Sceptical Reasoning About Defaults

In this paper we investigate preference-based logics for sceptical reasoning about defaults. In preference-based default logics a default is either formalized by a strong or by a weak preference, instances of what we call the ordering and minimizing usages of preference orderings. In a previous paper, we showed how ordering and minimizing can be formalized in Boutilier’s modal logic CT40 and how they can be combined in a two-phase default logic. In this paper, we extend these results from the credulous case to the more complex sceptical case.