Abstract: A method of controlling the operation of a computer database system. The method of the invention is particularly adapted for use in processing a query in a computer system which has an inference engine, query processing means, and a set of object theories and metatheories. The method includes providing a system theory to define coupling relationships between the object theories and metatheories, selecting an object theory and associated metatheories according to the system theory in dependence on parameters of a query to be processed, and processing the query by the theories so chosen. The coupling relationship may be an interpreter, an assimilator or an attribute metatheory.