An ontology is an explicit specification of a conceptualization.
The data described by an ontology in the OWL family is interpreted as a set of "individuals" and a set of "property assertions" which relate these individuals to each other. An ontology consists of a set of "axioms which place constraints on sets of individuals (called "classes") and the types of relationships permitted between them. These axioms provide semantics by allowing systems to infer additional information based on the data explicitly provided. A full introduction to the expressive power of the OWL is provided in the W3C's OWL Guide.
OWL ontologies can import other ontologies, adding information from the imported ontology to the current ontology.
An ontology describing families might include axioms stating that a "hasMother" property is only present between two individuals when "hasParent" is also present, and individuals of class "HasTypeOBlood" are never related via "hasParent" to members of the "HasTypeABBlood" class. If it is stated that the individual Harriet is related via "hasMother" to the individual Sue, and that Harriet is a member of the "HasTypeOBlood" class, then it can be inferred that Sue is not a member of "HasTypeABBlood".
The W3C-endorsed OWL specification includes the definition of three variants of OWL, with different levels of expressiveness. These are OWL Lite, OWL DL and OWL Full (ordered by increasing expressiveness). Each of these sublanguages is a syntactic extension of its simpler predecessor. The following set of relations hold. Their inverses do not.
- Every legal OWL Lite ontology is a legal OWL DL ontology.
- Every legal OWL DL ontology is a legal OWL Full ontology.
- Every valid OWL Lite conclusion is a valid OWL DL conclusion.
- Every valid OWL DL conclusion is a valid OWL Full conclusion.
OWL Lite was originally intended to support those users primarily needing a classification hierarchy and simple constraints. For example, while it supports "cardinality constraints, it only permits cardinality values of 0 or 1. It was hoped that it would be simpler to provide tool support for OWL Lite than its more expressive relatives, allowing quick migration path for systems using "thesauri and other "taxonomies. In practice, however, most of the expressiveness constraints placed on OWL Lite amount to little more than syntactic inconveniences: most of the constructs available in OWL DL can be built using complex combinations of OWL Lite features, and is equally expressive as the description logic . Development of OWL Lite tools has thus proven almost as difficult as development of tools for OWL DL, and OWL Lite is not widely used.
OWL DL is designed to provide the maximum expressiveness possible while retaining computational "completeness (either φ or ¬φ holds), "decidability (there is an effective procedure to determine whether φ is derivable or not), and the availability of practical reasoning algorithms. OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, number restrictions may not be placed upon properties which are declared to be transitive). OWL DL is so named due to its correspondence with "description logic, a field of research that has studied the logics that form the formal foundation of OWL.
OWL Full is based on a different semantics from OWL Lite or OWL DL, and was designed to preserve some compatibility with RDF Schema. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right; this is not permitted in OWL DL. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. OWL Full is undecidable, so no reasoning software is able to perform complete reasoning for it.
In OWL 2, there are three sublanguages of the language. OWL 2 EL is a fragment that has polynomial time reasoning complexity; OWL 2 QL is designed to enable easier access and query to data stored in databases; OWL 2 RL is a rule subset of OWL 2.
The OWL family of languages supports a variety of syntaxes. It is useful to distinguish high level syntaxes aimed at specification from exchange syntaxes more suitable for general use.
These are close to the ontology structure of languages in the OWL family.
OWL abstract syntax
This high level syntax is used to specify the OWL ontology structure and semantics.
The OWL abstract syntax presents an ontology as a sequence of annotations, axioms and facts. Annotations carry machine and human oriented meta-data. Information about the classes, properties and individuals that compose the ontology is contained in axioms and facts only. Each class, property and individual is either anonymous or identified by an "URI reference. Facts state data either about an individual or about a pair of individual identifiers (that the objects identified are distinct or the same). Axioms specify the characteristics of classes and properties. This style is similar to "frame languages, and quite dissimilar to well known syntaxes for "description logics (DLs) and "Resource Description Framework (RDF).
Sean Bechhofer, et al. argue that though this syntax is hard to parse, it is quite concrete. They conclude that the name abstract syntax may be somewhat misleading.
OWL2 functional syntax
This syntax closely follows the structure of an OWL2 ontology. It is used by OWL2 to specify semantics, mappings to exchange syntaxes and profiles.
|"Internet media type||
|Developed by||"World Wide Web Consortium|
|"Standard||OWL 2 XML Serialization October 27, 2009 ,
OWL Reference February 10, 2004
Syntactic mappings into "RDF are specified for languages in the OWL family. Several RDF "serialization formats have been devised. Each leads to a syntax for languages in the OWL family through this mapping. RDF/XML is normative.
OWL2 XML syntax
OWL2 specifies an "XML serialization that closely models the structure of an OWL2 ontology.
The Manchester Syntax is a compact, human readable syntax with a style close to frame languages. Variations are available for OWL and OWL2. Not all OWL and OWL2 ontologies can be expressed in this syntax.
- The W3C OWL 2 Web Ontology Language provides syntax examples.
Consider an ontology for tea based on a Tea class. But first, an ontology is needed. Every OWL ontology must be identified by an "URI (http://www.example.org/tea.owl, say). This is enough to get a flavour of the syntax. To save space below, preambles and prefix definitions have been skipped.
- OWL2 Functional Syntax
Ontology(<http://example.com/tea.owl> Declaration( Class( :Tea ) ) )
- OWL2 XML Syntax
<Ontology ontologyIRI="http://example.com/tea.owl" ...> <Prefix name="owl" IRI="http://www.w3.org/2002/07/owl#"/> <Declaration> <Class IRI="Tea"/> </Declaration> </Ontology>
- Manchester Syntax
Ontology: <http://example.com/tea.owl> Class: Tea
- RDF/XML syntax
<rdf:RDF ...> <owl:Ontology rdf:about=""/> <owl:Class rdf:about="#Tea"/> </rdf:RDF>
<http://example.com/tea.owl> rdf:type owl:Ontology . :Tea rdf:type owl:Class .
Relation to description logic
In the beginning, IS-A was quite simple. Today, however, there are almost as many meanings for this inheritance link as there are knowledge-representation systems.
Early attempts to build large ontologies were plagued by a lack of clear definitions. Members of the OWL family have "model theoretic formal semantics, and so have strong "logical foundations.
"Description logics (DLs) are a family of logics that are decidable fragments of "first-order logic with attractive and well-understood computational properties. OWL DL and OWL Lite semantics are based on DLs. They combine a syntax for describing and exchanging ontologies, and formal semantics that gives them meaning. For example, OWL DL corresponds to the description logic, while OWL 2 corresponds to the logic. Sound, complete, terminating "reasoners (i.e. systems which are guaranteed to derive every consequence of the knowledge in an ontology) exist for these DLs.
Relation To RDFS
OWL Full is intended to be compatible with "RDF Schema (RDFS), and to be capable of augmenting the meanings of existing "Resource Description Framework (RDF) vocabulary. A "model theory describes the formal semantics for RDF. This interpretation provides the meaning of RDF and RDFS vocabulary. So, the meaning of OWL Full ontologies are defined by extension of the RDFS meaning, and OWL Full is a semantic extension of RDF.
Open world assumption
[The closed] world assumption implies that everything we don’t know is false, while the open world assumption states that everything we don’t know is undefined.— Stefano Mazzocchi, Closed World vs. Open World: the First Semantic Web Battle
The languages in the OWL family use the "open world assumption. Under the open world assumption, if a statement cannot be proven to be true with current knowledge, we cannot draw the conclusion that the statement is false.
Contrast to other languages
A "relational database consists of sets of "tuples with the same "attributes. "SQL is a query and management language for relational databases. "Prolog is a "logical programming language. Both use the "closed world assumption.
Languages in the OWL family are capable of creating classes, properties, defining instances and its operations.
An instance is an object. It corresponds to a description logic individual.
A class is a collection of objects. It corresponds to a description logic (DL) concept. A class may contain individuals, instances of the class. A class may have any number of instances. An instance may belong to none, one or more classes.
A class may be a subclass of another, inheriting characteristics from its parent superclass. This corresponds to "logical "subsumption and DL concept inclusion notated .
All classes are subclasses of owl:Thing (DL "top notated ), the root class.
All classes are subclassed by owl:Nothing (DL "bottom notated ), the empty class. No instances are members of owl:Nothing. Modelers use owl:Thing and owl:Nothing to assert facts about all or no instances.
Class and their members can be defined in OWL either by "extension or by "intension. An individual can be explicitly assigned a class by a Class assertion, for example we can add a statement Queen elizabeth is a(n instance of) human, or by a class expression with ClassExpression statements every instance of the human class who has a female value to the sex property is an instance of the woman class.
Let's call human the class of all humans in the world is a subclass of owl:thing. The class of all women (say woman) in the world is a subclass of human. Then we have
The membership of some individual to a class could be noted
ClassAssertion( human Barack_Obama )
and class inclusion
SubClassOf( woman human )
The first means "Barack Obama is a human" and the second "every woman is human".
A property is a characteristic of a class - a directed binary relation that specifies some attribute which is true for instances of that class. Properties correspond to a description logic role. Properties sometimes act as data values, or links to other instances. Properties may exhibit logical features, for example, by being transitive, symmetric, inverse and functional. Properties may also have domains and ranges.
Datatype properties are relations between instances of classes and RDF literals or XML schema datatypes. For example, modelName (String datatype) is the property of Manufacturer class. They are formulated using owl:DatatypeProperty type.
Object properties are relations between instances of two classes. For example, ownedBy may be an object type property of the Vehicle class and may have a range which is the class Person. They are formulated using owl:ObjectProperty.
Languages in the OWL family support various operations on classes such as union, intersection and complement. They also allow class enumeration, "cardinality, and disjointness.
"Metaclasses are classes of classes. They are allowed in OWL full or with a feature called class/instance punning.
- Marine Metadata Interoperability Project (MMI) Ontology Registry and Repository
- "SchemaWeb - RDF Schemas Directory". September 2005. Archived from the original on 10 August 2011.
- "Suggested Upper Merged Ontology
- PROV-O, the ontology version of the "W3C's PROV-DM
The following tools include public ontology browsers:
- No direct language support for n-ary relationships. For example, modelers may wish to describe the qualities of a relation, to relate more than 2 individuals or to relate an individual to a list. This cannot be done within OWL. They may need to adopt a pattern instead which encodes the meaning outside the formal semantics.
- "Agris: International Information System for the Agricultural Sciences and Technology
- "Common logic
- "FOAF + DOAC
- "Frame language
- "Geopolitical ontology
- "IDEAS Group
- "Meta-Object Facility (MOF), a different standard for the "Unified Modeling Language (UML) of the "Object Management Group (OMG)
- "Multimedia Web Ontology Language
- "Semantic reasoner
- "SSWAP: Simple Semantic Web Architecture and Protocol
- "metaclass (semantic web), a featured allowed by OWL to represent knowledge
- Knublauch, Holger; Oberle, Daniel; Tetlow, Phil; Wallace, Evan (2006-03-09). "A Semantic Web Primer for Object-Oriented Software Developers". "W3C. Retrieved 2008-07-30.
- "OWL 2 Web Ontology Language Document Overview". W3C. 2009-10-27.
- "XML and Semantic Web W3C Standards Timeline" (PDF).
- "OWL". W3.org. Retrieved 2017-02-23.
- "Submission Request to W3C: OWL 1.1 Web Ontology Language". W3C. 2006-12-19.
- "W3C Standard Facilitates Data Management and Integration". W3.org. 2009-10-27. Retrieved 15 October 2013.
- Sirin, E.; Parsia, B.; Grau, B. C.; Kalyanpur, A.; Katz, Y. (2007). "Pellet: A practical OWL-DL reasoner" (PDF). Web Semantics: Science, Services and Agents on the World Wide Web. 5 (2): 51–53. "doi:10.1016/j.websem.2007.03.004.
- "RACER - Home". Racer-systems.com. Retrieved 2017-02-23.
- Tsarkov, D.; Horrocks, I. (2006). "FaCT++ Description Logic Reasoner: System Description". Automated Reasoning (PDF). Lecture Notes in Computer Science. 4130. pp. 292–297. "doi:10.1007/11814771_26. "ISBN "978-3-540-37187-8.
- "Google Code Archive - Long-term storage for Google Code Project Hosting". Code.google.com. Retrieved 2017-02-23.
- "Home". HermiT Reasoner. Retrieved 2017-02-23.
- Berners-Lee, Tim; James Hendler; Ora Lassila (May 17, 2001). "The Semantic Web A new form of Web content that is meaningful to computers will unleash a revolution of new possibilities". Scientific American. 284: 34–43. "doi:10.1038/scientificamerican0501-34.
- John Hebeler (April 13, 2009). Semantic Web Programming. "ISBN "978-0470418017.
- Segaran, Toby; Evans, Colin; Taylor, Jamie (July 24, 2009). Programming the Semantic Web. O'Reilly Media. "ISBN "978-0596153816.
- Lacy, Lee W. (2005). "Chapter 10". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. "ISBN "1-4120-3448-5.
- "Baader, Franz; "Horrocks, Ian; "Sattler, Ulrike (2005). "Description Logics as Ontology Languages for the Semantic Web". In Hutter, Dieter; Stephan, Werner. Mechanizing Mathematical Reasoning: Essays in Honor of Jörg H. Siekmann on the Occasion of His 60th Birthday. Heidelberg, DE: Springer Berlin. "ISBN "978-3-540-25051-7.
- Horrocks, Ian; Patel-Schneider, Peter F.; van Harmelen, Frank. "From SHIQ and RDF to OWL: the making of a Web Ontology Language". Web Semantics: Science, Services and Agents on the World Wide Web. 1 (1): 7–26. "doi:10.1016/j.websem.2003.07.001.
- World Wide Web Consortium (2010-02-06). "W3C Semantic Web Activity". Retrieved 18 April 2010.
- World Wide Web Consortium (2002-08-23). "Metadata Activity Statement". World Wide Web Consortium. Retrieved 20 April 2010.
- World Wide Web Consortium (2002-08-23). "RDF Vocabulary Description Language 1.0: RDF Schema". RDF Vocabulary Description Language 1.0. World Wide Web Consortium. Retrieved 20 April 2010.
- Lacy, Lee W. (2005). "Chapter 9 - RDFS". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. "ISBN "1-4120-3448-5.
- "Web-Ontology (WebOnt) Working Group (Closed)". W3C.
- Patel-Schneider, Peter F.; Horrocks, Ian (2006-12-19). "OWL 1.1 Web Ontology Language". World Wide Web Consortium. Retrieved 26 April 2010.
- Grau, B. C.; "Horrocks, I.; Motik, B.; Parsia, B.; Patel-Schneider, P. F.; "Sattler, U. (2008). "OWL 2: The next step for OWL" (PDF). Web Semantics: Science, Services and Agents on the World Wide Web. 6 (4): 309–322. "doi:10.1016/j.websem.2008.05.001.
- Herman, Ivan. "Why OWL and not WOL?". Tutorial on Semantic Web Technologies. World Wide Web Consortium. Retrieved 18 April 2010.
- "Re: NAME: SWOL versus WOL". message sent to W3C webont-wg mailing list on 27 December 2001.
- "Ian Horrocks (2012). "Ontologe Reasoning: The Why and The How" (PDF). p. 7. Retrieved January 28, 2014.
- Wang, T. D.; Parsia, B.; Hendler, J. (2006). "A Survey of the Web Ontology Landscape". The Semantic Web - ISWC 2006. Lecture Notes in Computer Science. 4273. p. 682. "doi:10.1007/11926078_49. "ISBN "978-3-540-49029-6.
- "Gruber, Tom (1993); "A Translation Approach to Portable Ontology Specifications", in Knowledge Acquisition, 5: 199-199
- "W3C (ed.). "OWL Web Ontology Language Guide".
- Patel-Schneider, Peter F.; Horrocks, Ian; "Patrick J., Hayes (2004-02-10). "OWL Web Ontology Language Semantics and Abstract Syntax". World Wide Web Consortium. Retrieved 18 April 2010.
- Bechhofer, Sean; Patel-Schneider, Peter F.; Turi, Daniele (2003-12-10). "OWL Web Ontology Language Concrete Abstract Syntax". "University of Manchester. Retrieved 18 April 2010.
- Motik, Boris; Patel-Schneider, Peter F.; Parsia, Bijan (2009-10-27). "OWL 2 Web Ontology Language Structural Specification and Functional-Style Syntax". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
- "A. Swartz (September 2004). "application/rdf+xml Media Type Registration (RFC3870)". IETF. p. 2. Archived from the original on 2013-09-17. Retrieved 15 October 2013.
- Patel-Schneider, Peter F.; Motik, Boris (2009-10-27). "OWL 2 Web Ontology Language Mapping to RDF Graphs". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
- Motik, Boris; Parsia, Bijan; Patel-Schneider, Peter F. (2009-10-27). "OWL 2 Web Ontology Language XML Serialization". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
- Horridge, Matthew; Patel-Schneider, Peter F. (2009-10-27). "OWL 2 Web Ontology Language Manchester Syntax". W3C OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
- Hitzler, Pascal; Krötzsch, Markus; Parsia, Bijan; Patel-Schneider, Peter F.; Rudolph, Sebastian (2009-10-27). "OWL 2 Web Ontology Language Primer". OWL 2 Web Ontology Language. World Wide Wed Consortium. Retrieved 15 October 2013.
- Brachman, Ronald J. (1983); What ISA is and isn't: An analysis of taxonomic links in semantic networks, IEEE Computer, vol. 16, no. 10, pp. 30-36
- Horrocks, Ian; Patel-Schneider, Peter F. "Reducing OWL Entailment to Description Logic Satisfiability" (PDF).
- Hitzler, Pascal; Krötzsch, Markus; Rudolph, Sebastian (2009-08-25). Foundations of Semantic Web Technologies. CRCPress. "ISBN "1-4200-9050-X.
- McGuinness, Deborah; "van Harmelen, Frank (2004-02-10). "OWL Web Ontology Language Overview". W3C Recommendation for OWL, the Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
- "Hayes, Patrick (2004-02-10). "RDF Semantics". Resource Description Framework. World Wide Web Consortium. Retrieved 18 April 2010.
- Patel-Schneider, Peter F.; Hayes, Patrick; Horrocks, Ian (2004-02-10). "OWL Web Ontology Language Semantics and Abstract Syntax Section 5. RDF-Compatible Model-Theoretic Semantics". W3C Recommendation for OWL, the Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
- Mazzocchi, Stefano (2005-06-16). "Closed World vs. Open World: the First Semantic Web Battle". Retrieved 27 April 2010.
- Lacy, Lee W. (2005). "Chapter 12". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. "ISBN "1-4120-3448-5.
- OBO Technical WG. "The OBO Foundry". The OBO Foundry. Retrieved 2017-02-23.
- "GBIF Community Site: Section 1: a review of the TDWG Ontologies". Community.gbif.org. 2013-02-12. Retrieved 2017-02-23.
- "PROV-O: The PROV Ontology". W3.org. Retrieved 2017-02-23.
- "PROV-DM: The PROV Data Model". W3.org. Retrieved 2017-02-23.
- "protégé". Protege.stanford.edu. Retrieved 2017-02-23.
- Noy, Natasha; "Rector, Alan (2006-04-12). "Defining N-ary Relations on the Semantic Web". World Wide Web Consortium. Retrieved 17 April 2010.
- Horrocks, Ian (2010); SemTech 2010 tutorial part 1 and part 2 on Description Logics and OWL
- Tutorial on OWL at the University of Manchester Computer Science Department
- Introduction to Description Logics DL course by Enrico Franconi, Faculty of Computer Science, Free University of Bolzano, Italy
- OWL API API for Using OWL 2, at SourceForge