A Computational Semantics for Natural Language

A Computational Semantics for Natural Language Lewis G. Creary and Carl J. Pollard Hewlett-Packard Laboratories 1501 Page Mill Road Palo Alto, CA 94304, USA Abstract In the new Head-driven Phrase Structure Grammar (HPSG) language processing system that is currently under development at Hewlett-Packard Laboratories, the Montagovian semantics of the earlier GPSG system (see [Gawron et al. 19821) is replaced by a radically different approach with a number of distinct advantages. In place of the lambda calculus and standard first-order logic, our medium of conceptual representation is a new logical for- realism called NFLT (Neo-Fregean Language of Thought); compositional semantics is effected, not by schematic lambda expressions, but by LISP procedures that operate on NFLT expressions to produce new expressions. NFLT has a number of features that make it well-suited {'or nat- ural language translations, including predicates of variable arity in which explicitly marked situational roles supercede order-coded argument positions, sortally restricted quan- tification, a compositional (but nonextensional) semantics that handles causal contexts, and a princip[ed conceptual raising mechanism that we expect to lead to a computation- ally tractable account of propositional attitudes. The use of semantically compositional LiSP procedures in place of lambda-schemas allows us to produce fully reduced trans- lations on the fly, with no need for post-processing. This approach should simplify the task of using semantic infor- mation (such as sortal incompatibilities) to eliminate bad parse paths. I. Introduction Someone who knows a natural language is able to use utterances of certain types to give and receive information about the world, flow can we explain this? We take as our point of departure the assumption that members of a language community share a certain mental system -- a grammar -- that mediates the correspondence between ut- terance types and other things in the world, such as individ- u~ds, relations, and states of ~ffairs, to a large degree, this system i~ the language. According to the relation theory of meaning (Barwise & Perry !1983!), linguistic meaning is a relation between types of utterance events and other as- pects of objective reality. We accept this view of linguistic meaning, but unlike Barwise and Perry we focus on how the meaning relation is mediated by the intersubjective psycho- logical system of grammar. [n our view, a computational semantics ['or a natural language has three essential components: 172 a. a system of conceptual representation for internal use as a computational medium in processes of information retrieval, inference, planning, etc. b. a system of linkages between expressions of the natural language and those of the conceptual representation, and c. a system of linkages between expressions in the concep- tual representation and objects, relations, and states of affairs in the external world. [n this paper, we shall concentrate almost exclusively on the first two components. We shall sketch our ontologi- cal commitments, describe our internal representation lan- guage, explain how our grammar (and our computer im- plementation) makes the connection between English and the internal representations, and finally indicate the present status and future …

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