Basic formalism

Each rule-based system consists of two parts: a rule base and an interpreter, often called an inference engine. The rules define behavior, while the inference engine provides methods to use the rules. It takes care about selection and execution. The format of rules is usually defined as follows:

$$IF \; \langle antecendent \rangle \; THEN \; \langle consequent \rangle$$

and it is considered to be a production rule, in practice equivalent to logical implication: if-then rule. The rules provide Intensional Knowledge, while Extensional is given as the input for the system. Extensional Knowledge can also be taken from other data sources, as external databases. Rule-based Systems are designed on the basis of Logic as a data model, tailored to the needs of particular application. Such systems focus mainly on the rules, which are quantitatively the core part, while facts are in minority. The main part of database systems are facts, while the main part of Rule-based systems are rules.