Logical Forms in the Core Language Engine

LOGICAL FORMS IN THE CORE LANGUAGE ENGINE Hiyan Alshawi & Jan van Eijck SRI International Cambridge Research Centre 23 Millers Yard, Mill Lane, Cambridge CB2 11ZQ, U.K. Keywords: logical form, natural language, semantics ABSTRACT This paper describes a 'Logical Form' target language for representing the literal mean- ing of English sentences, and an interme- diate level of representation ('Quasi Logical Form') which engenders a natural separation between the compositional semantics and the processes of scoping and reference resolution. The approach has been implemented in the SRI Core Language Engine which handles the English constructions discussed in the paper. INTRODUCTION The SRI Core Language Engine (CLE) is a domain independent system for translat- ing English sentences into formal represen- tations of their literal meanings which are capable of supporting reasoning (Alshawi et al. 1988). The CLE has two main lev- els of semantic representation: quasi logical forms (QLFs), which may in turn be scoped or unscoped, and fully resolved logical forms (LFs). The level of quasi logical form is the target language of the syntax-driven seman- tic interpretation rules. Transforming QLF expressions into LF expressions requires (i) fixing the scopes of all scope-bearing opera- tors (quantifiers, tense operators, logical op- erators) and distinguishing distributive read- ings of noun phrases from collective ones, and (ii) resolving referential expressions such as definite descriptions, pronouns, indexical ex- pressions, and underspecified relations. The QLF level can be regarded as the nat- ural level of sentence representation resulting 25 from linguistic analysis that applies composi- tional semantic interpretation rules indepen- dently of the influence of context. Sentence ~, syntax rules Parse trees semantic rules QLF ezpressions ~, context LF expressions The QLF expressions are derived on the ba- sis of syntactic structure, by means of se- mantic rules that correspond to the syntax rules that were used for analysing the sen- tence. Having QLFs as a well-defined level of representation allows the problems of com- positional semantics to be tackled separately from the problems of scoping and reference resolution. Our experience so far with the CLE has shown that this separation can ef- fectively reduce the complexity of the system as a whole. Also, the distinction enables us to avoid multiplying out interpretation possibil- ities at an early stage. The representation languages we propose are powerful enough to give weU-motiwted translations of a wide range of English sentences. In the current version of the CLE this is used to provide a systematic and coherent coverage of all the major phrase types of English. To demon- strate that the semantic representations are also simple enough for practical natural lan- guage processing applications, the CLE has been used as an interface to a purchase order processing simulator and a database query system, to be described elsewhere. In summary, the main contributions of the work reported in this paper are (i) the intro- duction of the QLF level to achieve a natural separation between compositional semantics and the processes of scoping and reference resolution, and (ii) the integration of …

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