One of the primary methods of representing knowledge in CLIPS is a rule. A rule is a collection of conditions and the actions to be taken if the conditions are met. The developer of an expert system defines the rules which describe how to solve a problem. Rules execute (or fire) based on the existence or nonexistence of facts or instances of userdefined classes. CLIPS provides the mechanism (the inference engine) which attempts to match the rules to the current state of the system (as represented by the factlist and instancelist) and applies the actions.

Throughout this section, the term pattern entity will be used to refer to either a fact or an instance of a userdefined class.

5.1 DEFINING RULES