Definitions In Knowledge Sets

Certain constraints that are interpretable as definitions or quasi-definitions cause prediction problems for logics of action. Standard ways to avoid them rely on notions of priority in the minimization of fluent change. We offer an alternative approach based on belief revision. Revision is performed on the reduct of the system theory after isolating definitions, and the revised theory is then expanded by restoring the definitions. The basis for this is a weak monotonicity result for AGM contraction. Examples from the literature are used to illustrate the approach.