OntoClean is a methodology for analyzing ontologies based on formal, domain-independent properties of classes (the metaproperties) developed by Nicola Guarino and Chris Welty.
Overview and History OntoClean was the first attempt to formalize notions of ontological analysis for information systems. The idea was to justify the kinds of decisions that experienced ontology builders make, and explain the common mistakes of the inexperienced. Alan Rector, during a debate at the KR-2002 conference in Toulouse, said, "What you have done is reduce the amount of time I spend arguing with medics." The notions Guarino & Welty focused on were drawn from philosophical ontology. They were not after the seemingly endless arguments about what the right ontology of the universe is, but rather the techniques these philosophers use to analyze, support, and criticize each other's arguments. These techniques make very little, if any, commitment to a particular ontology, instead they expose what are often very subtle distinctions. The ideas underlying OntoClean appeared first in the literature in a series of three papers published in 2000. The name OntoClean does not appear in the literature until 2002. According to Thompson-ISI, work on OntoClean was the most cited of academic papers on Ontology. OntoClean was important as it was the first formal methodology for ontology engineering, applying scientific principles to a field whose practice was mostly art.
Note on terminology In logic, a property is a unary predicate in intension, in other words a property is what it means to be a member of a class. For example, we say that instances of the Person class have the property of "being a person." In the semantic web, a property is a binary relation. The distinction between property and class is subtle, and probably not critical to understanding OntoClean, however this article, follows the OntoClean publications and consistently uses "property" according to its original meaning, and one can treat "property" and "class" as synonymous. Thus a metaproperty is a property of a property or class.
Metaproperties The basis of OntoClean are the domain-independent properties of classes, the OntoClean metaproperties: identity, unity, rigidity, and dependence. Later work by Welty & Andersen has added two more metaproperties: permanence and actuality.
Identity Identity is fundamental to ontology, and especially to information systems ontologies. Identity is well known in metaphysics and in database conceptual modeling. In the latter case, it is an accepted best practice to specify a primary key for rows in a table. If "two" rows have identical primary keys, they are considered the same row. More importantly for ontology are questions of identity that expose the existence of, or at least the need to represent, other entities. Here the issue at stake is finding the conditions under which a proposed entity would be both the same and different. The classic example is an amount of clay that is shaped into a statue. If you use the same clay but reshape it into a different statue, is it the same entity? If so, how could it be different? If not, how could it be the same. In conceptual modeling, it is understood that when such an ambiguity arises, one should treat it as two different entities to account for a situation where one changes and the other stays the same. In OntoClean, identity criteria are associated with, or carried by, some classes of entities, called sortals. A sortal is a class all of whose instances are identified in the same way. In information systems, these criteria are often extrinsic, like a social security number or universally unique id, which is not interesting from an ontological point of view. Identity criteria should be informative, they should help us and others understand what a class means. A triangle, for example, can be identified by the length of its three sides, or by two sides and an interior angle, etc. This says a lot about what is intended by the triangle class here, e.g. the same triangle could be in many places at the same time. Someone else may have an ontology in which the triangle class has different identity criteria, such that different drawings are always different triangles, even if they are the same size. Identity criteria (and OntoClean, for that matter) do not tell you that one of these definitions of triangle is right or wrong, just that they are different and thus that the classes are different. Identity criteria and sortals are intuitively meant to account for the linguistic habit of associating identity with certain classes. In the classical statue and clay example, we naturally say "the same clay" or "the same statue", indicating that there are identity criteria that are peculiar to each class. Being a sortal is the first OntoClean metaproperty, indicated with the +I superscript (−I for non-sortals) on a class in the original notation. +I (but not −I) is inherited down the class hierarchy, if a class is a sortal then all its subclasses are as well.
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