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The first definition dealt with describing and representing an area of knowledge. What about the second definition of ontology? What does the specification of a conceptualization mean? Let's try to clarify that definition by referring to the different parts of the first definition.3
A conceptualization is a way of thinking about part of the world. When we conceive of the world or a part of the world, we have in mind, literally, a mental model of that part of the world. For example, when we conceive of automobile repair, we probably have a set of mental images of automobiles, their subsystems and parts, an automobile garage or repair shop, mechanics in uniforms, and so on. If we were to describe these images, we would probably do so according to the first definition—in terms of the things that are important to the notion of automobile repair, and their properties and relationships. Given a particular way of thinking about a part of the world (a subject area or domain), in other words, a conceptualization (we conceive it to be this way or that way, and not some other way), when we seek to describe it to ourselves or another person in a fairly detailed and precise way, we say we are specifying it.
Table 8.1 displays some of the key terminology you'll learn about in this chapter, along with shorthand definitions. You may want to refer back to this table periodically as you encounter one of the terms in the text.
3See Guarino and Giaretta (1995) for elaboration of various definitions of ontology.
Table 8.1 Terminology
Common Logic (CL) Cyc and OpenCyc
Frame-based knowledge representation
The name for the ISO standard knowledge representation language based on KIF.
The first ontology-based knowledge representation system, whose development began in 1984 at the Microelectronics & Computer Technology Corporation (MCC) in Austin, Texas, and that is now commercialized by Cycorp (http:// www.cyc.com/). Cyc is a repository of machine-interpretable commonsense knowledge represented as mostly first-order predicate logic-based ontologies (with some second-order logic extensions) and a reasoning engine. OpenCyc is the open source version of the Cyc technology (http://www.opencyc.org/).
DARPA (Defense Advanced Research Program) Agent Markup Language-Ontology Inference Layer: These are two XML- and Web-based languages to support the Semantic Web, which have recently fused. DAML originated from a US DARPA-sponsored program; OIL originated from a European Union-sponsored program. Together they constitute the most semantically expressive language available for WWW documents. The combined language is now supported by the W3C Web standards consortium.
A knowledge representation formalism (sometimes called a terminological logic, classification logic, concept logic, or term subsumption logic) based on a subset of first-order predicate logic that is a declarative formalism for the representation and expression of knowledge and sound, tractable reasoning methods founded on a firm theoretical (logical) basis.
A knowledge representation formalism for expressing ontological information derived originally from the artificial intelligence (AI) language called KL-1, which itself is one of the earliest formalizations of the notion of semantic network. The notion of a frame comes from the early LISP programming language terminology used by early KR languages. In frame terminology, a concept is a class, and a relation is a slot. Attributes (sometimes called properties) are just slots defined on a domain (a specific class subtree) or one of its subdomains (a subclass of a domain class).
Table 8.1 (continued)
Knowledge Interchange Format (KIF) This is a knowledge representation language developed prior to the emergence of the Semantic Web. KIF is based on first-order predicate logic, with a slight extension into second-order logic (because it includes quantifying over sequence variables, i.e., predicates in a sequence). KIF has a LISP-like syntax. There is now an ISO-KIF standard called Common Logic (CL). CL has multiple syntaxes, including an XML and a Conceptual Graph syntax.
Open Knowledge Base Connectivity (OKBC) language This is a language for knowledge access and interchange (an API) derived from the Generic Frame Protocol, developed in the early 1990s by knowledge representation technologists under the support of the DARPA Knowledge Sharing Effort (Patil et al., 1992). This protocol became the OKBC under the support of the DARPA High Performance Knowledge Base (HPKB) program, 1996-1999 (Cohen et al., 1998).
Ontolingua A knowledge representation language based on KIF that is used for expressing ontologies (http:// www.ksl.stanford.edu/software/ontolingua/). See Gruber (1993). Ontolingua was the first ontology language. It is currently also an ontology system with reasoning methods and is supported by the Knowledge Systems Laboratory at Stanford University.