Quasi-Indexical Reference in Propositional Semantic Networks

QUASI-INDEXICAL REFERENCE IN PROPOSITIONAL SEMANTIC NETWORKS William J. Rapaport Department of Philosophy, SUNY Fredonia, Fredonia, NY 14063 Departmeot of Computer Science, SUNY Buffalo, Buffalo, NY 14260 Stuart C. Shapiro Department of Computer Science, SUNY Buffalo, Buffalo, NY 14260 ABSTRACT We discuss how a deductive question-answering sys- tem can represent the beliefs or other cognitive states of users, of other (interacting) systems, and of itself. In particular, we examine the representation of first-person beliefs of others (e.g., the ~/v.~'~ representation of a user'A belief that he himself is rich). Such beliefs have as an essential component "'quasi-indexical pronouns" (e.g., 'he himself'), and, hence, require for their analysis a method of represent- ing these pronominal constructions and performing valid inferences with them. The theoretical jus- tification for the approach to be discussed is the representation of nested "'de ditto" beliefs (e.g., the system's belief that user-I believes that system-2 believes that user-2 is rich). We dis- cuss a computer implementation of these represen- tations using the Semantic Network Processing Sys- tem (SNePS) and an ATN parser-generator with a question-answering capability. I- INTRODUCTION Consider a deductive knowledge-representation system whose data base contains information about various people (e.g., its users), other (perhaps interacting) systems, or even itself. In order for the system to learn more about these entities--to expand its "'knowledge" base--it should contain information about the beliefs (or desires, wants, or other cognitive states) of these entities, and it should be able to reason about them (cf. Moore 1977, Creary 1979, Wilks and Bien 1983, Barnden 1983, and Nilsson 1983: 9). Such a data base constitutes the "knowledge" (more accurately, the beliefs) of the system about these entities and about their beliefs. Among the interrelated issues in knowledge representation that can be raised in such a context are those of multiple reference and the proper treatment of pronouns. For instance, is the person named 'Lucy' whom John believes to be rich the same as the person named 'Lucy' who is believed by the system to be young? How can the system (a) represent the person named 'Lucy' who is an object of its own belief without (b) confus- ing her with the person named 'Lucy' who is an object..of. John'~ belief, yet (c) be. ..able to merge its representations of those two people if it is later determined that they are the same? A solution to this problem turns out to be a side effect of a solution to a subtler problem in pro- nominal reference, namely, the proper treatment of pronouns occurring within belief-contexts. i. OUASI-INDICATORS Following Casta~eda (1967: 85). an indic,tot is a personal or demonstrative pronoun or adverb used to make a strictly demonstrative reference. and a ouasi-indicator is an expression within a 'believes-that' context that represents a use of an indicator by another person. Consider the fol- lowing statement by person A addressed to person at time ~ and ~lace ~: A says, "I am going to kill you here now. Person ~, who overheard this, calls the police …

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