1Chapter 4OWL Based on slides from Grigoris Antoniou and Frank van HarmelenOutline1. A bit of history2. Basic Ideas of OWL 3. The OWL Language4. Examples5. The OWL Namespace6. Future ExtensionsJoint EU/US CommitteeDAMLOntoKnowledge+OthersThe OWL Family TreeFramesDescription LogicRDF/RDF(S)OILDAML-ONTDAML+OIL OWLW3CSHOELogic ProgrammingA Brief History of OWL: SHOEz Simple HTML Ontology Extensionsz Sean Luke, Lee Spector, and David Rager, 1996SHOE allows World-Wide Web authors to annotate their pages with ontology-based knowledge about page contents. We present examples showing how the use of SHOE can support a new generation of knowledge-based search and knowledge discovery tools that operate on the World-Wide Web.z Supported adding “semantic” tags defined in an ontology plus prolog-like rules to web pages.2A Brief History of OWL: SHOE<META HTTP-EQUIV="Instance-Key" CONTENT="http://www.cs.umd.edu/~george"> <USE-ONTOLOGY "our-ontology" VERSION="1.0" PREFIX="our" URL="http://ont.org/our-ont.html"> …<CATEGORY "our.Person"><RELATION "our.firstName" TO="George"><RELATION "our.lastName" TO="Cook"><RELATION "our.marriedTo" TO="http://www.cs.umd.edu/~helena"><RELATION "our.employee" FROM="http://www.cs.umd.edu"> A Brief History of OWL: OILz Developed by group of (largely) European researchers (several from EU OntoKnowledgeproject)z Based on frame-based languagez Strong emphasis on formal rigour. z Semantics in terms of Description Logicsz RDFS based syntaxFramesDescription LogicsRDF/RDF(S)OILDAML-ONTDAML+OIL OWLA Brief History of OWL: DAML-ONTz Developed by DARPA DAML Program. – Largely US based researchersz Extended RDFS with constructors from OO and frame-based languagesz Rather weak semantic specification– Problems with machine interpretation– Problems with human interpretationFramesDescription LogicsRDF/RDF(S)OILDAML-ONTDAML+OIL OWLA Brief History of OWL: DAML+OILz Merging of DAML-ONT and OIL z Basically a DL with an RDFS-based syntax.z Development was carried out by “Joint EU/US Committee on Agent Markup Languages”z Extends (“DL subset” of) RDFz Submitted to W3C as basis for standardisation– Web-Ontology (WebOnt)Working Group formedFramesDescription LogicsRDF/RDF(S)OILDAML-ONTDAML+OIL OWL3A Brief History of OWL: OWL z W3C Recommendation (February 2004)z Based largely on the March 2001 DAML+OIL specificationz Well defined RDF/XML serializationsz Formal semantics– First Order– Relationship with RDFz Comprehensive test cases for tools/implementationsz Growing industrial take up.FramesDescription LogicsRDF/RDF(S)OILDAML-ONTDAML+OIL OWLOutline1. A bit of history2. Basic Ideas of OWL3. The OWL Language4. Examples5. The OWL Namespace6. Future ExtensionsRequirements for Ontology Languagesz Ontology languages allow users to write explicit, formal conceptualizations of domain modelsz The main requirements are:– a well-defined syntax – efficient reasoning support – a formal semantics – sufficient expressive power – convenience of expressionExpressive Power vs Efficient Reasoningz There is always a tradeoff between expressive power and efficient reasoning supportz The richer the language is, the more inefficient the reasoning support becomesz Sometimes it crosses the noncomputabilityborderz We need a compromise:– A language supported by reasonably efficient reasoners – A language that can express large classes of ontologies and knowledge.4Kinds of Reasoning about Knowledgez 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 Dz 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, tooz Consistency– X instance of classes A and B, but A and B are disjoint– This is an indication of an error in the ontologyz 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 AUses for Reasoning z Reasoning support is important for– checking the consistency of the ontology and the knowledge– checking for unintended relationships between classes– automatically classifying instances in classesz Checks like the preceding ones are valuable for – designing large ontologies, where multiple authors are involved– integrating and sharing ontologies from various sourcesReasoning Support for OWLz Semantics is a prerequisite for reasoning supportz 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 formalismsz OWL is (partially) mapped on a description logic, and makes use of reasoners such as FaCT, RACER and Pelletz Description logics are a subset of predicate logic for which efficient reasoning support is possibleRDFS’s Expressive Power Limitationsz 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, too5RDFS’s Expressive Power Limitationsz Disjointness of classes– Sometimes we wish to say that classes are disjoint (e.g. male and female)z 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 femaleRDFS’s Expressive Power Limitationsz Cardinality restrictions– E.g. a person has exactly two parents, a course is taught by at least one lecturerz Special characteristics of properties– Transitive property (like “greater than”)– Unique property (like “is mother of”)– A property is the inverse of another property (like “eats” and “is eaten by”)Combining OWL with RDF Schemaz Ideally, OWL would extend RDF Schema– Consistent with the layered architecture of the Semantic Webz But simply extending RDF Schema would work against obtaining expressive power and efficient reasoning – Combining RDF Schema with logic leads to uncontrollable computational properties Three Species of OWLz W3C’sWeb Ontology Working Group defined OWL as three different sublanguages:– OWL Full– OWL DL– OWL Litez Each sublanguage geared toward fulfilling different aspects of requirements6OWL
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