Notwithstanding substantial progress in the logic of law, it is hard to get a coherent view on the different opinions and approaches that have been proposed. A reason can be that there still is no common set of primitive notions that has turned out sufficiently flexible and expressive to accommodate the different approaches. Here a model of dialectical argumentation with argumentation schemes is presented in an attempt to fill this gap.
The presented approach combines three ideas. First, in legal decision making any statement can be just as well supported (by a reason for it) as attacked (by a reason against it). It turns out that this seemingly trivial point becomes a powerful tool when it is noted that it applies also to the warrants of arguments, i.e., the statements that express which statement supports or attacks which other statement (in analogy with, and extending Toulmin's 1958 notion of warrant).
Second, legal decision making uses dedicated argu-mentation schemes (like rule application, precedent distinction and analogy) and these schemes tend to be defeasible or even contingent. The modeling of argumentation schemes results in a blurred border between the logical object and meta-level, which provides philosophical insight into the relation between concrete, special-purpose legal logics and abstract, general-purpose logics (cf. also Verheij 1999b).
Third, legal decision making is a kind of dialectical theory construction. Initial assumptions and conclusions are critically addressed by adducing reasons for and against them. The result is a gradually changing dialectical theory in which statements can at one time be justified and at others defeated or unsettled. This process is heuristically guided by the data and argumentation schemes available.
In the full paper, the approach to the formal modeling of legal decision making is presented in reply to a critique of two existing approaches, viz. Prakken's (1997) and Hage's (1997). Five themes are discussed that are relevant for the formal modeling of legal decision making, and it is discussed to what extent and with how much success these are covered by Prakken's and Hage's approach. The themes are the logic of argument defeat, the logic of rules, the logic of law, the empirical adequacy of the approaches, and their heuristic value.
The approach to the formal modeling of legal decision making summarized here involves two central elements. First, the abstract logic DEFLOG has been proposed as a 'bottom line dialectical logic', i.e., one that just allows the modeling of dialectical argu-ment by means of dialectical theory construction. Second, it has been shown how contingent, defeasible argumentation schemes can be embedded in the bottom line logic for the flexible and empirically ade-quate modeling of concrete kinds of reasoning, such as statute-based and precedent-based reasoning in the law.
Verheij, Bart (2001). Legal decision making as dialectical theory construction with argumentation schemes. The 8th International Conference on Artificial Intelligence and Law. Proceedings of the Conference, pp. 225-226. ACM, New York. The full paper is available at http://www.ai.rug.nl/~verheij/publications.htm.
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