Special Workshop Abstract

The answer to the first question is affirmative. Thee are two reasons why legal reasoning is defeasible, namely if reasons must be balanced, and when rules are over-inclusive. It is theoretically possible that a legal system does not allow these causes of defeasibility to occur in practice. For instance, a legal system might use an extreme entrenched model of rule application, that does not allow any exceptions to applicable rules. In this sense, reasoning with rules is not necessarily defeasible. However, there are good reasons why exceptions to rules should be allowed and to my knowledge every legal systems allows exceptions under exceptional circumstances. This means that the logic of the application of rules as they actually exist is non-monotonic.

The answer to the second question must be more subtle. In a sense we do not need non-monotonic logic to deal with defeasibility, because defeasibility of arguments can also be handled by belief revision. However, it turns out that the beliefs that must be revised are often beliefs regarding the quality of a defeasible argument. They would be the belief that the truth of the other premises of the argument guarantees the truth of the argument’s conclusion, or the belief that if the other premises of the argument are justified, the conclusion is also justified. If this is the real function of the belief revision, belief revision is nothing else than argument defeat in disguise. Then replacement of non-monotonic logic by belief revision is at best superfluous, and arguably even misleading. From this perspective, non-monotonic logic is necessary to deal with defeasible arguments in a clear way.

In the course of trying to answer the two main questions, the paper also discusses some related issues. One issue is the distinction between two views of logic, called the abstract and the embedded view. According to the abstract view, logic deals with relations between sets of sentences (the premises) and single sentences (conclusions), and has as such nothing to do with making inferences. According to the embedded view, logic deals with the question when it is justified to believe a particular conclusion on basis of the belief in a set of premises, or when it is justified to derive the conclusion from these premises. Defeasible reasoning and non-monotonic logic only fit in this embedded view, not in the abstract view of logic.

Using the distinction between views of logic the paper argue that the view of defeasible reasoning as reasoning with incomplete knowledge is misleading. Defeasible reasoning, starting from the embedded view of logic, deals with the question whether a conclusion is justified relative to a set of premises. To answer this question, no more information is needed that that embodied in these premises. To call that information incomplete would be misleading. It is only incomplete with regard to the question whether the conclusion of the argument is true, but that is not the issue at stake in defeasible reasoning.