Special Workshop Abstracts

Special Workshop Title:	Law, Morality, Politics, Defeasibility
Author:	Henry Prakken, Institute of Information and Computing Sciences, Utrecht University; Chris Reed, Department of Applied Computing, University of Dundee; Douglas Walton, Department of Philosophy, University of Winnipeg, Winnipeg, Manitoba
Paper Title:	Argumentation Schemes and Generalisations in Reasoning about Evidence
Abstract:	This paper addresses the computational modelling of legal reasoning about evidence within general theories of defeasible reasoning and argumentation. Rather than taking our starting point in mathematical or computational models, such as probability theory or model-based reasoning, we follow a more cognitively inspired approach, aiming to stay close to the evidential reasoning forms used in practice. In particular, we take our starting point in legal theory, studying a method for structuring evidential reasoning that goes back to John Henry Wigmore (1931), and which was recently rediscovered by modern Anglo-American legal evidence theorists such as Anderson, Schum, Tillers and Twining; see e.g. Schum (1994) for an overview. Wigmore developed a graphical method for charting legal evidence, a tool for making sense of a large body of evidence. Stripped to the bones, his charts depict the arguments that can be constructed from a given body of evidence, as well as possible sources of doubt with respect to these arguments. At the time, Wigmore’s charting method went largely unnoticed, but today his method looks surprisingly modern, anticipating recent AI models of defeasible reasoning as well as recent software for argument visualisation. Our aim is to explore both these directions. First we will apply the argument visualisation system Araucaria (Reed & Rowe, 2002) to a Wigmore-style analysis of legal evidence, and then we will capture some of the ‘logic’ underlying Wigmore’s charts in an outline of a formal account of evidential reasoning in terms of John Pollock’s work on defeasible argumentation (e.g. Pollock, 1987). The element that unites these analyses is the notion of argumentation schemes. When looking at evidential reasoning (or indeed at reasoning in general), one sees that many arguments, as well as attacks on them, are instances of recurring patterns, such as inferences from witness or expert testimonies, causal arguments, or temporal projections. The idea of systematising reasoning in terms of such argumentation schemes rather than just in terms of abstract principles of inference is the subject of much current study in argumentation theory (e.g. Walton, 1996). In fact, much AI (& Law) work can also be regarded as studying argumentation schemes (although it is usually not presented as such). For instance, we think that John Pollock’s work on prima facie reasons is of this kind, as is much AI & Law research on modelling legal argument. In light of this, the aim of this paper is twofold: (1) Showing how recent developments in legal evidence theory can be analysed in formal models of defeasible reasoning and utilised in software for visualising legal arguments; (2) Arguing for the relevance of the idea of argumentation schemes to models of evidential reasoning, and indicating a possible formalisation of reasoning with such schemes. References: [1] Pollock, J.L. (1987), Defeasible reasoning. Cognitive Science 11:481-518. [2] Reed, C.A. & Rowe, G.W.A (2002), Araucaria: Software for Puzzles in Argument Diagramming and XML. Department of Applied Computing, University of Dundee Technical Report. [The Araucaria software can be downloaded from www.computing.dundee.ac.uk/staff/creed/araucaria/] [3] Schum, D.A. (1994), The Evidential Foundations of Probabilistic Reasoning. New York: John Wiley & Sons. [4] Schum, D.A. & Tillers, P. (1991), Marshalling evidence for adversary litigation. Cardozo Law Review 13:657-704. [5] Walton, D.N. (1996), Argumentation Schemes for Presumptive Reasoning. Mahwah, N. J.: Erlbaum. [6] Wigmore, J.H. (1931), The Principles of Judicial Proof, 2^nd ed., Boston: Little, Brown and Company.