Why rules on the Semantic Web?

• There are conditions that ontologies (ie, OWL) cannot express (or only with difficulties)

• • a well known examples is Horn rules: (P1 ∧ P2 ∧ …) → C

• There are conditions that are complicated in rules and ontologies are better (eg, complex classification of terms)

• Simple rule engines might be easier to implement (eg, on top of database engines)

• A different way of thinking — people may feel more familiar in one or the other