1. A computer-implemented method for processing rules in an inference system in order to determine the value of a target variable by an inference engine, wherein: the value of the target variable is determined on the basis of a set of rules to define conclusions depending on premises, the premises being functions of values of variables, wherein the variables can be: source variables, having values defined by costs stored in a database, or regulated variables, having values dependent on conclusions of the rules, and wherein the operation of the inference engine ( 110 ) is controlled by an inference control module ( 130 ) configured to determine a reasoning sequence based on values of costs defined for source variables in the database ( 140 ).