OWL: an ontology whose conclusions compute
On 10 February 2004 the W3C made OWL, a web ontology language, a Recommendation, and with it a reworked RDF specification. OWL added classes, restrictions on properties and three levels of expressiveness to the graph: Lite, DL and Full.
Why it matters
A graph of triples infers nothing by itself: it stores statements but does not know what follows from them. OWL gave a way to write rules such that, for part of the language, a machine can be proved to reach a conclusion in finite time. The trade between expressiveness and decidability was here made explicit and written into a standard for the first time.
The editors of the OWL overview were Deborah L. McGuinness of the Knowledge Systems Laboratory at Stanford University and Frank van Harmelen of the Vrije Universiteit in Amsterdam. The document separates the three sublanguages by what each promises. OWL Lite is for those needing a classification hierarchy and simple constraints. OWL DL is for those wanting maximum expressiveness while keeping, verbatim, "computational completeness (all conclusions are guaranteed to be computable) and decidability (all computations will finish in finite time)". OWL Full offers maximum expressiveness and the syntactic freedom of RDF but, again verbatim, "with no computational guarantees". A new edition of RDF appeared the same day. Its own text says what it is: "one document in a set of six (Primer, Concepts, Syntax, Semantics, Vocabulary, and Test Cases) intended to jointly replace the original Resource Description Framework specifications, RDF Model and Syntax (1999 Recommendation) and RDF Schema (2000 Candidate Recommendation)". The same document writes down for the first time what a graph means: "the meaning of an RDF graph is the conjunction (logical AND) of the statements corresponding to all the triples it contains". What this record does not claim. The completeness guarantee belongs to OWL DL, not to OWL: the document itself denies it to Full. Nor does the record attribute to the standard a description in terms of the description logics SHIF(D) and SHOIN(D): those correspondences were established in separate academic work, and both strings occur zero times in the Recommendation.