Web Ontology Language

Early ontology languages There is a long history of ontological development in philosophy and computer science. Since the 1990s, a number of research efforts have explored how the idea of knowledge representation (KR) from artificial intelligence (AI) could be made useful on the World Wide Web. These included languages based on HTML (called SHOE), based on XML (called XOL, later OIL), and various frame-based KR languages and knowledge acquisition approaches. ==Ontology languages for the web==

In 2000 in the United States, DARPA started development of DAML led by James Hendler. In March 2001, the Joint EU/US Committee on Agent Markup Languages decided that DAML should be merged with OIL. Semantic web standards RDF schema In the late 1990s, the World Wide Web Consortium (W3C) Metadata Activity started work on RDF Schema (RDFS), a language for RDF vocabulary sharing. The RDF became a W3C Recommendation in February 1999, and RDFS a Candidate Recommendation in March 2000. Though RDFS provides some support for ontology specification, the need for a more expressive ontology language had become clear. Web-Ontology Working Group The World Wide Web Consortium (W3C) created the Web-Ontology Working Group as part of their Semantic Web Activity. It began work on November 1, 2001, with co-chairs James Hendler and Guus Schreiber. The first working drafts of the abstract syntax, reference and synopsis were published in July 2002. was made to the W3C. The W3C chartered the OWL Working Group as part of the Semantic Web Activity in September 2007. In April 2008, this group decided to call this new language OWL2, indicating a substantial revision. OWL 2 became a W3C recommendation in October 2009. OWL 2 introduces profiles to improve scalability in typical applications. Adoption A 2006 survey of ontologies available on the web collected 688 OWL ontologies. Of these, 199 were OWL Lite, 149 were OWL DL and 337 OWL Full (by syntax). They found that 19 ontologies had in excess of 2,000 classes, and that 6 had more than 10,000. The same survey collected 587 RDFS vocabularies. ==Ontologies==

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. Example An ontology describing families might include axioms stating that a "hasMother" property is only present between two individuals when "hasParent" is also present, and that 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". This is, however, only true if the concepts of "Parent" and "Mother" only mean biological parent or mother and not social parent or mother. Logic To choose a subset of first-order logic that is decidable, propositional logic was used, increasing its power by adding logics represented by convention with acronyms: ==Species==

OWL dialects 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 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 \mathcal{SHIF}(\mathbf{D}). • OWL2 EL is a fragment that has polynomial time reasoning complexity. It is based on the description logic \mathcal{EL}. • OWL2 QL is designed to enable easier access and query to data stored in databases. It is based on the DL-Lite family of description logics. • OWL2 RL is a rule subset of OWL 2 (the acronym "RL" stands for "Rule Language"). It is based on the so-called description logic programs (DLP). Later, a logic called \mathcal{RL} corresponding to this profile has been introduced. ==Syntax==

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. High level These are close to the ontology structure of languages in the OWL family. OWL abstract syntax 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 DLs and Resource Description Framework (RDF). 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. Exchange syntaxes RDF syntaxes 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. Manchester Syntax 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. Examples • The W3C OWL 2 Web Ontology Language provides syntax examples. Tea ontology Consider an ontology for tea based on a Tea class. First, an ontology identifier is needed. Every OWL ontology must be identified by a URI (http://www.example.org/tea.owl, say). This example provides a sense of the syntax. To save space below, preambles and prefix definitions have been skipped. ;OWL2 Functional Syntax: Ontology( Declaration( Class( :Tea ) ) ) ;OWL2 XML Syntax: ;Manchester Syntax: Ontology: Class: Tea ;RDF/XML syntax: ;RDF/Turtle: rdf:type owl:Ontology . :Tea rdf:type owl:Class . ==Semantics==

Relation to description logics OWL classes correspond to description logic (DL) concepts, OWL properties to DL roles, while individuals are called the same way in both the OWL and the DL terminology. 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 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 \mathcal{SHOIN}^\mathcal{(D)} description logic, while OWL 2 corresponds to the \mathcal{SROIQ}^\mathcal{(D)} 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 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. ==Public ontologies==

Libraries Biomedical • OBO Foundry • NCBO BioPortal • NCI Enterprise Vocabulary Services Standards • Suggested Upper Merged Ontology (SUMO) • TDWG • PROV-O, the ontology version of the W3C's PROV-DM • Basic Formal Ontology (BFO) • European Materials Modelling Ontology (EMMO) Browsers The following tools include public ontology browsers: • Protégé OWL Search • Swoogle ==Limitations==

• 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. ==See also==