Defaults and revision in structured theories
Mark Dermot Ryan · 2002
Starting from a logic which specifies how to make deductions from a set of sentences (a flat theory), a way to generalize this to a partially ordered bag of sentences (a structured theory) is given. The partial order is used to resolve conflicts. If phi occurs below psi , then psi is accepted only insofar as it does not conflict with phi . The study starts with a language L, a set of interpretations M and a satisfaction relation. The key idea is to define, for each structured theory, a preorder on interpretations. Models of the structured theory are defined to be maximal interpretations in the ordering. A revision operator that takes a structured theory and a sentence and returns a structured theory is defined. The consequence relation has the properties of weak monotonicity, weak cut, and weak reflexivity with respect to this operator, but fails their strong counterparts.>