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ISBN 0-471-43257-1
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Classification. We classify things to establish groupings by which generalizations can be made. Just as we classify files on our personal computer in a directory structure, we will continue to better classify resources on corporate intranets and even the Internet. Chapter 7 discusses taxonomy concepts and specific taxonomy models like XML Topic Maps (XTM). The concepts for classification have been around a long time. Carolus Linnaeus developed a classification system for biological organisms in 1758. An example is displayed in Figure 1.4.
The downside of classification systems is evident when examining different people's filesystem classification on their personal computers. Categories (or folder names) can be arbitrary, and the membership criteria for categories are often ambiguous. Thus, while taxonomies are extremely useful
6Tim Berners-Lee, Weaving the Web, Harper San Francisco, p. 13.
What Is the Semantic Web?
Felis domesticus
Figure 1.4 Linnaean classification of a house cat.
for humans browsing for information, they lack rigorous logic for machines to make inferences from. That is the central difference between taxonomies and ontologies (discussed next).
Formal class models. A formal representation of classes and relationships between classes to enable inference requires rigorous formalisms even beyond conventions used in current object-oriented programming languages like Java and C#. Ontologies are used to represent such formal class hierarchies, constrained properties, and relations between classes.
The W3C is developing a Web Ontology Language (abbreviated as OWL). Ontologies are discussed in detail in Chapter 8, and Figure 1.5 is an illustrative example of the key components of an ontology. (Keep in mind that the figure does not contain enough formalisms to represent a true ontology. The diagram is only illustrative, and a more precise description is provided in Chapter 8.)
Figure 1.5 shows several classes (Person, Leader, Image, etc.), a few properties of the class Person (birthdate, gender), and relations between classes (knows, is-A, leads, etc.). Again, while not nearly a complete ontology, the purpose of Figure 1.5 is to demonstrate how an ontology captures logical information in a manner that can allow inference. For example, if John is identified as a Leader, you can infer than John is a person and that John may lead an organization. Additionally, you may be interested in questioning any other person that "knows" John. Or you may want to know if John is depicted in the same image as another person (also known as co-depiction). It is important to state that the concepts described so far (classes, subclasses, properties) are not rigorous enough for inference.
To each of these basic concepts, additional formalisms are added. For example, a property can be further specialized as a symmetric property or a transitive property. Here are the rules that define those formalisms:
If x = y, then y = x. (symmetric property)
If x = y and y = z, then x = z. (transitive property)
Chapter 1
Figure 1.5 Key ontology components.
An example of a transitive property is "has Ancestor." Here is how the rule applies to the "has Ancestor" property:
If Joe hasAncestor Sam and Sam hasAncestor Jill, then Joe hasAncestor Jill.
Lastly, the Web ontology language being developed by the W3C will have a UML presentation profile as illustrated in Figure 1.6.
The wide availability of commercial and open source UML tools in addition to the familiarity of most programmers with UML will simplify the creation of ontologies. Therefore, a UML profile for OWL will significantly expand the number of potential ontologists.
Rules. With XML, RDF, and inference rules, the Web can be transformed from a collection of documents into a knowledge base. An inference rule allows you to derive conclusions from a set of premises. A well-known logic rule called "modus ponens" states the following:
If P is TRUE, then Q is TRUE.
P is TRUE.
Therefore, Q is TRUE.
Figure 1.6 UML presentation of ontology class and subclasses.
What Is the Semantic Web?
An example of modus ponens is as follows:
An apple is tasty if it is not cooked. This apple is not cooked. Therefore, it is tasty.
The Semantic Web can use information in an ontology with logic rules to infer new information. Let's look at a common genealogical example of how to infer the "uncle" relation as depicted in Figure 1.7:
If a person C is a male and childOf a person A, then person C is a "sonOf" person A.
If a person B is a male and siblingOf a person A, then person B is a "brotherOf" person A.
If a person C is a "sonOf" person A, and person B is a "brotherOf" person A, then person B is the "uncleOf" person C.
Aaron Swartz suggests a more business-oriented application of this. He writes, "Let's say one company decides that if someone sells more than 100 of our products, then they are a member of the Super Salesman club.