‘Ontology’ (with captial O) has a philosophical meaning and ‘ontology’ bears a knowledge engineering sense. Ofcourse, it is well-known that the term is borrowed from philosophy. The philosophers often use ontology as synonym of metaphysics. The term ‘ontology’ originated in 1613. Please see the comment for an explanation on what is ontology in philosophy..
Classification (categorization) has been the essence of knowledge representation since the times of Aristotle, Kant. Aristotle’s system of categories were—substance, quality, quantity, relation, action, passion, having, situatedness, place and time. Kant developed the framework of categorization based on the key question “what structures does our mind use to capture the reality?” According to Kant, the categories are organized into four classes, each with three sub-categories: quantity (unity, plurality, totality), quality (reality, negation, limitation), relation (inherence, causality, community), and modality (possibility, existence, necessity).
Now in the knowledge engineering model, ontology is applied to model declarative knowledge. It is really interesting to know that there are around 10 definitions for the term ‘ontology’ in the KR model. Below are the definitions:
- An ontology defines the basic terms and relations comprising the vocabulary of a topic area as well as the rules for combining terms and relations to define extensions to the vocabulary. (Neches and colleagues, 1991).
- An ontology is an explicit specification of a conceptualization. (Gruber, 1993).
- Ontologies are defined as a formal specification of a shared conceptualization. (Borst, 1997).
- An ontology is a formal, explicit specification of a shared conceptualization. (Studer and colleagues, 1998).
- Ontology as— a philosophical discipline; an informal conceptual system; a formal semantic account;a specification of a conceptualization; a representation of a conceptual system via a logical theory (characterized by specific formal properties, characterized only by its specific purposes); the vocabulary used by a logical theory; a meta-level specification of a logical theory. (Guarino and Giaretta, 1995).
- A logical theory which gives an explicit, partial account of a conceptualization. (Guarino and Giaretta, 1995).
- A set of logical axioms designed to account for the intended meaning of a vocabulary. (Guarino, 1998).
- An ontology provides the means for describing explicitly the conceptualization behind the knowledge represented in a knowledge base. (Bernaras and colleagues, 1996).
- An ontology is a hierarchically structured set of terms for describing a domain that can be used as a skeletal foundation for a knowledge base. (Swartout, 1997).
- An ontology may take a variety of forms, but it will necessarily include a vocabulary of terms and some specification of their meaning. (Uschold and Jasper, 1999).
Ontological Engineering — Asuncion Gomez-Perez, Mariano Fernandez-Lopez, Oscar Corcho
Ontology — Barry Smith, Chapter in in L. Floridi (ed.), Blackwell Guide to the Philosophy of Computing and Information, Oxford: Blackwell, 2003, 155–166.
Knowledge Representation — John Sowa