Ontological Engineering: What Are Ontologies and How Can We Build Them?
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44 Chapter III Ontological Engineering: What Are Ontologies and How Can We Build Them? Oscar Corcho University of Manchester, UK Mariano Fernández-López Universidad San Pablo CEU and Universidad Politécnica de Madrid, Spain Asunción Gómez-Pérez Universidad Politécnica de Madrid, Spain ABSTRACT Ontologies are formal, explicit specifications of shared conceptualizations. There is much literature on what they are, how they can be engineered and where they can be used inside applications. All these literature can be grouped under the term “Ontological Engineering,” which is defined as the set of ac- tivities that concern the ontology development process, the ontology lifecycle, the principles, methods and methodologies for building ontologies, and the tool suites and languages that support them. In this chapter we provide an overview of Ontological Engineering, describing the current trends, issues and problems. INTRODUCTION the knowledge bases upon which they are based did not have to be constructed from scratch, but The origin of ontologies in computer science by assembling reusable components. This reuse can be referred back to 1991, in the context of applies both to static knowledge, which is modeled the DARPA Knowledge Sharing Effort (Neches, by means of ontologies, and dynamic problem- Fikes, Finin, Gruber, Senator, & Swartout, 1991). solving knowledge, which is modeled by means The aim of this project was to devise new ways of of problem solving methods. constructing knowledge-based systems, so that Copyright © 2007, Idea Group Inc., distributing in print or electronic forms without written permission of IGI is prohibited. Ontological Engineering Since then, considerable progress has been WHAT IS AN ONTOLOGY AND made in this area. Ontologies are now considered WHICH ARE ITS COMPONENTS? as a commodity that can be used for the develop- ment of a large number of applications in different There are two different views about the use of the fields, such as knowledge management, natural term “ontology,” considering whether the person language processing, e-commerce, intelligent who uses that term is interested in its philosophical integration information, information retrieval, roots or in its application to Computer Science. database design and integration, bio-informatics, For philosophers, the term Ontology (normally education, and so forth. typed with uppercase) refers to the “the essence The emergence of the Semantic Web (Bern- of things through the changes.” Greek philoso- ers-Lee, 1999) has caused a growing need for phers, from Parmenides of Elea to Aristotle, were knowledge reuse, and has strenghtened its poten- interested in these aspects. In the 18th century, tial at the same time. Therefore, ontologies and Kant worked also on these ideas. More recently, problem-solving methods (which in some cases people working in the area of formal ontologies are considered as the precursors of Semantic are also interested in these philosophical ideas Web Services) are playing an important role in and its application in the context of Computer this context. Science. As described in the chapter title, we will pres- On the other side, ontology engineers in the ent the what and how of ontologies, describing context of computer science are more interested the activities that should be carried out during the in how ontologies (typed with lowercase) can be ontology development process, the principles to used to represent reusable and sharable pieces of be followed in ontology design, and the methods, domain knowledge and how they can be used in ap- methodologies, software tools and languages that plications. In this context, ontologies are reusable give support to each one of these activities. The and sharable artifacts that have to be developed second section defines the word “ontology” and in a machine interpretable language ��������(Gruber, explains which are the main components that can 1993; Studer, ��������������������������������Benjamins, & Fensel�������������, 1998)������. This be used to model ontologies. The third section point of view is clearly addressed in the defini- focuses on methods and methodologies for the tion given by Studer and colleagues (1998): An development of ontologies, either used for the ontology is a formal, explicit specification of a whole ontology development process or only for shared conceptualization. We consider that this specific activities. The fourth section focuses on definition is one of the most complete ones from ontology tools, which normally give support to those available in the literature. the previous methodological approaches. The Once we have analysed these different defini- fifth section describes ontology languages that tions of the term “ontology,” we will focus on the can be used to implement ontologies. All these second use of this term, that is, on what is normally sections are structured in a similar way: first we known as Ontological Engineering (Gómez-Pérez, give a brief overview of their evolution, then we Fernández-López, & Corcho����������������������, 2003). First we will describe the current trends, and finally we pay discuss about the components that are used to attention to the open issues and practical aspects. create an ontology. Finally, conclusions and future lines of work are Different knowledge representation formal- presented in the last section. isms (and corresponding languages) exist for the 45 Ontological Engineering fomalisation (and implementation) of ontologies. According to Gruber (1993), formal axioms Each of them provides different components that serve to model sentences that are always true. can be used for these tasks. However, they share They are normally used to represent knowledge the following minimal set of components.1 that cannot be formally defined by the other Classes represent concepts, which are taken components. In addition, formal axioms are used in a broad sense. For instance, in the traveling to verify the consistency of the ontology itself domain, concepts are: locations (cities, vil- or the consistency of the knowledge stored in a lages, etc.), lodgings (hotels, camping, etc.) and knowledge base. Formal axioms are very useful means of transport (planes, trains, cars, ferries, to infer new knowledge. An axiom in the traveling motorbikes and ships). Classes in the ontology domain would be that it is not possible to travel are usually organised in taxonomies through from the America to Europe by train. which inheritance mechanisms can be applied. Instances are used to represent elements or We can represent a taxonomy of entertainment individuals in an ontology. An example of instance places (theater, cinema, concert, etc.) or travel of the concept AA7462 is the flight AA7462 that packages (economy travel, business travel, etc.). arrives at Seattle on February 8, 2006 and costs In the frame-based KR paradigm, metaclasses 300 (US Dollars, Euros, or any other currency). can also be defined. Metaclasses are classes Besides formalisms and languages specifically whose instances are classes. They usually allow designed for representing knowledge, ontologies for gradations of meaning, since they establish can be formalised with other approaches coming different layers of classes in the ontology where from the areas of Software Engineering, such as they are defined. the Unified Modeling Language (UML) (Rum- Relations represent a type of association be- baugh, Jacobson, & Booch, 1998) or Entity-Re- tween concepts of the domain. They are formally lationship (ER) Diagrams (Chen, 1976). defined as any subset of a product of n sets, that In this context, the Object Management Group is: R ⊂ C1 x C2 x ... x Cn. Ontologies usually (OMG)2 is working on a specification to define the contain binary relations. The first argument is meta-models of some of the diagram types and known as the domain of the relation, and the languages used in ontology representation. This second argument is the range. For instance, the specification is known as ontology description binary relation arrivalPlace has the concept Travel model (ODM, 2005), and uses a common formal as its domain and the concept Location as its range. notation to describe the metamodels. Such meta- Relations can be instantiated with knowledge from models (defined for UML, Entity-Relationship, the domain. For example, to express that the flight OWL, RDF(S), etc.) can be considered formalisa- AA7462-Feb-08-2002 arrives in Seattle we must tions of knowledge representation ontologies. All write: (arrivalPlace AA7462-Feb-08-2002 Seattle) these correspondences are formally described in Binary relations are sometimes used to ex- the ODM document (ODM, 2005). press concept attributes (i.e., slots). Attributes The purpose of ODM documents is to allow are usually distinguished from relations because software engineers to model ontologies with their range is a datatype, such as string, number, familiar notations for them, for example, UML and so forth, while the range of relations is a and ER, and to transform their conceptual models concept. The following code defines the attribute into formal ontologies represented in ontology flightNumber, which is a string. We can also express languages. relations of higher arity, such as “a road connects two different cities.” 46 Ontological Engineering METHODS AND METHODOLOGIES 1994, 1996, 2001, 2004; Guarino, 2004; Gua- FOR THE DEVELOPMENT OF rino & Welty, 2002; Kalfoglou & Robertson, ONTOLOGIES 1999a, 1999b; Welty & Guarino, 2001), ontology evolution (Klein & Fensel,