Re: Principles of ontology

Michael Uschold (
Fri, 5 Dec 1997 16:35:32 -0800


The relationship between an Ontology and a Knowledge Base has long been a
source of confusion. Michael Gruninger and I have taught an introductory
tutorial on ontologies for the past few years, and this question always comes
up and is never satisfactorily resolved. John's comments about this brings up
only some of the relavent issues:

KNOWLEDGE BASE. An informal term for a collection of information that
includes an ontology as one component. Besides an ontology,
a knowledge base may contain information specified in a
declarative language such as logic or expert-system rules, but
it may also include unstructured or unformalized information
expressed in natural language or procedural code.

First, a knowledge base may or may not have an explicit ontology in it or
associated with it. You can say every KB *has* one, but often it is only
implicit, and thus more usually referred to as a conceptualisation, not an
ontology. So I do not think it is a good idea to define a Knowledge Base in
terms of an ontology. A very simple and hopefully non-controversial
definition: of a Knowledge Base is:

KNOWLEDGE BASE: a set of sentences in some knowledge representation language.

I believe these is consistent with the Norvis/Russel AI text. More
clarification can then be given about the relationship between a knowledge base
and an ontology.

Very important, is to consider the *purpose* of an ontology -- this seems to
give rise to different relationships between the ontology and the knowledge
base. Purposes vary widely. This has been addressed in some detail in a paper
"Knowledge Level Modelling: Concepts and Terminology" to come out in the Jan 98
issue of Knowledge Engineering Review. It is a glossary of terms, based on a
group effort with several European partners and was peer-reviewed.

In it we attempt to identify the most important concepts in the areas of
ontologies and problem solving models. We identify terms for these concepts,
and define them to reflect the most common usage; we also note where
differences and conflicts arise.

For the most part, John's terms and definitions seem to be compatible with
those in this paper (as it should be), as well as offering further
clarification reflecting the growing understanding of these issues. Below are
a few annotated quotes from that paper. Pardon the .tex format

We identify *purpose* as one of a few dimensions along which ontologies vary
(another is 'degree of formality' which John addresses)

\paragraph{{\sc Purpose ---}}

The literature is currently rich with descriptions of ontologies and their
intended purposes. At a high level, most seem to be intended for some manner
of reuse. Some of these purposes are implicit in the various interpretations
of the word `ontology' that are commonly found in the literature, as noted in
\cite{guarino95}; ({\em e.g.} a vocabulary for \cite{gruber_93} vs a meta-level
specification of, a logical theory \cite{KACTUS-O-word,wielinga94}).

Other factors include the nature of the software with which the ontology will
be used, whether it is intended to be shared within a small group and reused
within that context for a variety of applications, or whether it is intended to
be reused by a larger community. Some view their ontologies mainly as a means
to structure a knowledge base; others conceive an ontology to be used as part
of a knowledge base, {\em e.g.} by loading it in as a set of sentences which
will be added to as appropriate; still others view their ontology as an
application-specific inter-lingua (e.g. ATOS \cite{fuchs95,jones_95} and the
Enterprise Ontology \cite{uschold98}).

On the topic of Ontologies and Knowledge Bases in particular, we say:

There is an important distinction between a {\em language} for expressing
knowledge, and the knowledge itself, as expressed in the language. Some
ontologies are conceived and used as the vocabulary with which a knowledge base
will be specified [this is the situation I belive John is referring to, the
point I'm making is that this is not the *only* relationship between an
ontology and a KB]. For example, a medical ontology might include terms for
disease, symptom, treatment etc. A particular knowledge base built using these
terms might include a taxonomy of particular diseases, examples of treatments,
etc. In this situation, one often refers to the ontology {\em of} or {\em for}
the knowledge base.

KNOWLEDGE BASE, if both are specified in the same language. Furthermore, {\em
it does not matter!} The difference may be merely one of what part of the
knowledge base is shared and agreed on, and what part is more specific.
Furthermore, this may change over time.


Mike Uschold


N.~Guarino and P~Giaretta.
\newblock Ontologies and knowledge bases- towards a terminological
\newblock In N.J. Mars, editor, {\em Towards Very Large Knowledge Bases -
Knowledge Building and Knowledge Sharing 1995}, pages 25--32. IOS Press,
Amsterdam, 1995.

Fuchs. J. and J.~Wheadon.
\newblock Prospective applications of ontologies for future space missions.
\newblock In {\em The Impact of Ontologies on Reuse, Interoperability and
Distributed Processing}, pages 83--96. Unicom Seminars, London, 1995.

M.~Jones, J.~Wheadon, D.~Whitgift, M.~Niezatte, R.~Timmermans, I.~Rodriguez,
and R.~Romero.
\newblock An agent based approach to spacecraft mission operations.
\newblock In N.J. Mars, editor, {\em Towards Very Large Knowledges Bases -
Knowledge Building and Knowledge Sharing 1995}, pages 259--269. IOS Press,
Amsterdam, 1995.

\newblock A translation approach to portable ontology specifications.
\newblock {\em Knowledge Acquisition}, 5(2):199--220, 1993.

G.~Schreiber, B.~Wielinga, and W.~Jansweijer.
\newblock The kactus view on the `o' word.
\newblock In {\em Workshop on Basic Ontological Issues in Knowledge Sharing}.
International Joint Conference on Artificial Intelligence, 1995.

R.~Wielinga, G.~Schreiber, W.~Jansweijer, A.~Anjewierden, and F.~van Hamelen.
\newblock Framework and formalism for expressing ontologies.
\newblock Technical report, University of Amsterdam, 1994.
\newblock Esprit Project 8145 Deliverable DO1b1, available from

M.~Uschold, M.~King, S.~Moralee, and Y.~Zorgios.
\newblock The enterprise ontology.
\newblock {\em Knowledge Engineering Review}, 13(1), 1998.
\newblock Also available as AIAI-TR-195 from AIAI, The University of Edinburgh.
This ontology was developed as part of the Enterprise Project, see {\em}$\sim${\em entprise/enterprise/} for further