Logics for the Semantic Web

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.