Abstract: An expert system wherein the rules are of three classes: (1) retrieval rules, which each associate one of several attributes to an object in accordance with the values of inputs; (2) analysis rules, which selectively associate an attribute with an object, and which are somewhat analogous to the natural-language inference rules which would be used in communications between domain experts; and (3) action rules, which selectively carry out the output and control actuation options, based on the attributes associated with objects by the other rules.Preferably only the action rules can enable execution of an external command procedure. Preferably each of each of the action rules requires no logical operations other than a test for association between an attribute and an object. Preferably none of the action rules can associate an attribute with an object. Preferably only the retrieval rules include numeric operations.