what is an ontology

‘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


4 Responses to what is an ontology

  1. gnowgi says:

    The following statement is too naive and incorrect.

    “In philosophy, its meaning is quite plain and simple—a systematic classification of entities of all beings.” I never knew that any thing in philosophy can be quite plain and simple.


  2. Ya, may be i should state that in philosophy it has atleast one meaning, but in KR it has around 10 definitions.


  3. gnowgi says:

    this is not correct either. there is no one meaning for ontology in
    philosophy. different philosophers gave different interpretations to
    it. plato, aristotle, leibniz, kant, husserl, hegel, bolzano, and so
    on … each of them considered and modeled the world in a different
    way. what exists? and what does’nt exist? what is a phenomena and
    what is an appearance? what is apparant and what is real? how many
    basic kinds of things are there? what is real and what is not real?
    what are basic and what are derived? what are primary and what are secondary?
    is the world made of universals
    or particulars? etc. when you try to answer these questions, each
    philosopher gives a different view point, most times, each of them are
    coherent enough that it builds a world view. that is what is roughly
    the engagement in philosophy in the name of ontology.

  4. hmm thanx nagarjuna for the explanation.


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