Invited Talk
Logical and Legal Relevance
Douglas Walton
University of Windsor
This paper discusses the relationship between logical relevance and legal relevance in order to help provide a foundation for helping computational argumentation
models of relevance move ahead based on some clear and coherent notion of relevance that is, broadly speaking, logical in nature. This argumentation-based notion ofrelevance is logical, in that it uses forms of argument (argumentation schemes) tojudge relevance or irrelevance of an argument (or of a proposition in an argument,such as a premise). But it is also pragmatic, meaning that it needs to take the use of anargument in a communicative context, such as a trial setting, into account.
Some recent computational argumentation systems in AI and law have used thenotion of relevance in abstract argumentation systems to provide a way for cutting
down the argumentation in a formal dialogue that might go on endlessly. But relevance has long been a contested concept in many disciplines, including law. Peter
Tillers, and a series of long footnotes in one edition of J. H. Wigmore’s monumental
work on evidence law, Evidence in Trials at Common Law, has summarized the notvery well-known history of the subject. On Wigmore’s theory, logical relevance is theunderlying foundation of the legal concept of relevant evidence. He thought there wasa science of proof, which constituted a structure of logical reasoning underlying thetrial rules, which should be used to judge relevance in legal cases. But what is that
logic? One answer is deductive logic, another is the logic of probability (referring tosome form of statistical/Bayesian reasoning). This paper will outline a third alternative, a logical and pragmatic notion of relevance derived from recent work in argumentation theory and informal logic. It is shown that this third alternative approach ispromising by applying some argumentation tools to some examples where judging
relevance or irrelevance is a problem.
The concept of relevance is fundamental to rules of evidence in law, such as theFederal Rules of Evidence, where relevance is defined in terms of an increased probative weight transferred to the conclusion of an argument from its premises. But it
remains unclear whether this notion of probative weight is better analyzed by Bayesian methods of probability calculations or along other lines that use inference to thebest explanation, where an explanation can be made more plausible by argument supporting it, or less plausible by arguments attacking it.
This paper will consider some legal examples (and one example form a logictextbook) where relevance is an issue, and use argumentation tools and theories toanalyze the arguments in them, making special use of argumentation schemes andargument diagrams. Wigmore himself was one of the founders of the use of argument
diagramming (argument mapping). He used evidence charts to evaluate evidential
reasoning in some legal cases. This approach died out for a long time, but has now
returned to some prominence in AI. Now many formal argumentation models in AI
are based on graph structures that are essentially argument diagrams.
At this present point in the development of computational argumentation systems in AI and law, therefore, it is timely to pay more systematic attention to exploring relevance by examining legal cases to show what the problems are in defining andapplying some useful notion of relevance. The work in this paper is part of a larger
research project on relevance in argumentation that has the essential prerequisite ofstarting with making some progress on the goal of illuminating the distinction between logical and legal relevance.