First-Order Default Logic Revisited

ABSTRACT. It is well known that Reiter's original proposal for default logic in the first-order case is problematic because of Skolemization. This paper reconsiders this long-standing open problem, and proposes a new world view semantics for first-order default logic. Roughly speaking, a world view of a first-order default theory is a maximal collection of structures satisfying the default theory where the default part is fixed by the world view itself. We show that how this semantics generalizes propositional/closed default logic, classical first-order logic and first-order answer set programming, and we argue that first-order default logic under the world view semantics provides a rich framework for integrating classical logic and rule-based formalism in the first-order case.