Similarity preservation in default logic

The paper identifies a problem in default reasoning in Reiter's Default Log ic and related systems: elements which are similar given the axioms only, b ecome distinguishable in extensions. We explain why, sometimes, this is con sidered undesirable. Two approaches are presented for guaranteeing similari ty preservation: One approach formalizes a way of uniformly applying the de faults to all similar elements by introducing generic extensions, which dep end only on similarity types of objects. According to the second approach, for a restricted class of default theories, a default theory is viewed as a "shorthand notation" to what is "really meant" by its formulation. In this approach we propose a rewriting of defaults in a form that guarantees simi larity preservation of the modified theory. It turns out that the above two approaches yield the same result.