguists engaged in building Question-Ans- wering systems should find these examples thought-provoking. Two sequences consisting of a statement followed by a question which appear to be parallel syntactically (transitive use of a verb in the statement, intransitive use of the same verb in the question) elicit quite different responses. The simple syntax of these pairs is unlikely to pose a challenge for the parsers used in most existing systems. The problem is that the intransitive uses of the two verbs, eat and dress, receive very different interpre- tations. Thus the intransitive use of eat found in the question "Did Sally eat?" implies the existence of an understood but un- expressed 'object that is interpreted as a prototyplcal type of food or a meal: (1) Sally ate a pear. ~ Sally ate. (i.e., Sally ate some food.) The question "Did David dress?" on the other hand does not mean ~Did David dress something one typically dresses?', it means 'Did David dress himself?': (2) David dressed the baby, =7~ David dressed (i.e., David dressed himself.) Natural language systems should be able to recognize that the relationship between transitive and intransitive dress is not the same as that between transitive and intransitive eat. A large number of English verbs have both transitive and intransitive uses. Interchanges parallel to the one described for eat are possible with a wide range of verbs: (3) Jessiea typed a letter. Did Jessica type? Yes. (4) Gabriella swept the floor. Did she sweep? Yes. (5) Miriam read the book. Did Miriam read? Yes. But the behavior of the verb dress is not exceptional. Another set of verbs including bathe, change, shave, shower, and wash behave like it. For example, these verbs show the same entail- ments as dress: (6) Carla bathed the dog. :=~ Carla …

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