Summary

In developing the PHYSSYS ontology we found that constructing an ontology from smaller ontologies leads to an ontology that because of its structure is easy to understand and well suited for reuse. Three types of ontologies have been distinguished:

`Super'theories

which are general and abstract ontologies such as mereology, topology, systems theory.

Viewpoint or base ontologies

that formalize a conceptual category of concepts in a domain. For the physical domain at least three of such categories exist: that of a configuration of components, physical processes underlying behaviour and the engineering mathematics that is used to describe the processes.

Domain ontologies

that form an integral and coherent conceptualization of a domain. The conceptualization of the domain of physical systems offered by PHYSSYS is nothing other than a combination of the three viewpoints plus the formalization of the interdependencies between the concepts in different viewpoints.

To construct a large ontology from smaller ontologies, the dependencies between concepts and relations in different ontologies are formalized as ontology projections. Three types of ontology projections were used and are named according to the way they can be implemented.

Include and extend:

An imported ontology is extended with new concepts and relations. The result has the same level of abstraction as the included ontology. An example is the extension of mereology to topology that was described in this section.

Include and specialize:

An abstract theory is imported and applied to the contents of the importing ontology. Doing this, abstract concepts are specialized. An abstract `super'theory can be considered generic when there are many useful specializations that can be made. For instance, systems theory is used twice in PHYSSYS. It is used as an abstraction of system components as well as an abstraction of physical process descriptions.

Include and project:

Different viewpoints on a domain are joined by including the views in the domain ontology and formalization of their interdependencies. In contrast to the previous ontology projections, these projections contain a great deal of domain knowledge and can therefore be considered to be ontologies of their own.