Reasoning on the Web

The techniques that we can use to construct rational agents were presented in Chapters 3 and 4. In these chapters, we identified a variety of general purpose reasoning techniques that can be used by agents to accomplish goal-directed behaviour. In this chapter, we turn our attention to the construction of agent-based reasoning processes that are directed towards the Semantic Web. These processes are based on the general techniques that we described previously, but are tailored specifically for the Semantic Web. In particular, our reasoning processes are designed to operate directly on Semantic Web knowledge expressed in RDF, RDFS, and OWL documents. These reasoning processes are essentially specialized kinds of deductive reasoning systems. It is important to appreciate that the reasoning processes that we describe in this chapter have certain limitations. For each technique, we will only be able to solve certain classes of problems. These limitations are a direct consequence of the representation that we use. Thus, it is necessary that we understand what kinds of problems we can solve in each approach, as this will determine the kinds of Semantic Web applications that we can construct. In effect, we are seeking to answer two key questions: 1. What kinds of reasoning can we perform with our knowledge? 2. How do we specify the problems that we wish to solve? The first question concerns the representation of the knowledge. In Chapter 2 we stated that there is a trade-off between expressibility and efficient reasoning. In general, the more features that we have in the representation language, the more difficult it is to reason with the language. The second question concerns the definition of the reasoning process itself. As we show in this chapter, there is a further trade-off between specification complexity and reasoning power. In general, the more complex our specification formalism, the more difficult it is to reason efficiently. In this chapter, we discuss two main approaches that we can use to define reasoning processes for the Semantic Web: query languages and logic-based formalisms.