The Building of Oscar

You all know about the importance of being Ernest. This paper is about the importance of being Oscar. Who, might you ask, is Oscar? Oscar is the little man who lives in my computer and reasons just like we do, or, at least, almost just like we do. Let me tell you about Oscar. One of the major accomplishments of contemporary epistemology has been the recognition that most reasoning is defeasible, in the sense that reasons may justify a conclusion when taken alone, but no longer justify that conclusion when additional defeating information is added to them. This is what computer scientists working in Al call 'non-monotonic reasoning'. Philosophers know a lot about some aspects of defeasible reasoning. We know about prima facie reasons and defeaters, and we know quite a bit about what prima facie reasons there are. But we do not have a good understanding of precisely how these constituents are put together in reasoning to arrive at conclusions. Our situation is analogous to knowing what primitive logical entailments there are, but not knowing the principles for constructing deductive arguments out of those entailments. The purpose of this paper is to investigate the structure of defeasible reasoning. Given an array of defeasible and nondefeasible reasons, how are they to be used in drawing conclusions? A satisfactory theory of defeasible reasoning ought to be sufficiently