Translating English Into Logical Form

TRANSLATING ENGLISH INTO LOGICAL FORM' Stanley J. Rosenscbein Stuart M, Shieber ABSTRACT A scheme for syntax-directed translation that mirrors com- positional model-theoretic semantics is discussed. The scheme is the basis for an English translation system called PArR and was used to specify a semantically interesting fragment of English, including such constructs as tense, aspect, modals, and various iexically controlled verb complement structures. PATR was embedded in a question-answering system that replied appropriately to questions requiring the computa- tion of logical entailments. I INTRODUCTION When contemporary linguists and philosophers speak of "semantics," they usually mean m0del-theoretic semantics-- mathematical devices for associating truth conditions with Sentences. Computational linguists, on the other hand, often use the term "semantics" to denote a phase of processing in which a data structure (e.g., a formula or network) is constructed to represent the meaning of a sentence and serve as input to later phases of processing. {A bet- ter name for this process might be "translation" or "traneduction.") Whether one takes "semantics" to be about model theory or translation, the fact remains that natural languages are marked by a wealth of com- plex constructions--such as tense, aspect, moods, plurals, modality, ad- verbials, degree terms, and sententiai complemonts--that make seman- tic specification a complex and challenging endeavor. Computer scientists faced with the problem of managing software complexity have developed strict design disciplines in their programming methodologies. One might speculate that a similar re- quirement for manageability has led linguists (since Montague, at least) to follow a discipline of strict compositiouality in semantic specification, even though model*theoretic semantics per me does not demand it. Compositionaiity requires that the meaning of a pbrase be a function of the meanings of its immediate constituents, a property that allows the grammar writer to correlate syntax and semantics on a rule-by-rule basis and keep the specification modular. Clearly, the natural analogue to compositionality in the case of translation is syntax-directed trans- lation; it is this analogy that we seek to exploit. We describe a syntax-directed translation scheme that bears a close resemblance to model-theoretic approaches and achieves a level of perspicuity suitable for the development of large and complex gram- mars by using a declarative format for specifying grammar rules. In our formalism, translation types are associated with the phrasal categories of English in much the way that logical-denotation types are associated Artificial Intelligence Center SRI International 333 Raveoswood Avenue Menlo Park, CA 94025 with phrasal categories in model-theoretic semantics. The translation 'types are classes of data objects rather than abstract denotations, yet they play much the same role in the translation process that denotation types play in formal semantics. In addition to this parallel between logical types and trans- lation types, we have intentionally designed the language in which translation rules are stated to emphasize parallels between the syntax- directed translation and corresponding model-theoretic interpretation rules found in, say, the GPSG literature [Gazdar, forthcoming]. In the GPSG approach, each syntax rule has an associated semantic rule (typically involving functional application) that specifies …

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