Towards Discourse-Oriented Nonmonotonic System

'TOWARDS DISCOURSE-0RIENTED NONMONOTONIC SYSTEM ,Barbara Dunin-K~plicz Witold Lukaszewicz ,Institute of Informatics Warsaw University P.O.Box 1210 00-901 Warszawa, POLAND ABSTRACT The purpose of this paper is to analyse the phenomenon of nonmonotonicity in a natural lan- guage and £o formulate a number of general prin- ciples which should be taken in%o consideration while constructing a discourse oriented nonmo- notonic formalism. INTRODUCTION F'or the purpose of this paper we assume that to understand a discourse is to specify all the conclusions derivable from the discourse itself knowledge about the world and knowledge of language use. To complete discourse-connected conclusions, a number of various linguistic phenomena must be resolved. The solution of such problems as anaphora, quantification, negation and so on, should be regarded as full right conclusions. The reason is that this purely linguistic informa- tion is necessary for obtaining the essential one, describing the external world under conside- ration. The measure of quality of any discourse analysis system is the adequacy of its inference capabilities to those of human language users. This implies that any such a high quality system must provide some mechanism modelling common sense reasoning. In everyday life we are continously forced to accept various conclusions which we are pre- pared to reject when our knowledge increases. The ability of drawing such cancellableinferences, beliefs in AI terminology, makes common sense reasoning nonmonotonic in the sense that the set of derivable conclusions does not increase mono- tonically with the set of premises, as in standard logics. Earlier experiences have proved that ad hoc nonmonotonic tools were ineffective. They have seemed to work for the simplest cases only. Since they lacked theoretical founds[ions, their behaviour has been unclear in more complex situations. Recently there have been a number of attempts to formalize various nonmonotonic mech- anisms (see (iI, 1980), (AAAI, 1984[)). In this paper we analyse the phenomenon of nonmonotonicity in a natural language. We also formulate a number of general principles which should be taken into account while specifying a discourse-oriented nonmonotonic formalism. It is well recognized that ordering of dis- course utterances is essential for its understand- ing~ On the other hand, logical systems lack mechanisms capturing this property. If follows, therefore~ that to model the dynamic nature of a discourse, some kind of struciuralization is needed. A very natural, both computationally and conceptually, discourse structuralization has been suggested by Kamp (Kamp, 1981). According to Kamp, a discourse is represented aS a D (iscourse) R (epresentation) S (tructure). Reughly speaking, DRS is a sequence of D (iscourse)R (epresentations)~ DR is nothing else than a partial modeI~ describing discourse objects and their tel[aliens. A ORS is constructed as follows. V~'e start with the empby DR. Each discourse utterance extends the actual DR by adding appropriate information contained in the utterance, qFo construct any DR, some kind of reasoning mechanism is needed. V~'e postulate the application of the nonmonotonic inference system for that purpose. NONMONOTONICITY IN DISCOURSE UNDERSTANDING q~o show the universality of nonmonotonicity in a natural language 5 we …

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