Expressing preferences in default logic

We address the problem of reasoning about preferences among properties (out comes, desiderata, etc.) in Reiter's default logic. Preferences are express ed using an ordered default theory, consisting of default rules, world know ledge, and an ordering, reflecting preference, on the default rules. In con trast with previous work in the area, we do not rely on prioritised version s of default logic, but rather we transform an ordered default theory into a second, standard default theory wherein the preferences are respected, in that defaults are applied in the prescribed order. This translation is acc omplished via the naming of defaults, so that reference may be made to a de fault rule from within a theory. In an elaboration of the approach, we allo w an ordered default theory where preference information is specified withi n a default theory. Here one may specify preferences that hold by default, in a particular context, or give preferences among preferences. In the appr oach, one essentially axiomatises how different orderings interact within a theory and need not rely on metatheoretic characterisations. As well, we c an immediately use existing default logic theorem provers for an implementa tion. From a theoretical point of view, this shows that the explicit repres entation of priorities among defaults adds nothing to the overall expressib ility of default logic. (C) 2000 Elsevier Science B.V. All rights reserved.