The Case for Explicit Exceptions

Most of the work on inheritance hierarchies in recent years has had as its goal the design of general purpose algorithms that depend only on the topology of the inheritance network. This research has produced some important observations about the various strategies used in human common sense reasoning, but it has also produced a proliferation of incompatible systems. In this paper, we resurrect the alternative technique, originally proposed by Etherington and Reiter, of explicitly encoding exceptions to default rules. The main technical innovation is the use of a different logical framework: a logic programming language based on intuitionistic logic. Using a combination of full intuitionistic negation plus negation-as-failure to encode default rules, we obtain analogues of the normal, seminormal and nonnormal defaults of Reiter’s default logic. The advantage of our approach is that, whereas there is no adequate proof theory in classical logic for seminormal defaults, the analogous queries to an intuitionistic default rulebase can be answered by a simple top-down goal-directed interpreter. The claim that a default rulebase with explicit exceptions is easy to write and debug has been substantiated by encoding more than 40 standard examples from the literature.