Informal Logic: Issues and Techniques

324 Pages·1997·15.28 MB·English

by  Wayne Grennan

Informal Logic
Issues and Techniques
In this insightful critique of informal logic and critical thinking strate-
gies and techniques, Wayne Grennan examines argument evaluation
techniques currently used in both formal and informal logic texts and,
finding them lacking, proposes a new system of evaluation.
Grennan bases his evaluation of arguments on two criteria: logical
adequacy and pragmatic adequacy. He asserts that the common formal
logic systems, while logically sound, are not very useful for evaluating ev-
eryday inferences, which are almost all deductively invalid as stated.
Turning to informal logic, he points out that while more recent informal
logic and critical thinking texts are superior in that their authors recog-
nize the need to evaluate everyday arguments inductively, they typically
cover only inductive fallacies, ignoring the inductively sound patterns
frequently used in successful persuasion. To redress these problems,
Grennan introduces a variety of additional inductive patterns.
Concluding that informal logic texts do not encourage precision in
evaluating arguments, Grennan proposes a new argument evaluation
procedure that expresses judgments of inferential strength in terms of
probabilities. Based on theories of Stephen Toulmin, Roderick Chis-
holm, and John Pollock, his proposed system allows for a more precise
judgment of the persuasive force of arguments.
WAYNE GRENNAN is associate professor of philosophy, St Mary's Uni-
versity.
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Informal Logic
Issues and Techniques
WAYNE GRENNAN
McGill-Queen's University Press
Montreal & Kingston • London • Buffalo
McGill-Queen's University Press 1997
ISBN 0-7735-1542-9
Legal deposit second quarter 1997
Bibliotheque nationale du Quebec
Printed in the United States on acid-free paper
This book has been published with the help of a grant from the
Humanities and Social Sciences Federation of Canada, using funds
provided by the Social Sciences and Humanities Research Council
of Canada. Funds have also been provided by Saint Mary's University.
Canadian Cataloguing in Publication Data
Grennan, Wayne, 1938-
Informal logic: issues and techniques
Includes bibliographical references and index.
ISBN 0-7735-1542-9
i. Logic. I. Title.
80177.0741997 160 9€ 7-900139-0
This book was typeset by Typo Litho Composition Inc.
in 10/12 Baskerville.
Contents
Preface xiii
1 Arguments 3
i. i Arguments as Speech Acts 4
1.1.1 The Speech Act Concept of Argument 5
1.1.2 Argument Utterance Effects i o
1.2 Arguments as Speech Act Products 13
1.3 Conclusion 15
2 Current Argument-Evaluation Techniques 16
2.1 The Traditional Approach 17
2.1.1 Evaluation Techniques 17
2.1.2 Traditional Texts: Deficiencies of Omission 20
2.2 The New Approach: Informal Logic 26
2.2.1 Origins of the New Approach 27
2.2.2 The Elements of an Argument-Evaluation
System 2 8
2.2.3 Summary 32
3 An Argument-Evaluation System 34
3.1 Outline of a Theory of Argument Evaluation 34
3.2 An Argument Rating Scheme 36
3.2.1 The Need For a Rating Scheme 37
3.2.2 Two Extant Rating Schemes 40
3.2.3 Developing a New Rating Scheme 42
vi Contents
3.2.4 A Premiss Rating Scheme 45
3.2.5 Developing an Inference Rating Scheme 46
3.2.6 A Conclusion-Support Rating Scheme 52
3.3 The Evaluation Procedure 54
3.3.1 Scriven's Procedure 54
3.3.2 The Proposed Argument-Evaluation
Procedure 57
3.3.3 Variations 60
3.3.4 A Difficulty for the Rating System 61
4 Diagramming Arguments 64
4.1 Toulmin's Model of Argument 66
4.2 The "T2" Model of an Argument 68
4.2.1 One-Premiss Arguments 71
4.2.2 Convergent Arguments 71
4.2.3 Linked-Premiss Arguments 71
4.3 Distinguishing Warrants from Grounds 77
4.3.1 The van Eemeren Challenge 77
4.3.2 Freeman's Challenge 78
4.3.3 Identifying Syllogism Warrants 80
4.3.4 Further Criteria for Individuating Warrants 82
4.3.5 Warrants in Deductive Logic 83
4.3.6 How Crucial Is the Ground/Warrant
Distinction? 84
4.4 Divergent Arguments 86
4.5 Serial Arguments 86
4.6 Depiction of Modal Qualifiers in Diagrams 86
4.7 Comparison of Diagram Formats 89
4.8 Conclusion 92
5 Evaluation of Deductive Inference 93
5.1 Testing for Formal Validity 93
5.1.1 Evaluating Prepositional Reasoning 93
5.1.2 Evaluating Class Reasoning 111
5.2 Testing for Material Validity 120
5.2.1 Constitutive Rule-Based Inferences 121
5.2.2 Regulative Rule-Based Inferences 124
5.2.3 Hitchcock's Approach to Material Validity 125
6 Inductive Inference Evaluation 128
6.1 Evaluating Inductive Inferences 128
6. i. i Evaluating Inferences with No Qualifiers 129
6. i.2 Evaluating Inferences with Qualified
Assertions 130
vii Contents
6.2 The Rebuttal-Factor Approach 133
6.2.1 Derivation of the Rebuttal-Factor Approach 134
6.2.2 Inference Evaluation by Rebuttal-Factor
Method 146
6.2.3 Interim Summary 147
6.2.4 The Rebuttal-Factor Method vs
Counterexampling 147
6.2.5 The Rebuttal Factor Method vs the Bayesian
Method 148
7 An Inductive Argument Typology 151
7.1 Argument-Pattern Typologies 152
7.1.1 The Perelman and Olbrechts-Tyteca
Typology 152
7.1.2 Argument Fields 153
7.2 The Ehninger and Brockreide Typology 154
7.2.1 Warrant Classification 155
7.2.2 The Ehninger and Brockreide Claim
Classification 158
7.3 A Revised Typology 163
7.3.1 Arguments for Obligation Claims 165
7.3.2 Arguments for Supererogatory Claims 170
7.3.3 Arguments for Prudential Claims 173
7.3.4 Arguments for Evaluative Claims 177
7.3.5 Arguments for Physical-World Claims 186
7.3.6 Arguments for Mental-World Claims 190
7.3.7 Arguments for Constitutive-Rule Claims 194
7.3.8 Arguments for Regulative-Rule Claims 197
7.4 Argument from Authority 201
7.4.1 Inference Evaluation 202
7.4.2 Observer Testimony 212
7.4.3 Expert Testimony 213
7.4.4 Summary 214
7.5 Analogical Arguments 215
7.5.1 Arguments Relying on Predictive Analogies 215
7.5.2 Proportional-Analogy Arguments 216
7.6 Synopsis 218
8 Premiss Evaluation 220
8.1 Evaluation Strategies 220
8.2 Categorical Claims 222
8.2.1 Obligation Claims 223
8.2.2 Supererogatory Claims 224
8.2.3 Prudential Claims 225
viii Contents
8.2.4 Evaluative Claims 226
8.2.5 Physical-World Claims 229
8.2.6 Mental-World Claims 230
8.2.7 Constitutive-Rule Claims 231
8.2.8 Regulative-Rule Claims 232
8.3 Generalizations 233
8.3.1 Universal Generalizations 233
8.3.2 Qualified Generalizations 234
8.3.3 Nonqualified Generalizations 239
8.4 Alternations (Disjunctions) 240
8.5 Conditionals 243
8.6 "p(Py and Arguer Credibility 246
8.6.1 Assessing the Impact of Credibility 246
8.6.2 Examples 247
8.6.3 Conclusion 250
9 Missing Premisses 251
9.1 Missing-Premiss Concepts 251
9. i. i Missing Premisses as Gap Fillers 252
9.1.2 Diagramming Syllogisms 254
9.1.3 Are Enthymemes Complete Arguments? 256
9.1.4 Machina on Missing Premisses 260
9.2 Arguments with Missing Premisses 262
9.3 Evaluation Using Supplementary Premisses 264
9.3.1 A Procedure for Argument Evaluation 269
9.3.2 Sufficiency, Preservation, and Plausibility 271
9.3.3 Concluding Comments 273
10 Conclusion 275
10.1 The Argument-Evaluation System 275
10.2 Problems of Informal Logic Addressed by
This Book 277
Notes 285
Bibliography 297
Index 305
Tables
1 The Thomas Rating Scheme 40
2 Grennan's (1984) Rating Scheme 41
3 Premiss Rating Scheme 46
4 Truth Table Definition of Material Conditional 48
5 Inference Rating Scheme 53
6 Conclusion-Support Ratings 54
7 Truth Table Definitions of the Logical Connectives 98
8 Test of a Valid Inference Pattern 99
9 Test of an Invalid Inference Pattern 99
i o Test of a Valid Pattern by the RTT Method i o i
11 RTT Test Requiring Value Choice 102
12 Completed RTT Test after Choice 103
13 Alternative Version of Test 103
14 RTT Test of an Invalid Pattern 104
15 Full Truth Table Version of RTT Method 104
16 Pattern Requiring Value Choice 105
17 Completed Test after Value Choice 105