Knowledge Representation with Ontologies: The Present and Future

72 1094-7167/04/$20.00 specifications of what exists, or what we can say about the world—have been around at least since Aristotle. At various times, philosophers have wondered whether the present King of France is bald or whether existence is a predicate. Just as scientists have grappled with the reality of negative numbers, subatomic particles, or the vital force, so have theologians and mystics grappled with the reality of God and inner spiritual experiences. The nature of knowledge is an abiding question and has resulted in people’s continuous attempts to find ways to express, word, or convey their own “knowledge.” Physics and mathematics depend on specific symbolic languages, and many approaches to AI regard finding the problem’s optimal representation as most of the solution. Recently, we have seen an explosion of interest in ontologies as artifacts to represent human knowledge and as critical components in knowledge management, the Semantic Web, business-to-business applications, and several other application areas. Various research communities commonly assume that ontologies are the appropriate modeling structure for representing knowledge. However, little discussion has occurred regarding the actual range of knowledge an ontology can successfully represent. How adequate a conception of knowledge is this? Clearly, we can’t easily represent certain types of knowledge (for example, skills or distributed knowledge). We can’t easily transform certain types of representation into ontology-appropriate formats (for example, diagrammatic knowledge). Other types of knowledge are extremely suited to ontological representation, such as taxonomic information. Most, but not all, definitions of ontology insist that an ontology specifically represents common, shared conceptual structures. Does this requirement for publicity help guarantee adequacy? And if so, can we talk of personal ontologies?1 In this installment of Trends and Controversies, we bring together several practitioners to debate these issues. We have tried to secure a range of perspectives, from the philosophical to the practical, because the question of ontology is so multilayered. Indeed, we hope these essays exhibit that very multifariousness. Sociologist and epistemologist Steve Fuller kicks off our debate by distinguishing between two views of ontology, which he calls the Newtonian and the Leibnizian. The former refers to views of ontology as finding elegant simplifying principles; the latter hopes to do justice to the extreme complexity of experience. Clearly, in the context of the Semantic Web and other knowledge management contexts, the two approaches offer contrasting advantages. Elegant ontologies might be easier to manage, but scruffy ones might be easier to apply. Lining up for the scruffy, Leibnizian team is Yorick Wilks, who combines philosophy and linguistics to argue that you can’t take a lofty, unengaged view of what exists. Every ontological theory has a viewpoint and is associated with a set of interests, and through the terms used—just like any dictionary or thesaurus—involves covert ontological commitment. Newtonian abstraction will always be a chimera. Computer scientist Enrico Franconi represents the elegant Newtonians. Franconi, while subscribing to a belief in ontologies with unambiguous semantics, argues that in the absence of sound and complete inference engines, ontologies’ full formal semantics can’t be exploited. In this case, ontologies revert to being mere data structures, and Franconi sees the development of inference engines as the Semantic Web’s major challenge and therefore an essential research topic. “Ontologies with unambiguous meanings” are clear Platonic descendants as opposed to Wilks’ scruffy, relativistic, and task-dependent ontologies, which are more Aristotelean in their practicality. The question is whether the ontology used depends on what you want to do with it—in other words, the task for which it is developed. Aristotle’s followers answer “yes,” and the Platonists say “no.”2 Indeed, although beyond this introduction’s scope, it’s fascinating to muse about the relationship between Newton and Leibniz and Plato and Aristotle; are they the same distinction? And if not, how do they influence each other? Surely they can’t be orthogonal. The articles we’ve selected make many suggestive points. If ontologies are irrevocably task-relative (that is, we characterize the world differently depending on what we’re trying to do), several heterogeneous ontologies should exist. But then will the modeling overhead be too high? Mark Musen, from his experience in medical informatics, reminds us of when the