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1. Modelling techniques
2. Ontology elements
3. Ontology principles
4. Ontology types
According to Gomez-Perez (2004), the definition of Ontology has evolved over the last twenty years. In 1995, Guarino analyzed seven definitions about this concept to propose a new one. This author defines ontology as “a set of logical axioms designed to account for the intended meaning of a vocabulary” (Guarino, 1998).
One of the reasons for disagreement is the broad definition that has been proposed to group all current ontologies. Wikipedia defines ontology as a “Formal representation of a set of concepts within a domain and the relationships between those concepts”. This definition might well describe any other Knowledge Organization System or terminological resource. Wikipedia adds that ontologies are “used to reason about the properties of that domain, and may be used to define the domain”.
The best known definition was proposed by Gruber: “a formal, explicit specification of a shared conceptualization.” (1991).
1. Modeling techniques
The two most commonly used techniques are:
2. Ontology elements
Depending on the technique that has been used, the vocabulary for designing some elements might differ.
To achieve inference, the existence of true assertions has to be established: these assertions are then used to express restrictions, rules and axioms.
Finally, events are a way to represent how the values of attributes and their relationships might change.
First order logic usually names these elements as: classes, relations, attributes (slots), functions, instances, and axioms.
Description Logic has the following elements: concepts (equivalent to classes); roles (equivalent to relations and properties of concepts); and Individuals (equivalent to the instances of concepts and their properties).
3. Principles for the design of Ontologies
To be able to share knowledge, interoperability is required. Many principles have been proposed (Gruber 1993):
4. Types of Ontologies
There are different types of ontologies:
Regarding the usability and reusability, most abstract ontologies are highly reusable (e.g. Knowledge Representation Ontologies and Upper Ontologies) but their usability is poor. Application and Domain Ontologies have a low reusability but a high level of usability.
Modeling techniques use different languages. As an example, one of the languages related to first order logic is KIF. Whereas OWL is the language widely used to represent ontologies on the Web. OWL serialization is based on RDF/RDFS.
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Jorge Morato (27/9/2009)
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