Innovation and Ontologies

170 Elements of Ontologies
RDF (Manola & Miller, 2004; McBride, 2004a) was developed by the World Wide Web
Consortium (W3C) as a semantic-network based language to describe internet resources. It was
proposed as a W3C Recommendation in 1999. RDF Schema (McBride, 2004b), an extension to
RDF with frame-based primitives, was made a W3C Recommendation in 2003. The combination
of RDF 204 and RDF Schema 205 is mostly referred to as RDF(S). According to Berners-Lee &
Fischetti (1999), these languages constitute the foundations of the Semantic Web.
In this context three more languages have been developed as extensions to RDF(S):
• OIL (Horrocks et al., 2000). Based on description logic, OIL 206 was developed within the EU
project On-To-Knowledge; the ontology development methodology of this project is
described in part III. OIL adds frame-based knowledge representation primitives to RDF(S).
• DAML+OIL (Horrocks & van Harmelen, 2001). DAML+OIL was created between 2000 and
2001 in the context of the DARPA project DAML 207 . It adds knowledge representation
primitives which base on description logic to RDF(S).
• OWL (Dean & Schreiber, 2004).
Because the Web Ontology Language (OWL), a W3C recommendation since 2004, includes many
currently accepted aspects of ontology specification, it will be described in some more detail.
OWL resulted of the efforts of a W3C working group 208 dedicated to create a new ontology
markup language for the Semantic Web. It is organized in three layers of “increasingly expressive
sublanguages” (McGuinness & van Harmelen, 2004) enabling ontology modelers to choose an
appropriate balance between expressivity and operation efficiency by choosing the according
version of OWL (Antoniou & van Harmelen, 2004; Laufs et al., 2006).
OWL covers most of the features of previously developed languages (i.e. DAML and OIL). It
was designed to (a) provide most of the commonly used modeling primitives of frame-based and
description logic oriented ontologies, (b) have a simple and well-defined semantics, and (c)
provide support for automated reasoning (Klein et al., 2002). Accordingly, the language is
estimated to have a high maturity and expressive power 209 (Antoniou & van Harmelen, 2004;
Koschmider & Oberweis, 2005).
204 More information on RDF can be retrieved from the following contributions: Gómez-Pérez & Corcho (2002), Gómez-Pérez, Fernández-
López & Corcho (2004), Klein et al. (2002), Maedche (2003), Maedche & Motik (2003), McBride (2004a), Mizoguchi (2004b), Staab &
Maedche (2001).
205 More information on RDF Schema can be retrieved from the following contributions: Gómez-Pérez & Corcho (2002), Gómez-Pérez,
Fernández-López & Corcho (2004), Klein et al. (2002), McBride (2004a), Staab & Maedche (2001).
206 More information on OIL can be retrieved from the synopsis in Gómez-Pérez, Fernández-López & Corcho (2004).
207 More information is available online at: http://www.daml.org/ (2007/07/05). For details on the DAML+ONT specification which has
preceded DAML+OIL (cf. McGuinness et al. (2002) and Gómez-Pérez, Fernández-López & Corcho (2004)).
208 More information on the Web-Ontology Working Group can be found at http://w3.org/2001/sw/WebOnt/ (2008/06/05).
209 Although OWL supports various possibilities to specify axioms (e.g. restrictions on data types and cardinalities), there is no support for
arbitrary rules (Antoniou & van Harmelen, 2004; Gómez-Pérez & Corcho, 2002). This motivated the extension of the Semantic Web Stack
with a rules layer that resides upon the ontology layer. There is some ongoing discussion on how the resulting architecture should be
organized and what rule languages (e.g. SWRL 209) and concepts are suitable for efficient reasoning support (Horrocks et al., 2005).