EL zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA : A Formal, Yet Natural, Compre Knowledge Representation* Chung Hee Hwang & Lenhart K. Schubert Department of Computer Science, University of Rochester Rochester, New York 14627-0226 {hwang,schubert}@cs.rochester.edu Abstract zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA We present Episodic Logic (EL), a highly expressive knowledge representation well-adapted to general com- monsense reasoning as well as the interpretive and in- ferential needs of natural language processing. One of the distinctive features of EL is its extremely permis- sive ontology, which admits situations (episodes, events, states of affairs, etc.), propositions, possible facts, and kinds and collections, and which allows representation of generic sentences. EL is natural language-like in ap- pearance and supports intuitively understandable infer- ences. At the same time it is both formally analyzable and mechanizable as an efficient inference engine. Introduction zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA One of the requirements on knowledge representation is that it should sup ort efficient inference (cf., [Brachman & Levesque, 1985 ). 7 Our basic methodological assump- tion is that this demand on the representation is best met by using a highly expressive logic closely related to natural language itself. The possibility of handling sit- uations, actions, facts, beliefs, attitudes, causes, effects, and general world knowledge simply and directly de- pends on the expressiveness of the representation. These remarks apply as much to semantic representation of English sentences, as to knowledge representation. In fact, the simplest assumption is that the two are one and the same. On that premise, we have been develop- ing Episodic Logic (EL), a highly expressive knowledge and semantic representation well-adapted to common- sense reasoning as well as the interpretive and inferential needs of natural language processing. EL is a first order, intensional logic that incorpo- rates from situation semantics the idea that sentences describe situations [Barwise & Perry, 1983; Barwise, 19891. A distinctive feature of the logic, responsible for its name, is the inclusion of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA episodic (situational) variables. (Episodes, as the term is construed in EL, subsume events, states of affairs, circumstances, even- tualities, etc.) The adjective “episodic” is intended to emphasize the fact that reasoning about the world and the agents in it often involves inference of the temporal *This research was supported in part by NSF Re- search G rant IRI-9013160, ONR/DARPA Research Con- tracts No. N00014-82-K-0193 and No. N00014-92-J-1512, NSERC Operating G rant A 8818, and the Boeing Co. in Seat- tle under Purchase Contract W 288104. 676 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Hwang and causal connections among transient types (as op- posed to eternal types) of situations, i.e., occurrences or state changes. EL is related to natural language through a Montague- style coupling between syntactic form and logical form, allowing the relationship between surface form and log- ical form to be specified in a modular, transparent way. EL representations derived from English text are natu- ral and close to English surface form. Episodic variables implicit in English sentences and temporal relations be- tween those episodes are automatically introduced into the logical form in the process of deindexing. Very gen- eral inference rules, rule instantiation and goad chaining, have been developed that allow for deductive and prob- abilistic inferences. We first describe the ontology of EL, which provides the necessary ingredients for interpreting an expressive representation, and then show how some of the more unusual kinds of objects are represented using these in- gredients. After that we briefly discuss how inferences are made in EL. EL and its Liberal Ontology A distinctive feature of EL is its very permissive ontol- ogy, which supports the interpretation of a wide range of constructs that are expressible in English. EL can rep- resent conjoined predicates by means of X-abstraction (e.g. 9 crack longer than 3 inches); restricted quantifiers (e.g., most aircrafts manufactured by Boeing); predicate modifiers (e.g., severe damage); perception (e.g., “Mary heard the bomb explode”); attitudes and possible facts (e.g., “Mary believes that gasoline is heavier than wa- ter”); actions (e.g., “John thought Mary’s dropping the glass was intentional”); opaque contexts (e.g., “John wants to design a new engine” ); kinds (e.g., “the two kinds of precious metal, gold and platinum” ), etc. We now describe the ontological basis of this wide expressive range of EL. Model structures for EL are based on an ontology of possible individuals 2). Like Hobbs [1985], we believe it is better to expand one’s ontology to allow more kinds of entities than complicating the logical form. Possibde in- dividuals include not only real or actual individuals but also imaginary or nonexistent ones (e.g., “ Tomorrow’s lecture has been cancelled” [Hirst, 19911). As shown in Figure 1, 2) includes many unusual types of individuals From: AAAI-93 Proceedings. Copyright © 1993, AAAI (www.aaai.org). All rights reserved.