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.