A Situation Theoretic Approach to Computational Semantics PhD Thesis V N I E R U S E I T H Y T O H F G E R D I N B U Alan W Black Department of Articial Intel ligen ce Faculty of Science and Engineering University of Edinburgh Foreword This do cument consists of reedited version of my PhD thesis submitted to Department of Articial Intelligence University of Edinburgh December An implementation of the computational language astl describ ed and used through out this thesis is available by anonymous ftp The software is free and may b e freely redistributed sub ject to the attached GNU General public licence The software is available from scottcogsciedacuk pubawbastltarZ Note that this is still an exp erimental version later versions will b e b etter The ab ove version includes the example astl descriptions describ ed in Chapters and which are also repro duced in App endix A Alan W Black awbedacuk Centre for Cognitive Science University of Edinburgh March ii Abstract This thesis presents an approach to the description of natural language semantic theo ries within a situation theoretic framework In recent years research has pro duced a number of semantic theories of natural language that primarily deal with very similar phenomena such as quantication and anaphora Although these theories often deal with similar data it is not always p ossible to see dierences b etween theories treat ments due to dierences in the theories syntax notations and denitions In order to allow b etter comparison of theories the idea of a general semantic metalanguage is discussed and a suitable language in presented Astl is a computational language which is formally dened It is based on funda mental asp ects of situation theory It oers representations of individuals relations parameters facts types and situations It also oers intersituation constraints and a set of inference rules is dened over them In order to show astls suitability as a computational metalanguage three contemporary semantic theories are describ ed wit hin it Situation Theoretic Grammara situation semantic based theory Discourse Representation Theory and a form of dynamic semantics The results show that at least core parts of these semantic theories can b e describ ed in astl Because astl has an implementation it directly oers implementation of the theories describ ed in it The three descriptions can b e closely compared b ecause they are describ ed in the same framework Also this introduces the p ossibility of sharing treatments of semantic phenomena b etween theories Various extensions to astl are discussed but even in its simplest form it is p owerful and useful b oth as an implementation language and sp ecication language Finally we try to identify what essential prop erties of astl make it suitable as a computational metalanguage for natural language semantic theories iii Acknowledgements Firstly I would like to thank Robin Co op er His comments and guidance through this work has added greatly to it as well as helping me understand what I am doing Graeme Ritchie has now for many years given me help in my research and career b oth at a high and low level for which I thank him Ian Lewin has also contributed to my work through long discussions in which I would try to explain to him what I was trying to do and more often he could tell me I am also indebted to various funding b o dies who have made my studies p ossible The SERC funded the ma jority of this work through a p ostgraduate studentship number Also towards the end of this work I have b een more than adequately funded by Esprit Basic Research Action Pro ject DYANA In addition to the ma jor contributions I also wish to acknowledge funding for travel from Esprit Basic Research Action Pro ject DYANA and Department of Articial Intelligence which allowed me to attend conferences and workshops at which some of this work was presented At these events I gained much useful exp erience and background Thanks also go to Gail Anderson of AIAI for arranging use of a workstation during most of this pro ject I would also like to thank Richard Tobin and Je Dalton who have put up with me for some years now oering cheap accommo dation fo o d and home based computing services b oth for work and diversion I also cannot forget my fellow students who I have served my time with John Beaven Matt Cro cker Flavio Correa da Silva Carla Pedro Gomes Ian Frank Ian Lewin Nelson Ludlow Suresh Manandhar Dave Moat Keiichi Nakata Brian Ross Rob Scott Wamberto Vasconcelos and others who have passed through E Without them my time in Edinburgh would not have b een so enjoyable There are others to o who have contributed to my views and work in my previous incarnations in the Edinburgh research community I thank them I am grateful to have had the opp ortunity to b e part of such a very stimulating community iv Contents Foreword ii Abstract iii Acknowledgements iv Introduction Outline of chapters Computational Semantics Introduction Montague Grammar Some semantic phenomena Some semantic theories Discourse Representation Theory Dynamic semantics Situation Theory A general computational semantic language Feature systems Semantic abstraction Thesis aims Summary A Computational Situation Theoretic Language v Introduction astla situation theoretic language Syntax of astl Semantics of astl Inference in astl Extended Kamp Notation Simple example Some formal prop erties Soundness of astl Computational complexity Implementation Comparison with other systems astl and situation theory astl and prosit astl and feature systems Summary Pro cessing Natural Language and STG Introduction Situations and language pro cessing A simple grammar fragment Situation Theoretic Grammar Quantication Summary Discourse Representation Theory and Threading Introduction Discourse Representation Theory DRT in astl DRSs in astl vi Threading Constructing the threading information Pronouns and accessibility Other instantiations of DRT Summary Dynamic Semantics and Situation Theory Introduction Background and justication Denition of DPL DPL in astl Assignments DPL expressions in astl DPL and natural language Comparison of DPLNL and DRT Summary Extensions Introduction Extending DRT in astl Pronouns and Situation Theoretic Grammar Extending astl Abstraction parameters and anchoring in astl Using semantic translations Summary Conclusions Final comments Bibliography A Examples vii A Introduction A Ro oth Fragment A STG description A DRT description A DPLNL description viii Chapter Introduction Since the development of computers one of the many areas of research has b een the automatic pro cessing of human language In the b eginning it was hop ed that natural language pro cessing would not b e to o dicult and the exp ectations were high The translation of one natural language automatically into another was thought to b e p os sible and many pro jects were started However it was quickly discovered that it would not b e as simple as rst thought First b etter theories of natural language were nee ded and secondly b etter theories of programming were needed in order to implement language theories eciently It is not unconnected that during this time there was an increase in the study of theoretical linguistics which oered theories of language more suitable for computer implementation Work in Articial Intelligence however often tried to develop its own computational theories of language which were concerned more with computation than with linguistics Although the overall goal of high p erformance automatic natural language pro cessing was shared b etween the theorists and the pragmatists