Logics for the Semantic Web
Introduction
A major international research effort is currently under way to improve the existing World Wide Web (WWW), with the intention to create what is often called the Semantic Web [[Berners-Lee et al., 2001], [Hitzler et al., 2010]]. Driven by the World Wide Web Consortium (W3C) and its director Sir Tim Berners-Lee (inventor of the WWW), and heavily funded by many national and international research funding agencies, Semantic Web has become an established field of research. It integrates methods and expertise from many subfields of Computer Science and Artificial Intelligence [Studer, 2006], and it has now reached sufficient maturity for first industrial scale applications [[Hamby, 2012], [Hermann, 2010]]. Correspondingly, major IT companies are starting to roll out applications involving Semantic Web technologies; these include Apple’s Siri, IBM’s Watson system, Google’s Knowedge Graph, Facebook’s Open Graph Protocol, and schema.org as a collaboration between major search engine providers including Microsoft, Google, and Yahoo!.
The Semantic Web field is driven by the vision to develop powerful methods and technologies for the reuse and integration of information on the Web. While current information on the Web is mainly made for human consumption, it shall in the future be made available for automated processing by intelligent systems. This vision is based on the idea of describing the meaning—or semantics—of data on the Web using metadata—data that describes other data—in the form of so-called ontologies [Hitzler et al., 2010]. Ontologies are essentially knowledge bases represented using logic-based knowledge representation languages. This shall enable access to implicit knowledge through logical deduction [Hitzler and van Harmelen, 2010], and its use for search, integration, browsing, organization, and reuse of information.
Of course, the idea of adopting knowledge representation languages raises the question which of the many approaches discussed in the literature should be adopted and promoted to Web standards (officially called W3C Recommendations). In this chapter, we give an overview of the most important present standards as well as their origins and history.
The idea that the World Wide Web shall have capabilities to convey information for processing by intelligent systems, and not only by humans, has already been part of its original design [Berners-Lee, 1996]. The World Wide Web was initiated in 1990, and immediately showed exponential growth [Berners-Lee, 1996]. In the meantime, it has become a very significant technological infrastructure of modern society.
In the 1990s, the Semantic Web vision1 was mainly driven by the W3C Metadata Activity [W3C Metadata, revision of 23 August 2002] which produced the first version of the Resource Description Framework (RDF) which we will discuss in Section 2. The Semantic Web Activity [W3C Semantic Web, revision of 19 June 2013] replaced the Metadata Activity in 2001, and has installed several standards for representing knowledge on the Web, most noteably two revisions of RDF, the Web Ontology Language (OWL) discussed in Section 3, and the Rule Interchange Format (RIF) discussed in Section 4. In Section 5, we discuss some of the particular challenges which must be faced when adopting logic-based knowledge representation languages for the Semantic Web, and in Section 6 we discuss some of the more recent research developments and questions. Note that we give a more detailed technical account for RDF than for OWL and RIF, because the latter are closely related to description logics and logic programming, respectively, and the reader is refered to the corresponding chapters in this volume for additional background and introductions.
Section snippets
RDF and RDF Schema
The Resource Description Framework (RDF) [[Manola et al., 2004], [Hayes, 2004]] comprises a simple data format as well as a basic schema language, called RDF Schema [Brickley and Guha, 2004]. While historically often termed a “medadata” standard, that is, an exchange format for data about documents and resources, in the meantime, RDF has been well established as a universal data exchange format for classical data integration scenarios, and particularly for publishing and exchanging structured
A Brief History
While the previously described RDF and RDFS languages already allow to model domain knowledge, they are not very expressive and often insufficient for capturing the necessary relationships and constraints. Therefore, the development of richer representations was an early goal in the Semantic Web initiative, which eventually led to the OWL ontology language.
One of the main predecessors of OWL are frame based systems. While the notion of frames was previously introducted in different contexts,
Rules
Rules come in many guises. In one of their most basic forms, they consist of statements of the form
where B (the head of the rule) and all Ai (which form the body of the rule) are atomic formulas from first-order predicate logic, and all variables in the rule are considered universally quantified.7 Function symbols may or may not be allowed. Additional logical connectives may be allowed, e.g. disjunctions or existential quantifiers in the rule
Particular Challenges to Using Logic-Based Knowledge Representation on the Web
The use of logic-based knowledge representation and reasoning at the scale of the World Wide Web poses a number of particular challenges which have so far not received primary attention in logic research. We list some of them in the following.
The added value of a good machine-processable syntax for knowledge representation formalisms is easily underestimated. However, it is a fundamental basis for knowledge exchange and integration which needs to be approached carefully in order to obtain a
Recent Developments
Concerning more recent developments and investigations concerning the use of logic-based knowledge representation for the Semantic Web, it appears to make sense to distinguish between theoretical advances and advances concerning dissemination into practice and applications.
On the theory side, a convergence of different paradigms is currently happening, in particular with respect to the description-logic-based and the rule-based paradigm. The following are some of the most prominent recent
Acknowledgements
Pascal Hitzler acknowledges support by the National Science Foundation under award 1017225 “III: Small: TROn – Tractable Reasoning with Ontologies.” Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Jens Lehmann acknowledges support by grants from the European Union’s 7th Framework Programme provided for the projects GeoKnow (GA no. 318159) and LOD2 (GA no.
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