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11 A.I. & L. 1 (2003)

handle is hein.journals/artinl11 and id is 1 raw text is: Artificial Intelligence and Law 11: 1-43, 2003. # © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Is there a burden of questioning? DOUGLAS WALTON Department of Philosophy, University of Winnipeg, Winnipeg, Manitoba, R3B 2E9, Canada E-mail: d.walton@uwinnipeg.ca Abstract. In some recent cases in Anglo-American law juries ruled contrary to an expert's testimony even though that testimony was never challenged, contradicted or questioned in the trial. These cases are shown to raise some theoretical questions about formal dialogue systems in computational dialectical systems for legal argumentation of the kind recently surveyed by Bench-Capon (1997) and Hage (2000) in this journal. In such systems, there is a burden of proof, meaning that if the respondent questions an argument, the proponent is obliged to offer some support for it give it up. But what should happen in a formal system of dialogue if the proponent puts forward an argument and the respondent fails to critically question it, and simply moves on to another issue? Is this some kind of fault that should have implications? Should it be taken to imply that, by default, the respondent has conceded the argument? What, if anything, should be the outcome of such a failure to question in a formal dialogue system of argumentation? These questions are considered by examining some legal cases of expert opinion testimony in relation to rules for formal dialectical argumentation systems. Defeasible arguments are inconclusive. They hold only tentatively in an invest- igation, and are subject to defeat if new evidence should come in that rebuts the argument or undercuts it by posing criticisms of it (Pollock 1995). A central problem for argumentation studies is how such a defeasible argument, when it is good one, should be binding on a respondent.1 Should the respondent be free to ignore it altogether, with no penalty or loss of probative weight for his side? There is a burden of proof on the proponent's side. Should there also be a burden of questioning (or challenging) on the respondent's side? If not, it would seem that defeasible arguments need not be rationally binding on a respondent and thus may have no real force, weight or impact as reasons in the argumentation in a dialogue. The problem is how a rational argument put forward by one party in a dialogue should be binding on the other party. This problem arises for deductively valid arguments. It is a general one for formal or abstract models of argumentation of the kind proposed by Hamblin (1970, 1971), van Eemeren and Grootendorst (1984, 1987, 1992) and Walton and Krabbe (1995). But it arises in a form that is even more difficult to solve when defeasible arguments are considered. Legal evidence is largely made up of defeasible arguments (Verheij 1996; Bench-Capon 1997; Prakken 1997). One common defeasible argument of this sort is the appeal to expert opinion of the kind often used as testimony in trials (Walton

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