A Default Logic Approach to the Derivation of Natural Language Presuppositions By Robert Ernest Mercer B. Sc., The University of Alberta, 1972 M. Sc., The University of Alberta, 1977 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Computer Science We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 1987 © Robert E. Mercer, 1987 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of c^*0^^l*^ ew The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date /f <0C&le^ /f?1- DE-6(3/81) Abstract A hearer's interpretation of the meaning of an utterance consists of more than what is con• veyed by just the sentence itself. Other parts of the meaning are produced as inferences from three knowledge sources: the sentence itself, knowledge about the world, and knowledge about language use. One inference of this type is the natural language presupposition. This category of inference is distinguished by a number of features: the inferences are generated only, but not necessarily, if certain lexical or syntactic environments are present in the uttered sentence; normal interpretations of these presuppositional environments in the scope of a negation in a simple sentence produce the same inferences as the unnegated environment; and the inference can be cancelled by information in the conversational context. We propose a method for deriving presuppositions of natural language sentences that has its foundations in an inference-based concept of meaning. Whereas standard (monotonic) forms of reasoning are able to capture portions of a sentence's meaning, such as its entailments, non-monotonic forms of reasoning are required to derive its presuppositions. Gazdar's idea of presuppositions being consistent with the context, and the usual connection of presuppositions with lexical and syntactic environments motivates the use of Default Logic as the formal non• monotonic reasoning system. Not only does the default logic approach provide a natural means to represent presuppositions, but also a single (slightly restricted) default proof procedure is all that is required to generate the presuppositions. The naturalness and simplicity of this method contrasts with the traditional projection methods. Also available to the logical approach is the proper treatment of 'or' and 'if ... then ...' which is not available to any of the projection methods. The default logic approach is compared with four others, three projection methods and one non-projection method. As well as serving the function of demonstrating empirical and methodological difficulties with the other methods, the detailed investigation also provides the motivation for the topics discussed in connection with default logic approach. Some of the difficulties have been solved using the default logic method, while possible solutions for others have only been sketched. A brief discussion of a new method for providing corrective answers to questions is pre• sented. The novelty of this method is that the corrective answers are viewed as correcting presuppositions of the answer rather than of the question. ii Table of Contents Abstract ii Table of Contents iii List of Figures vii Acknowledgements viii 1 Introduction 1 1.1 Overview 1 1.2 Summary of Results 4 2 Relevant Language Issues 6 2.1 A Model of Communication 7 2.1.1 Situational Knowledge Sources 11 2.1.2 Background Knowledge Sources 11 2.1.3 Context 14 2.2 Non-standard Inferences 16 2.2.1 Grice's Conversational Theory 19 2.2.2 Implicatures - Conventional and Conversational 21 2.2.3 Reformulation of the Maxims 22 2.2.4 Summary 24 2.3 Presuppositions 25 2.3.1 Defining Presuppositions 26 2.3.2 Non-Presuppositions 30 2.3.3 Logical Presuppositions 32 iii 2.3.4 Semantic Presuppositions 33 2.3.4.1 Presuppositional Environments 34 2.3.4.2 Ambiguous Negation 38 2.3.4.3 Projection Problem for Presuppositions 39 2.3.5 Pragmatic Presuppositions 40 2.3.5.1 Speaker Beliefs 40 2.3.5.2 Kempson and Wilson 42 2.3.5.3 Projection Methods 45 3 A Default Logic Approach to Presuppositions 46 3.1 Logical and Representational Preliminaries 49 3.1.1 First-Order Representation 49 3.1.2 Importance of Inferencing 51 3.1.3 Representing Natural Language Negation 54 3.1.3.1 Ambiguity and Vagueness 56 3.1.3.2 Linguistic Tests for Ambiguity and Vagueness 58 3.1.3.3 Arguments Against the Ambiguity Hypothesis 60 3.1.4 Default Logic 66 3.2 Representing Presuppositions Using Default Rules 70 3.2.1 The General Scheme 72 3.2.2 Example 1 - Stop 74 3.2.3 Example 2 - Criterial and Noncriterial properties 76 3.2.4 Example 3 - Factive verbs 78 3.2.5 Example 4 - Focus 80 3.2.6 Correctly Interpreting the Inferences 81 3.2.7 Entailment vs Presupposition 83 3.2.8 Controlling the Application of the Default Rules 84 3.2.8.1 Constraining Default Rule Application in Case Analysis 84 3.2.8.2 Constraining Default Rule Application in General 86 3.3 Deriving Presuppositions in Complex Sentences 87 3.3.1 Example 1 - Or: No cancellation 91 3.3.2 Example 2 - Or: Intrasentential cancellation 93 iv 3.3.3 Example 3 - If... then No cancellation 97 3.3.4 Example 4 - If... then ...: Intrasentential cancellation 98 3.3.5 Example 5 - Possibly 100 3.4 A Definition of Presupposition 101 3.4.1 A Proof-Theoretic Definition of Presuppositions 101 3.4.2 A Model-Theoretic Definition of Presuppositions 102 Comparison of Various Approaches 105 4.1 Karttunen and Peters 107 4.1.1 Empirical Problems 109 4.1.1.1 Example 1 — Contradictory Presuppositions 110 4.1.1.2 Example 2 — Presuppositions cancelled by conversational im- plicatures Ill 4.1.1.3 Example 3 — Presuppositions cancelled by conversational im- plicatures 112 4.1.2 Methodological Problems 113 4.2 Gazdar 120 4.2.1 Empirical Counterexamples 124 4.2.1.1 Entailments Mislabelled as Presuppositions 125 4.2.1.2 Uncancelled Presuppositions 126 4.2.1.3 Non-Binary Features 128 4.2.2 Methodological Problems 130 4.2.3 Landman's Proposal 132 4.3 Gunji 133 4.3.1 Empirical Counterexamples 140 4.3.2 Methodological Problems 148 4.4 Soames 149 4.4.1 Empirical Problems 154 4.4.2 Methodological Problems 158 4.5 A Default Logic Approach 160 4.5.1 Methodological Similarities and Differences 160 4.5.2 Empirical Evidence 166 4.5.2.1 A Non-Binary Example 167 4.5.3 Sentential Adverbs 171 4.5.3.1 Too 172 4.5.3.2 Again 173 4.5.3.3 Default Logic Approach 174 4.5.4 Modified Phrases 176 4.5.5 Problems Requiring Further Analysis 180 4.5.5.1 Complex Indicative Conditionals 181 4.5.5.2 Multiple Extensions 183 4.6 Summary 185 5 Generating Corrective Answers 187 5.1 Questions and Answers 188 5.2 Description of the Two Methods 190 5.3 Differences between Px and PQ 193 6 Conclusion 195 6.1 Summary of Results 195 6.2 Future Work 197 References 199 vi List of Figures 2.1 A Model of Communication 8 3.1 Determining the Meaning of an Utterance 57 3.2 Two Types of Negation 61 3.3 Portion of H's ISA hierarchy 77 vii Acknowledgements To my supervisor, Richard Rosenberg, goes a great deal of gratitude for his guidance through the various stages of research and writing, especially since his job was complicated by doing much of it long distance, and, of course, for his having said the word presupposition at the right time. To the rest of my committee, Alan Mackworth, David Kirkpatrick, Paul Gilmore, and Michael Rochemont, I am grateful for their having taken the time to understand and criticize my research at various stages. I am especially indebted to Sarah Bell and Ray Reiter, both original committee members, whose special skills helped in the embryonic stages of my research. And I also thank the external examiner, C. Raymond Perrault, for his comments and criticisms. Thanks is also accorded to my fellow students, John Demco, Jay Glicksman, Jan Mulder, Alan Carter, and Jim Little for listening at various times to my rantings and ravings. But the most enduring gratitude is to David Etherington whose presence made my tenure as a student at UBC all the more worthwhile and whose influence will be felt long afterwards. Special thanks go to Nick Cercone of Simon Fraser University, whose faith in me was instru• mental in allowing me to get this far, and to Len Schubert of the University of Alberta, Bonnie Webber of the University of Pennsylvania, and Julia Hirschberg of A T & T Bell Laboratories who took the time to be interested in my work and make some crucial criticisms. Many others have made minor contributions to this thesis. They have been acknowledged elsewhere. And finally my most endearing gratitude goes to those who saw me at my worst; my wife, Nicole, and my children, Kelsey and Jonathan (whose births had been originally planned for after the thesis).