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This section looks at ontologies today, including some of the tools that are available, some issues concerning ontologies, and the emerging Semantic Web ontology languages.
Ontology development tools are now entering the market. Most of the tools until recently were research tools, such as Ontolingua/Chimaera (McGuinness et al., 2000) and Protégé (Noy et al., 2000). Both of these tools use frame-based knowledge representation languages developed for artificial intelligence, such as the Open Knowledge Base Connectivity (OKBC) language (Chaudhri et al., 1998). Two exceptions are Cyc (Lenat and Guha, 1990, 1991), which has been a commercial product for a number of years, and OntologyWorks's tool suite; both use a first-order logic (FOL) based language, with OntologyWorks using KIF/CL (which has second-order logic extensions).
Also, the Cyc upper ontology itself is freely available. What's an upper ontology? It's an ontology (or more appropriately, a set of integrated ontologies) that tries to characterize very basic commonsense knowledge notions that humans know so well we typically don't know we know them: that is, distinctions between kinds of objects in the world, events and processes, how parts constitute a whole and what that means, and general notions of time and space.
Other newer tools for creating ontologies include the commercially available OntoEdit (http://ontoserver.aifb.uni-karlsruhe.de/ontoedit/) and the research tool OilEd (http://img.cs.man.ac.uk/oil/). Both of these tools use knowledge representation languages which are being developed as standards under the W3C (http://www.w3.org/) to support the Semantic Web. Other, more generic tools that can help build an infrastructure for ontologies include both Java and Common Lisp (e.g., Allegro Common Lisp). See our Web site at http://www.wiley.com/compbooks/daconta for additional pointers to tools.
Levels of Ontologies: Revisited
Earlier in this chapter, we looked at levels of knowledge representation. In this section we look at levels briefly again, but this time with respect to the kinds of knowledge represented at different levels within the overall content level (what we had called the ontology concept and instance levels previously). This is the level of ontologies.
Ontologies really exist at three general levels: top level, middle level, and lower domain level. At the top level, the ontological information represented
concerns primary semantic distinctions that apply to every ontology under the sun: These concern primary distinctions between tangible and intangible objects (objects that can be touched or held and those that cannot; sometimes this distinction is called that between abstract and concrete objects), the semantics of parthood (i.e., what constitutes a part and what is the nature of those relations between parts and wholes; in many cases, there are multiple notions of parthood, some transitive, some not, some with other properties that need to be specified in an ontology and then inherited downward into the medium and lower domain levels of ontology representation.
In Figure 8.11, the three general levels of ontologies are depicted. At the top is the upper ontology. This represents the common generic information that spans all ontologies. In the middle is the middle ontology. This level represents knowledge that spans domains and may not be as general at the knowledge of the upper level. Finally, the lower levels represent ontologies at the domain or subdomain level. This is typically knowledge about more or less specific subject areas. In the figure, we point out the probable electronic commerce areas of interest, though we caution: In general, electronic commerce will be interested in all the ontology levels and areas, simply because commerce involves nearly everything.
Although we do not have space here to present ontology methodologies and the ways the different levels of ontologies are designed and developed by ontological engineers, we assure you that there are such methodologies and that in fact distinct methodologies and knowledge are required for each level.
But Also This!
E-commerce Area of Interest Mostly This
Figure 8.11 Ontology levels.
In general, ontologists and semanticists can address the upper and to some extent the middle ontology levels, but domain experts have to address the domain and lower levels, since only they know the specific knowledge about their domains. They can be guided by ontologists for semantic modeling issues, and in fact, must be guided by them. But the knowledge is theirs alone, and this knowledge must be provided to ontologists to represent their domains accurately.
Emerging Semantic Web Ontology Languages
This section introduces the emerging Semantic Web languages for representing ontologies. These languages include the Resource Description Framework (RDF) and RDF Schema (when referring to both, typically the abbreviation RDF/S or RDF(S) is used); Defense Advanced Research Projects Agency (DARPA) Agent Markup Language (DAML) + Ontology Inference Layer (OIL), usually abbreviated DAML+OIL; and the Web Ontology Language (OWL). Chapter 5 provided an introduction to RDF and RDFS, so we will not focus on RDF/S here.20 Instead, we will talk primarily about DAML+OIL and OWL, both of which are the most semantically expressive languages for defining ontologies for the Semantic Web, with emphasis on OWL in particular, because it builds on and is intended to supersede DAML+OIL.