Belief Change : Partial

In many database applications, designers can easily provide at least some information about the relative importance of the information to be stored and manipulated. While of potentially high value, the ordering information is typically only partial. Here we address the issue of updates in such partially ordered situations, which we call epistemically stratiied databases. In the current database theory literature, Alchourrr on, GG ardenfors and Makinson (AGM) have proposed a collection of rationality postulates that deene rational updates to deductively closed databases. We reformulate the AGM framework to accomodate empistemically stratiied databases, and to relax the closure requirement. Our immediate goal is to exploit the use of partial ordering information and to relax the logical omniscient avour of the closure condition. A more ambitious goal is only hinted at; it is motivated by a desire to develop a more general theory of updates that integrates the naturally related ideas in artiicial intelligence and deductive databases. Our approach begins with the deenition of a new contraction operator which uses the information provided by the partial order to make a rational choice amongst the various possible outcomes of the update operation. This operator is shown to satisfy a reformulated set of ratio-nality requirements. A second and similar contraction operator based on a logically omniscient framework is shown to satisfy most of the original AGM rationality postulates for contraction.