Logical Forms in the Core Language Engine

This paper describes a 'Logical Form' target language for representing the literal meaning of English sentences, and an intermediate 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. I N T R O D U C T I O N The SRI Core Language Engine (CLE) is a domain independent system for translating English sentences into formal representations of their literal meanings which are capable of supporting reasoning (Alshawi et al. 1988). The CLE has two main levels 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 semantic interpretation rules. Transforming QLF expressions into LF expressions requires (i) fixing the scopes of all scope-bearing operators (quantifiers, tense operators, logical operators) and distinguishing distributive readings of noun phrases from collective ones, and (ii) resolving referential expressions such as definite descriptions, pronouns, indexical expressions, and underspecified relations. The QLF level can be regarded as the natural level of sentence representation resulting 25 from linguistic analysis that applies compositional semantic interpretation rules independently of the influence of context. Sentence ~, syntax rules Parse trees semantic rules QLF ezpressions ~, context LF expressions The QLF expressions are derived on the basis of syntactic structure, by means of semantic rules that correspond to the syntax rules that were used for analysing the sentence. Having QLFs as a well-defined level of representation allows the problems of compositional 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 effectively reduce the complexity of the system as a whole. Also, the distinction enables us to avoid multiplying out interpretation possibilities 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 demonstrate that the semantic representations are also simple enough for practical natural language 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 introduction 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 a range of well-motivated semantic analyses for specific constructions in a single coherent framework. We will first motivate our extensions to first order logic and our distinction between LF and QLF, then describe the LF language, illustrating the logical form translations produced by the CLE for a number of English constructions, and finally present the additional constructs of the QLF language and illustrate their use.