CHAPTER4Incorporating Semantics’’Whatever we learn has a purpose and whatever we do affects everything andeveryone else, if even in the tiniest way. Why, when a housefly flaps his wings, abreeze goes round the world; when a speck of dust falls to the ground, the entireplanet weighs a little more; and when you stamp your foot, the earth moves slightlyoff its course ... And it’s much the same thing with knowledge, for whenever youlearn something new, the whole world becomes that much richer.’’—Norton Juster, The Phantom TollboothIn the previous chapter, you learned about the information model of theSemantic Web: the Resource Description Framework, or RDF. RDF providesa virtually limitless model for describing information. You can say anythingyou want about anything you want. The drawback of all the flexibility andexpressiveness of RDF is that used alone, it lacks explicit support for specifyingthe meaning, or semantics, behind the descriptions.Fortunately, the original vision for the S emantic Web considered the fact thatdevelopers and users need some way of specifying rich semantic descriptionsof concepts and relationships to exchange information effectively. RDF Schema(RDFS) and the OWL Web Ontology Language provide these capabilities.The purpose of this chapter is to provide you with the knowledge necessaryto add semantics to your web of RDF data. Semantics are the key to incorpo-rating domain knowledge into RDF data, making the descriptions richer andmore meaningful.The objectives of this chapter are:to understand the role of semantics in information modeling and theSemantic Webto introduce the elements of the OWL Web Ontology Language and howeach is used to add meaning to information in the Semantic Web9394 Part II ■ Foundations of Semantic Web ProgrammingThis chapter is m eant to be a primer for semantics in the Semantic Web andshould serve as an excellent reference for the OWL Web Ontology Language.Chapter 5, ‘‘Modeling Knowledge in the Real World,’’ builds on the contentsof this chapter by discussing ontology development and management and therole of inference in the Semantic Web through an in-depth, hands-on example.Semantics on the WebBefore diving head first into learning the technologies that will allow you toincorporate semantics into your web of data, it’s important to understand whysemantics are needed and how the various technologies of the Semantic Web fittogether. The following sections explore the motivations behind incorporatingsemantics into the RDF model and how RDFS and OWL combine to providethis capability.You’ll recall from Chapter 3, ‘‘Modeling Information,’’ that communicationinvolves semantics and syntax. Semantics refers to the meaning or concepts ofthe information that’s being shared. Syntax refers to the means by which theinformation is transferred. Consider a communication between a soccer playerwho has just scored a goal and her coach. The coach wishes to congratulatethe player, so he gives her a high five and shouts, ‘‘Great shot!’’ The coachis expressing approval and congratulations. The meaning of these conceptsis the semantics of the communication. The high five and verbal adulationare transfer mechanisms for communicating the semantics. They are thesyntax of the communication. The player understands the communication byinterpreting the syntax and its associated semantics.Adding semantics to a web of data requires the ability to define conceptsand relationships precisely in a manner that transcends syntax. Leveragingthis capability is what the Semantic Web is all about, and this chapter takes thefirst step toward understanding and applying semantics to any web of data,even the World Wide Web (WWW).NOTEYou can skip straight to the ‘‘Introduction to Ontologies’’ section if youare already familiar with the foundations of the Semantic Web and you want tobegin learning about the OWL Web Ontology Language and ontologies.Motivating FactorsThere are many motivating factors driving Semantic Web development. Theseinclude but are not limited to: making web-based information machine under-standable, providing a rich semantic model for expressing domain knowledge,and enabling cross-domain information exchange. The next couple of subsec-tions present two specific use cases to help you better understand the goals ofthe Semantic Web.Chapter 4 ■ Incorporating Semantics 95Understanding the World Wide WebThe World Wide Web is one of the largest public repositories of informationin the world. At the time this book was written, it was estimated that thereare on the order of tens of billions of web pages (www.worldwidewebsize.com).That is an extraordinary amount of information. Unfortunately, most of thatinformation is inaccessible to computers because it is designed for humanconsumption. Machines were designed to relay information, not to be awareof the concepts and relationships contained within it. This makes it verydifficult for applications to utilize the WWW as an information source in anykind of automated manner.It’s not immediatel y apparent that there are limitations with the currentdesign of the WWW. When you need some piece of information, it’s relativelyeasy to find it. You go to a search engine like Google or Yahoo, type in a searchstring, and then you review the results, refining your search as you go, untilyou find the page you’re looking for (as depicted in Figure 4-1).Figure 4-1 Searching on the Web involves an iterative process of refining search termsbased on the set of documents returned. This process relies heavily on the user’s ability toquickly interpret the resultsThis process works as well as it does because the user is in the loop. You,the user, do the hard p art. Yes, indexing the contents of pages and rankingtheir relevance to search terms are hard, but these tasks require computers todo what the y’re really good at: calculating, indexing, and sorting. The searchengine is really only performing a syntax-based p attern match between yourquery and the contents of documents on the Internet, with some added featuresfor i mproving performance and accuracy.96 Part II ■ Foundations of Semantic Web ProgrammingUnfortunately, the search engine isn’t aware of the semantics of the searchterms, nor does it understand the meaning of t he information contained ineach web page; therefore, it is unable to solve, or provide an answer,tothequery. All it can do is to try to help you, the