OntoClean is a methodology for analyzing ontologies based on formal, domain-independent properties of classes (the metaproperties) developed by Nicola Guarino and Chris Welty.
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."
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.
[5] OntoClean was important as it was the first formal methodology for ontology engineering, applying scientific principles to a field whose practice was mostly art.
More importantly for ontology are questions of identity that expose the existence of, or at least the need to represent, other entities.
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.
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 (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.
Being a sortal is the first OntoClean metaproperty, indicated with the +I superscript (−I for non-sortals) on a class in the original notation.
In formal ontology, wholes are often distinguished from mere sums, which are individuals whose boundaries are, in a sense, arbitrary.
Unity is the metaproperty, indicated by +U, of classes all of whose individuals are wholes under the same relation.
Imagine an ontology of mystical beliefs, for example, in which an entity changes from Person to Spirit upon death.
In the core OntoClean papers, Guarino & Welty used a kind of dependence that captures a meta-property of certain relational roles.