《高级Web技术》参考资料：语义Web课件 A Semantic Web Primer_Chapter4 Web Ontology Language：OWL

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

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

Lecture Outline Basic Ideas of owl 2. The oWL Language 3. Examples 4. The oWL Namespace 5. Future Extensions Chapter 4 A Semantic Web primer

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

Requirements for Ontology Languages o 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 3 Chapter 4 A Semantic Web primer

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

Tradeoff between Expressive Power and Efficient Reasoning Support o The richer the language is, the more inefficient the reasoning support becomes e 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

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

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 5 Chapter 4 A Semantic Web primer

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

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 6 Chapter 4 A Semantic Web primer

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

Uses for Reasoning e 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 7 Chapter 4 A Semantic Web primer

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

Reasoning Support for OWL Semantics is a prerequisite for reasoning support o 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 o Description logics are a subset of predicate logic for which efficient reasoning support is possible 8 Chapter 4 A Semantic Web primer

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

Limitations of the Expressive Power of rdf Schema e Local scope of properties rdfs: range defines the range of a property(e.g eats)for all classes In rdf Schema we cannot declare range restrictions that apply to some classes only E.g. We cannot say that cows eat only plants while other animals may eat meat, too 9 Chapter 4 A Semantic Web primer

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. eats) for all classes – In RDF Schema we cannot declare range restrictions that apply to some classes only – E.g. we cannot say that cows eat only plants, while other animals may eat meat, too

Limitations of the Expressive Power of RDF Schema(2) e Disjointness of classes Sometimes we wish to say that classes are disjoint(e.g. male and female) Boolean combinations of classes Sometimes we wish to build new classes by combining other classes using union, intersection and complement E.g. person is the disjoint union of the classes male and female 10 Chapter 4 A Semantic Web primer

Chapter 4 A Semantic Web Primer 10 Limitations of the Expressive Power of RDF Schema (2) ⚫ Disjointness of classes – Sometimes we wish to say that classes are disjoint (e.g. male and female) ⚫ Boolean combinations of classes – Sometimes we wish to build new classes by combining other classes using union, intersection, and complement – E.g. person is the disjoint union of the classes male and female