Legal Argumentation and Evidence

A leading expert in informal logic, Douglas Walton turns his attention in this new book to how reasoning operates in trials and other legal contexts, with special emphasis on the law of evidence. The new model he develops, drawing on methods of argumentation theory that are gaining wide acceptance in computing fields like artificial intelligence, can be used to identify, analyze, and evaluate specific types of legal argument. In contrast with approaches that rely on deductive and inductive logic and rule out many common types of argument as fallacious, Walton’s aim is to provide a more expansive view of what can be considered "reasonable" in legal argument when it is construed as a dynamic, rule-governed, and goal-directed conversation. This dialogical model gives new meaning to the key notions of relevance and probative weight, with the latter analyzed in terms of pragmatic criteria for what constitutes plausible evidence rather than truth.

“In this book Douglas Walton builds on his earlier research and shows how it has application to many of the difficult questions that arise in legal reasoning. He brings a dialectical theory of argumentation as well as a theory of plausible reasoning to bear on the traditional problems of legal evidence. Legal Argumentation and Evidence is an original and important contribution not only to legal reasoning but also to the development of argumentation theory, critical thinking, and reasoning in general. It will be of interest to legal scholars but also to argumentation and reasoning theorists who want to keep abreast of the most recent developments in the field.”—Hans V. Hansen, University of Windsor

“Walton makes a significant contribution to the understanding of legal argumentation and to the concept of relevance in evidence law. He goes beyond formal logic and adds an analysis of abduction and plausible inference to fill gaps in what a deductive system can accomplish. The resulting theory provides an important insight into the relationships among the steps of a legal argument. The dialogue structure on which it is based should prove to be of great value in understanding strategy, either for the advocate, the evaluator of evidence, or the student of the legal process.”—Kevin Saunders, University of Oklahoma Law School

“Impressively researched and clearly written, this book is a notable contribution to the study of legal argumentation.”—Derek Allen, University of Toronto Quarterly

Douglas Walton is Professor of Philosophy at the University of Winnipeg. He is the author of four other books published by Penn State Press: The Place of Emotion in Argument (1992), Arguments from Ignorance (1995), Appeal to Expert Opinion (1997), and Appeal to Popular Opinion (1998).

Contents

Acknowledgments

Introduction

1. SPECIAL FEATURES OF ARGUMENTATION IN A

LEGAL SYSTEM

Legal Rules and Particular Cases

Interpretation of Statutes and Documents

Stages of a Trial

Civil Law, Criminal Law, and Burden of Proof

Evidence

Relevance and Admissibility

Testimony of Witnesses

Expert Testimony

Examination

Dependence on Precedents

2. FORMS OF ARGUMENT COMMONLY USED IN LAW

Argument from Analogy

Argument from an Established Rule

Argument from Sign and Abductive Argument

Argument from Position to Know

Argument from Verbal Classification

Argument from Commitment

Practical Reasoning

Argument from Personal Attack (Ad Hominem Argument)

The Slippery Slope Argument

Other Important Forms of Argument

3. CIRCUMSTANTIAL EVIDENCE

The McCormick Criterion

The Jewish Classical Law Criterion

Bentham on Circumstantial Evidence

Patterson’s Criterion

Wigmore on Direct Evidence and Autoptic Proference

Wigmore on Circumstantial and Testimonial Evidence

The Hope Head Case

The Five Criteria Summarized

How Useful is the Concept of Circumstantial Evidence?

Logical Difficulties of Circumstantial Evidence

4. PLAUSIBILITY AND PROBABILITY

A Third Type of Reasoning

Plausibility and Probability

Wigmore on Logical Inference and Probative Value

Locke on Plausibility and Degrees of Assent

Bentham on Plausibility and Evidence

Plausibility and Casuistry

Plausible Reasoning in the Ancient World

Carneades’ Theory of Plausibility

Criteria and Applications of Carneades’ Theory

Why the Neglect of Plausible Reasoning?

5. THE DIALECTICAL FRAMEWORK OF LEGAL

ARGUMENTATION

Implicature and Conversational Postulates

Rational Persuasion in the Trial

Normative Models of Argumentation

Persuasion Dialogue

Other Types of Dialogue

Peirastic Dialogue and Extastic Dialogue

Relevance and Dialectical Shifts

The Fair Trial and the Witch-Hunt

A Dialectical Theory of Statutory Interpretation

Argumentation Schemes, Fallacies, and Legal Logic

6. A PLAUSIBILISTIC THEORY OF EVIDENCE

Components of the New Theory

Evidence and Argument

The Probative Function

Ancient Roots of the New Theory

Advantages of The Plausibilistic Theory

Scientific Evidence

Logical and Legal Relevance

Legal Evidence, Credibility, and Plausibility

Expert Testimony as Evidence

Problems and Conclusions

7. RELEVANCE IN PERSUASION DIALOGUE

Persuasion Dialogue

Chaining of Arguments

Rules of Dialogue and Fallacies

The Fallacy of Irrelevant Conclusion

The Method of Argument Extrapolation

Testing an Actual Example

How the Method Should be Applied

Questions Raised

Application to Legal Cases

Arguments and Explanations

8. MULTI-AGENT ARGUMENTATION AND CREDIBILITY

Formal Dialogue Systems in Logic

The Ad Hominem and Ad Verecundiam Fallacies

Labeled Deductive Systems

Multi-Agent Systems

Adding Agents to Formal Dialectical Structures

Evaluating Fallacies and Blunders

How Should ‘Agent’ be Defined in Formal Dialectic?

Dialectical Shifts and Relevance

The Solution to the Problem

Conclusions

9. HOW TO USE THE NEW METHOD

The New Method

Inference Forms and Critical Questions

Arguments Depending on Testimony and Credibility

Verbal Arguments and Critical Questions

The Trial as Persuasion Dialogue

Argument Diagramming

The Formal Structure of Diagramming

Formalizing the New System

The Subtleties of Peirastic Dialogue

The Current Problems with Relevance

Bibliography

Index

INTRODUCTION

The purpose of Legal Argumentation and Evidence is to provide a new method

for analyzing the structure of legal reasoning used in arguing a case and evaluating

legal argumentation in general. This book is intended to be of interest to

those who work in the legal profession, as well as to those who teach law, but

it is not a legal treatise. It takes a fresh look at the structure of legal reasoning

by exploring current developments in argumentation theory. This new viewpoint

is different from the traditional approaches of deductive and inductive

logic because it examines legal argumentation contextually. The relevance of

an argument, for example, is seen as depending on a conversational framework

in which an issue or conoict of opinions is to be resolved. A character-attack

argument, called an ad hominem argument in traditional logic, can be judged to

be irrelevant in the main argumentation stage of a criminal trial but relevant at

its sentencing stage.

In the new model of argumentation presented here, the form of the argument

(that is, its so-called argumentation scheme) is merely one factor in judging

its worth. How the argument is used in a specioc context of conversation

is also crucially important. This view of argumentation as dialectical (or dialogical)

is gradually becoming more widely accepted in current research in law

and artiocial intelligence. What is provided here are new foundations for this

growing and important research initiative and, at the same time, new and promising

ways of looking at legal argumentation and evidence that are of wider

interest.

This book sets out an abstract normative model of the fair trial as a goaldirected

structure that provides a forum for dispute resolution that, in turn,

does justice through the provision of due process. According to the model,

the trial contains an adversarial dispute. The purpose of the litigants and their

attorneys, as their agents, is to win the dispute. But the rules of evidence and

other procedural rules contain this struggle, so that relevant arguments on both

sides can be tested out. The trier (judge or jury) can use critical thinking to

evaluate the strengths and weaknesses of the arguments used on both sides.

The kind of fact-onding reasoning used in legal argumentation depends for its

evaluation on its use within the goal-directed conversational setting of the trial

as a normative structure. This model of the argument as a conversational exchange,

known as ‘‘dialectical argumentation,’’ can also be used to throw new

light on the kind of reasoning used in statutory interpretation that is important

in determining what law is. ‘‘Dialectical’’ is not meant here in the Hegelian or

Marxist sense but is used to evoke the dialectical method of question and answer

prized by the Greek philosophers, indicating a view of rational argumentation

as a conversational exchange.

The central purpose of the book is to provide a new dialectical foundation

or logical underpinning for evidence law. Especially with the recent televising

of several prominent trials, many people are expressing serious doubts about

whether the legal system and legal decisions are based on any kind of standards

of rational thinking. One fundamental concern is that evidence law in particular,

which is supposed to be the central pillar of good reasoning and fair procedure

in the law, is ‘‘adrift.’’ The underlying basis of such concerns is addressed

here in a new theory of evidence, a dialectical one that exhibits a kind of roleplaying

argumentation structure and bases the outcome of the argument on

conclusions drawn from what is reasonably taken to be evidence presented in

the case.

The dialectical theory offers an evaluation of legal arguments on a caseby-

case basis, relative to the given information at any particular stage in the

development of the case. This new theory is radical in at least two important

ways. First, it can be brought to bear on legal arguments that, in the past, have

been held to be fallacious by logicians, as well as show how, in the legal context

of use, such arguments are not fallacious but reasonable. Second, dialectical

theory explains the reasonableness of legal arguments in terms of their plausibility,

or what used to be called ‘‘probability’’ in an ancient Greek sense. I feel

it is beneocial to re-examine this notion of plausibility.

Legal Argumentation and Evidence seeks to vindicate John H. Wigmore’s theory

that there is a science of reasoning underlying the law of evidence, upon

which the evidence law needs to be based (Wigmore 1913). It does this not only

by providing a new logical framework in which Wigmore’s central concepts of

evidence can be expressed. It also traces the origins of these concepts back

through Locke and Bentham. Central to the new theory are Bentham’s ideas

of probative weight and the chaining together of inferences into sequences of

argumentation. Also central is Locke’s idea of degrees of assent.

Wigmore’s long-neglected method of diagramming the body of evidence in

a case is now shown to be the central method for legal argument evaluation in

the new dialectical system. Wigmore had contended that the logical structures

of reasoning underlying the law of evidence could be traced back to the ancient

logic of the Greek philosophers. In Chapter 4 it is shown how the notion of

plausible reasoning, known since the time of the Greek skeptical philosophers

but neglected for many centuries, is the key form of reasoning that underlies

evidence law and legal argumentation. The ancient theory of plausible reasoning

advocated cautious acceptance of an inference drawn plausibly from what

seems to be true, subject to reservations.

According to this view of plausible reasoning, we can provisionally accept

conclusions that seem to be true, even though we may be deceived occasionally.

It is a view that is fallible, however: we need to be open-minded enough to give

up the conclusion if new information comes in that indicates it is not true. For

example, if a murder victim did not cry out for help before she was struck, the

conclusion might be drawn by plausible reasoning that her assailant was not a

stranger. This kind of reasoning is based on assumptions about what we can

infer, given the way things can normally be expected to go in a kind of situation

familiar to all of us as reasonable persons.

Plausible reasoning was widely familiar to the Greek and Roman philosophers

and jurists but had pretty well disappeared from logic after the fall of

the ancient world. Only recently has plausible inference come back into some

prominence, through the reappearance of what is often called abductive reasoning

in artiocial intelligence (AI) studies in computer science. Plausible reasoning

is now widely used in AI, for example, in expert systems used for medical

diagnosis. The expert system makes a guess by abduction, or inference to the

best explanation, at a plausible diagnosis. But the hypothesis, or provisional

guess, can be changed if new information comes in that indicates a different

hypothesis is a better explanation of the facts known about the patient’s symptoms.

Many important instances of legal reasoning are based on plausible inferences

like those used in medical diagnosis.

Chapter 2 contains a list and description of the most common and important

forms of plausible inference used in legal argumentation. In the past, most

books on legal logic—with the notable exception of a few works like those of

Perelman and Olbrechts-Tyteca (1969) and Alexy (1989)1—have concentrated

almost exclusively on deductive or inductive logic in modeling legal argumentation.

Deductive and inductive forms of inference were seen as central models

of reasoning—or the only signiocant ones—by opinion leaders, evaluated from

a semantic standpoint that abstracts an argument from its context of use in a

particular case. The whole area of evidence law has been pretty well ignored,

1. New research initiatives in artiocial intelligence and law (Ashley 1990; Hage, Leenes, and

Lodder 1994; Gordon 1995; Verheij 1996; Prakken 1997; Vreeswijk 1997; Lodder 1999) are also

exceptions to this rule. Another exception is the work of Feteris (1999) on legal argumentation.

therefore, by logicians—perhaps because, in the past, it seemed an impossible

goal to model factors such as relevance of arguments. Even among those working

in the more recently explored area of informal logic, the question ‘‘What is

evidence?’’ has barely been asked.

I take a fresh approach to legal argumentation by treating evidence and relevance

as central to the whole understanding of constructing a framework for

the evaluation of legal arguments. The approach is dialectical in the ancient

Greek sense, meaning that an argument is evaluated with respect to how it was

supposedly used for a given purpose in a context of dialogue.2 Although forms

of inference are seen as vitally important, how chains of inferences are used by

a proponent and a respondent to persuade each other, or for various other

communicative purposes, are presented as the basis for the evaluation of arguments.

The central dialectical notion is that of commitment (rational acceptance

in dialogue). Commitment is dialogical, and can be retracted, but has

rational bite. If the respondent accepts the premises of the argument, and the

form of the argument meets the appropriate structural requirements, then the

respondent must accept the conclusion unless he can raise doubts about it by

asking appropriate critical questions. A participant’s commitment to a statement

is determined mostly by what he goes on record as saying in a dialogue

but also partly by the goals and rules of the dialogue.

The focus of this book is on certain types of plausible argument that have,

until recently, been generally considered fallacious in logic. Recent research is

tending to show, more and more, that these types of argument are not always

fallacious but that, in many instances, they are presumptively reasonable for

shifting a burden of proof in a case. They tend to be arguments that, even when

they are plausible, only shift a small weight of evidence from the premises to

the conclusion of the argument. Even so, such arguments can be extremely

important when lots of them are put together in a case.

For example, in a trial, one small argument may not prove much by itself.

But a whole lot of these small arguments can yield a body of evidence that

swings the burden of proof to one side or the other on a disputed issue. Using

new ondings and methods of informal logic and argumentation theory, I show

how to identify, analyze, and evaluate these kinds of plausible arguments that

are so common, both in legal argumentation and in everyday experiences of

conversational disputation. These types of arguments include familiar kinds of

everyday arguments such as argument from position to know, argument from

2. A dialogue is deoned as goal-directed type of conversational exchange in which two parties

reason together, taking turns to ask questions, give replies, and put forward arguments to each

other.

ignorance, argument from precedent, argument from a rule to a particular case,

appeal to expert opinion, appeal to pity, appeal to threat, argument against the

person, causal argument, argument based on a verbal classiocation, argument

from analogy, argument based on a deonition, and the slippery slope argument.

This fresh approach reveals not only that such arguments are very often reasonable

when used in a legal framework but are also most important and central

among the kinds of arguments used in legal argumentation. Legal Argumentation

and Evidence presents a new dialectical method of evaluating such arguments

as they are used in legal contexts, offering a method of determining in

each individual case of its use, whether the argument should be judged to be

reasonable or fallacious. If the argument is weak but not so bad that it is fallacious,

however, the new method tells you which critical questions should be

asked in order to pinpoint specioc weaknesses.

This book should be of interest to anyone who is attempting to make sense

of legal argumentation as a cooperative effort of several parties to reason with

each other as speech partners. Not only lawyers but professionals and students

in communication studies, linguistics (pragmatics), artiocial intelligence, logic,

critical thinking, and philosophy will be interested readers. Among specialized

groups, the analysis will appeal to those working in the law of evidence, and in

the oeld of artiocial intelligence and law. But the general reader with no specialized

training can easily read the book. Pre-law students, or anyone with an

interest in legal argumentation, will ond it helpful as an introduction to the

basic principle of legal reasoning and forms of argument. The presentation of

material clearly explains all legal and logical jargon that is used, and is written

in such a way that it can be read and understood by someone who does not

have legal or logical training or expertise, but who wants to learn something

about how argumentation works in the legal context. The orst chapter describes,

for such a reader, the special features of argumentation in a legal system.

The treatment is mainly set in the context of the Anglo-American

common law system, but from time to time, comments are made about other

systems. The book is meant to introduce people with a legal background to the

latest developments in argumentation theory, and show them how this new

oeld can be applied to many aspects of legal argumentation. But it is also useful

to introduce anyone interested in argumentation and critical thinking to basic

argumentation techniques used in many typical kinds of legal arguments often

used as examples in informal logic textbooks.

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Law

Philosophy