A Contradiction Management System

In most existing systems that give expert advice, efforts are made to ensure that the rules in the knowledge base do not give rise to contradictions. This is partly to avoid confusing the user, and partly for fear that a logical system that generates even a single contradiction must collapse. We argue that not only can contradictions be tolerated, but they can even be useful. In a legal context, it is in fact essential to allow contradictions and to handle them correctly. A legal expert system that can see only one side of an argument would not be useful in practice; an advocate needs to know the strengths and weaknesses of the opposing argument, too. Thus a legal expert system must allow for different interpretations of the underlying legal texts. With this in view, we propose a two level architecture, where the object level may include contradictory rules, and the metalevel resolves these conflicts. Furthermore, the ‘closed world assumption’ gives different results depending on which side one is arguing. Once such an architecture is in place, there are good reasons for allowing contradictions in wider contexts, too. In everyday reasoning, this is a common mechanism for testing an argument.