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The HTML for a page describes these generic document units and their order, and it’s the browser’s job to decide how to display them. You can give browsers suggestions, which they will normally try to honor as best they can. This approach has been wildly successful when the information is intended for people to read.
With XML, you can describe the structure of the information in other ways, not just in terms of generic document units. You can choose the kind of structure that’s most suitable for the particular information and anticipated use. Thus, XML is seen as the foundation layer of the Semantic Web.
The XML Schema layer provides the ability to specify structures and data types to be used by a particular XML document. The XML and XML Schema layers aren’t covered in this book, because they’re general-purpose technologies that aren’t specially related to the Semantic Web.
The RDF Schema layer describes those properties—what they are, which resources they can be assigned to, and so on. The Ontology layer takes this a step further: Not only does it describe the properties and terms that can be used, but it also can describe relationships between them.
These layers are used to describe, or represent, knowledge. Although the W3C version of the layer cake shows only RDF and RDF Schema, this book also discusses another candidate for representing knowledge: topic maps.
When I purchase a book and give my credit card number, the bookseller wants to know if the card is mine. If I’m there in person, I can show my driver’s license to give a kind of credence to my claim of identity. In essence, I’m saying, “If you don’t believe me, then trust the licensing authority.” This is acceptable as long as the seller believes that the identity card is not counterfeit. The seller might assess the validity of the card by its visual appearance, the match between the picture and my face, the degree of agreement between the age on the card and my apparent age, and any number of other clues. Here you see several principles in play: an appeal to authority, the trustworthiness of that authority, an inference of validity based on a set of facts, and beliefs about those facts.
When I buy the same book online, the seller likewise needs to have some assurance that the credit card is valid and that I’m authorized to use it. I need assurance that the web page really belongs to the bookseller and not to a criminal who wants to get access to my card. When software programs work with data they get over the Web, they face the same problems. The Trust layer will attempt to handle these issues. You can see how it will use all the other layers. The Trust machinery has to call on the Logic and Proof layer to analyze claims, make inferences, and draw conclusions. The Logic and Proof layer needs to know how the terms and properties relate to each other and whether they’re used correctly, which is the business of the Ontology layer. The Ontology layer needs to use the data structures defined and created by the RDF and XML Schema layers. These dependencies can also be viewed in the other direction: the RDF layer uses RDF Schema and Ontology as it assigns its properties, and the XML layer provides transportable data structures for the RDF information.
This layer cake structure sounds complicated, and it is. But it doesn’t all have to be in place before anything can be done. The important thing is to get widespread acceptance of relevant bits. That’s how the Web spread in the first place.
Basically, all the views assume that computers will be able to read and use data that today is mainly accessible to people. All views see computers as being able to use this data to perform tasks that help people. Within this broad range, certain themes show up repeatedly, as we’ve discussed in this chapter. The rest of this book covers each of these themes as they relate to the Semantic Web.
The current Web is the one success story we have of a very large, distributed, loosely connected, inconsistent system. It seems reasonable, therefore, that the Semantic Web should use the strengths of the current Web, especially key design patterns that have made it a success. In fact, it should be an extension of the current Web.
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Created: March 27 2003
Revised: October 4, 2004
URL: http://webreference.com/internet/semantic/1
