Reasoning on the Semantic Web: beyond Ontology Languages and Reasoners

This article discusses forms of reasoning (beyond ontology reasoning), and reasoning languages (beyond ontology languages), that are needed for a full deployment of the Semantic Web. We first outline a motivating application scenario, then discuss the logic languages needed on the Semantic Web and related issues. The views reported about in this article underly and have been developed within the EU research project REWERSE (REasoning on the WEb with Rules and SEmantics, cf. http://rewerse.net). 1 Motivating Application Scenario: Business Rules for Car Rental Consider business rules of a car rental company specifying ‘price regulations for company customers’. In a simplified form, such business rules, inspired from [1], can be as follows: • No more than 20 cars can be rented at once by a (company) customer at a reduced company price. • Reduced company prices are not applicable on Class A cars. Beside regulations for company customers, the car rental company might have specified other business rules expressing ‘general car renting regulations’: • No more than one car can be rented at once by a person. • Price reductions are not applicable to young drivers, i.e. drivers under 25 years. Note that the above two sets of business rules, the ‘price regulations for company customers’ and the ‘general car renting regulations’ can well be at distinct web sites, reflecting that two different departments of the car rental company at different locations are responsible for these two sets of business rules. A certain company customer of the car rental company might also well have its own ’car rental regulations’, e.g.: • Junior employees are at most 24 years old. • Every employee is junior or senior employee. • Senior employees rent only Class A cars. Note that the above regulations are expressible in OWL-DL but not in RDF, as quantifiers, cardinalities, and negation are needed. Consider now the forms of reasoning that might be needed in processing business rules like the above regulations: • To draw the conclusion that an employee older than 25 years is a senior employee, excluded middle is needed. • To draw the conclusion that no junior employees can rent a car on the (reduced) company price, refutation and monotonic negation are needed. • To check whether a regulation, say “No more than 20 cars can be rented at once by a (company) customer at a reduced company price.”, is currently enforced non-monotonic negation is needed. • To determine so-called ‘gold customers’, i.e. customers renting cars at least 5 times during the last 12 months, constructive reasoning suffices, i.e. excluded middle and refutation are not needed. 2 Reasoning Languages for the Seman-