Argumentation and Defeasible Reasoning in the Law

Different formalisms for defeasible reasoning have been used to represent knowledge and reason in the legal field. In this work, we provide an overview of the following logic-based approaches to defeasible reasoning: defeasible logic, Answer Set Programming, ABA+, ASPIC+, and DeLP. We compare features of these approaches under three perspectives: the logical model (knowledge representation), the method (computational mechanisms), and the technology (available software resources). On top of that, two real examples in the legal domain are designed and implemented in ASPIC+ to showcase the benefit of an argumentation approach in real-world domains. The CrossJustice and Interlex projects are taken as a testbed, and experiments are conducted with the Arg2P technology

Endogenous and Dangerous

Empirical studies show that courts frequently cite cases that the parties did not cite during briefing and oral arguments—endogenous cases. This Article shows the cognitive and rational dangers of endogenous cases and presents an empirical study of their use. I contend that judges should avoid using endogenous cases in their reasoning and opinions. This Article’s first significant contribution is to provide the first exhaustive treatment in the American legal literature of the rational bases upon which defeasible legal deductions and legal analogies may be built and the critical questions or defeaters that can weaken or bring them down. As far as we know, Aristotle was the first Westerner to formalize logic, about 2,500 years ago. He recognized immediately, however, that the valid de-ductive forms of reasoning he described do not take us far in reasoning about human affairs. Contemporary law scholars nevertheless talk about rule-based and deductive reasoning as if it exists in the law. But all legal arguments, even apparently deductive ones, are subject to defeat by counterargument—they are defeasible. This Article shows that legal analogies are in a sense more defeasible, and thus more complex, than legal deductions, and it suggests that legal policy arguments are more defeasible still. But what happens when judges build these arguments around cases that the parties have not cited—endogenous cases? Studies suggest that judges do so about half the time. The theory of defeasible arguments suggests judges should be reluctant to do so, and they should be most reluctant to do so with legal analogies and policy arguments. But until now, no study has examined how judges use en-dogenous cases. This Article’s second significant contribution is an empirical study of cases judges cited in a random sample of federal district court opinions, identifying where they found them and how they used them. It also identifies a hi-erarchy of badness among endogenous case uses, warning judges away from the most dangerous. It concludes that judges should avoid endogenous cases or, at a minimum, permit the parties to argue the cases before their application. Though the opinions studied here were from federal trial courts, the principles extend to any tribunal that uses opinions from previous cases to guide its decision-making

Endogenous and Dangerous

Empirical studies show that courts frequently cite cases that the parties did not cite during briefing and oral arguments—endogenous cases. This Article shows the cognitive and rational dangers of endogenous cases and presents an empirical study of their use. I contend that judges should avoid using endogenous cases in their reasoning and opinions. This Article’s first significant contribution is to provide the first exhaustive treatment in the American legal literature of the rational bases upon which defeasible legal deductions and legal analogies may be built and the critical questions or defeaters that can weaken or bring them down. As far as we know, Aristotle was the first Westerner to formalize logic, about 2,500 years ago. He recognized immediately, however, that the valid de-ductive forms of reasoning he described do not take us far in reasoning about human affairs. Contemporary law scholars nevertheless talk about rule-based and deductive reasoning as if it exists in the law. But all legal arguments, even apparently deductive ones, are subject to defeat by counterargument—they are defeasible. This Article shows that legal analogies are in a sense more defeasible, and thus more complex, than legal deductions, and it suggests that legal policy arguments are more defeasible still. But what happens when judges build these arguments around cases that the parties have not cited—endogenous cases? Studies suggest that judges do so about half the time. The theory of defeasible arguments suggests judges should be reluctant to do so, and they should be most reluctant to do so with legal analogies and policy arguments. But until now, no study has examined how judges use en-dogenous cases. This Article’s second significant contribution is an empirical study of cases judges cited in a random sample of federal district court opinions, identifying where they found them and how they used them. It also identifies a hi-erarchy of badness among endogenous case uses, warning judges away from the most dangerous. It concludes that judges should avoid endogenous cases or, at a minimum, permit the parties to argue the cases before their application. Though the opinions studied here were from federal trial courts, the principles extend to any tribunal that uses opinions from previous cases to guide its decision-making

Intuitions and the modelling of defeasible reasoning: some case studies

The purpose of this paper is to address some criticisms recently raised by John Horty in two articles against the validity of two commonly accepted defeasible reasoning patterns, viz. reinstatement and floating conclusions. I shall argue that Horty's counterexamples, although they significantly raise our understanding of these reasoning patterns, do not show their invalidity. Some of them reflect patterns which, if made explicit in the formalisation, avoid the unwanted inference without having to give up the criticised inference principles. Other examples seem to involve hidden assumptions about the specific problem which, if made explicit, are nothing but extra information that defeat the defeasible inference. These considerations will be put in a wider perspective by reflecting on the nature of defeasible reasoning principles as principles of justified acceptance rather than `real' logical inference.Comment: Proceedings of the 9th International Workshop on Non-Monotonic Reasoning (NMR'2002), Toulouse, France, April 19-21, 200

Computing Strong and Weak Permissions in Defeasible Logic

In this paper we propose an extension of Defeasible Logic to represent and compute three concepts of defeasible permission. In particular, we discuss different types of explicit permissive norms that work as exceptions to opposite obligations. Moreover, we show how strong permissions can be represented both with, and without introducing a new consequence relation for inferring conclusions from explicit permissive norms. Finally, we illustrate how a preference operator applicable to contrary-to-duty obligations can be combined with a new operator representing ordered sequences of strong permissions which derogate from prohibitions. The logical system is studied from a computational standpoint and is shown to have liner computational complexity

Defeasible Logic Programming: An Argumentative Approach

The work reported here introduces Defeasible Logic Programming (DeLP), a formalism that combines results of Logic Programming and Defeasible Argumentation. DeLP provides the possibility of representing information in the form of weak rules in a declarative manner, and a defeasible argumentation inference mechanism for warranting the entailed conclusions. In DeLP an argumentation formalism will be used for deciding between contradictory goals. Queries will be supported by arguments that could be defeated by other arguments. A query q will succeed when there is an argument A for q that is warranted, ie, the argument A that supports q is found undefeated by a warrant procedure that implements a dialectical analysis. The defeasible argumentation basis of DeLP allows to build applications that deal with incomplete and contradictory information in dynamic domains. Thus, the resulting approach is suitable for representing agent's knowledge and for providing an argumentation based reasoning mechanism to agents.Comment: 43 pages, to appear in the journal "Theory and Practice of Logic Programming