A Rgue ! - an Implemented System for Computer - Mediated Defeasible

This paper introduces the Argue!-system. It is an example of a system for computer-mediated defeasible argumentation, a new trend in the field of defeasible argumentation. In this research, computer systems are developed that can be used to mediate the process of argumentation of one or more users. Argument-mediation systems should be contrasted with systems for automated reasoning: the latter perform reasoning tasks for users, while the former play the more passive role of a mediator. E.g., mediation systems keep track of the arguments raised and of the justification status of statements. The argumentation theory underlying the system is based on CumulA, a procedural model of argumentation with arguments and counterarguments. The defeasibility of arguments is modeled in terms of argument structure and the attack relation between arguments, and completely independent of the underlying language. The process-model is free, in the sense that it allows not only inference (i.e., 'forward' argumentation, drawing conclusions), but also justification (i.e., 'backward' argumentation, adducing reasons). In the paper, the Argue!-system is presented from a 'bird's eye' view by focusing on the central ideas. The central user actions of the Argue!-system are inference, justification and attack. Conclusions are inferred from premises, issues are justified by adducing reasons, and arguments are attacked by counterarguments. Examples are given of the use of the Argue!-system as a philosophical tool. First, the question is raised whether the well-known difference between undercutters and rebutters still stands if defeat is purely interpreted in terms of argument structure (and answered positively). Second, it is argued that the indirect defeat of an argument (e.g., resulting from the defeat of an 'initial part' of the argument) should be terminologically distinguished from defeat by an undercutter. Third, the behavior of attack loops of even and odd length is studied. The paper ends with a comparison of the Argue!-system with existing systems, and a discussion of the relevance of computer-mediated defeasible argumentation. 1 Computer-mediated defeasible argumentation Defeasible argumentation can be regarded as a process of inference, justification and attack. When people argue, conclusions are inferred from premises, premises are justified by adducing reasons, and arguments are attacked by counterarguments. The understanding of defeasible argumentation has recently been significantly enhanced. Many researchers have devoted attention to the subject. The contributions of Reiter [1980], Pollock [1987, 1994, 1995], Loui [1991, 1992], Vreeswijk [1993, 1997], Dung [1993, 1995],