Chapter4 - Chapter 4 Web Ontology Language OWL Grigoris Antoniou Frank van Harmelen 1 Chapter 4 A Semantic Web Primer Lecture Outline 1 2 3 4 5 2 Basic

Chapter 4 A Semantic Web Primer 1 Chapter 4 Web Ontology Language: OWL Grigoris Antoniou Frank van Harmelen

Chapter 4 A Semantic Web Primer 2 Lecture Outline 1. Basic Ideas of OWL 2. The OWL Language 3. Examples 4. The OWL Namespace 5. Future Extensions

Chapter 4 A Semantic Web Primer 3 Requirements for Ontology Languages  Ontology languages allow users to write explicit, formal conceptualizations of domain models  The main requirements are: – a well-defined syntax – efficient reasoning support – a formal semantics – sufficient expressive power – convenience of expression

Chapter 4 A Semantic Web Primer 4 Tradeoff between Expressive Power and Efficient Reasoning Support  The richer the language is, the more inefficient the reasoning support becomes  Sometimes it crosses the border of noncomputability  We need a compromise: – A language supported by reasonably efficient reasoners – A language that can express large classes of ontologies and knowledge.

Chapter 4 A Semantic Web Primer 5 Reasoning About Knowledge in Ontology Languages  Class membership – If x is an instance of a class C, and C is a subclass of D, then we can infer that x is an instance of D  Equivalence of classes – If class A is equivalent to class B, and class B is equivalent to class C, then A is equivalent to C, too

Chapter 4 A Semantic Web Primer 6 Reasoning About Knowledge in Ontology Languages (2)  Consistency – X instance of classes A and B, but A and B are disjoint – This is an indication of an error in the ontology  Classification – Certain property-value pairs are a sufficient condition for membership in a class A; if an individual x satisfies such conditions, we can conclude that x must be an instance of A

Chapter 4 A Semantic Web Primer 7 Uses for Reasoning  Reasoning support is important for – checking the consistency of the ontology and the knowledge – checking for unintended relationships between classes – automatically classifying instances in classes  Checks like the preceding ones are valuable for – designing large ontologies, where multiple authors are involved – integrating and sharing ontologies from various sources

Chapter 4 A Semantic Web Primer 8 Reasoning Support for OWL  Semantics is a prerequisite for reasoning support  Formal semantics and reasoning support are usually provided by – mapping an ontology language to a known logical formalism – using automated reasoners that already exist for those formalisms  OWL is (partially) mapped on a description logic, and makes use of reasoners such as FaCT and RACER  Description logics are a subset of predicate logic for which efficient reasoning support is possible

Chapter 4 A Semantic Web Primer 9 Limitations of the Expressive Power of RDF Schema  Local scope of properties – rdfs:range defines the range of a property (e.g.