Applications of Ontology


John F. Sowa

Abstract:  For centuries, philosophers have sought universal categories for classifying everything that exists, lexicographers have sought universal terminologies for defining everything that can be said, and librarians have sought universal headings for storing and retrieving everything that has been written. Today, the semantic web has extended the task to the level of classifying, labeling, defining, finding, integrating, and using everything on the World Wide Web, which is rapidly becoming the universal repository for all the accumulated knowledge, information, data, and garbage of humankind. This talk surveys the issues involved, the approaches that have been successfully applied to small systems, and the ongoing efforts to extend them to distributed, interconnected, rapidly growing, heterogeneous systems.



Aristotle's Ontology

Aristotle's categories as arranged by Franz Brentano


Aristotle's Logic

Based on four sentence patterns:

Type Name Pattern
A Universal affirmative Every A is B.
I Particular affirmative Some A is B.
E Universal negative No A is B.
O Particular negative Some A is not B.


Aristotle's Syllogisms

First four inference patterns:

A:  Every animal is material.
A:  Every human is an animal.
A:  \ Every human is material.
E:  No spirit is a body.
A:  Every human is a body.
E:  \ No spirit is a human.
A:  Every beast is irrational.
I:  Some animal is a beast.
I:  \ Some animal is irrational.
E:  No plant is rational.
I:  Some body is a plant.
O:  \ Some body is not rational.


Using Logic with Ontology

Tree of Porphyry as drawn by Peter of Spain (1329)


Evaluation of Aristotle's System


  • Broad coverage:  every field of science and philosophy, including ethics, esthetics, and politics.

  • First system of formal logic (syllogisms).

  • Definitions by genus and differentiae.

  • Logic for building and analyzing ontology.


  • Obsolete physics.

  • Limited logic.

  • Limited form of definitions.

  • No support for revisions and exceptions.


Large Hand-Coded Ontologies

Three large ontologies:

  • Cyc: 100,000 concept types with over a million axioms.

  • Electronic Dictionary Research (EDR): 400,000 concept types with mappings to English and Japanese words.

  • WordNet: 166,000 English word senses with related projects for other languages.

Building these things requires a great deal of time and money.


Largest Formal Ontology

Top-level categories used in Cyc


Small Hand-Coded Ontologies

Can such simple systems coexist peacefully with the grand ontologies?


The Semantic Web

  • An attempt to add more semantics to the WWW.

  • Based on the Resource Definition Facility (RDF).

  • Specifes the types of resources and the types of relations that link them.

  • Uses a subset of logic (DAML + OIL) to define types and relations.


Ontology is a Prerequisite for RDF

Comment by Tim Bray (1998):

It seems unlikely that one PropertyType standing by itself is apt to be very useful. It is expected that these will come in packages; for example, a set of basic bibliographic PropertyTypes like Author, Title, Date, and so on. Then a more elaborate set from OCLC, and a competing one from the Library of Congress. These packages are called Vocabularies; it's easy to imagine PropertyType vocabularies describing books, videos, pizza joints, fine wines, mutual funds, and many other species of Web wildlife.

But Bray thinks that there will be many incompatible ontologies:

Nobody thinks that everyone will use the same vocabulary (nor should they), but with RDF we can have a marketplace in vocabularies. Anyone can invent them, advertise them, and sell them. The good (or best-marketed) ones will survive and prosper. Probably, most niches of information will come to be dominated by a small number of vocabularies, the way that library catalogues are today.


Multiple Standards  =  No Standards

Computers can make it more difficult to share information:

  • There are already thousands, if not millions of competing vocabularies. The tables and fields of every database and the lists of items in every product catalog for every business in the world constitute incompatible vocabularies.

  • When product catalogs were distributed on paper, any engineer or contractor could read the catalogs from different vendors and compare the specifications.

  • But minor variations in the terminology of computerized catalogs can make it impossible for a computer system to compare components from different vendors.



  • The subject of ontology is the study of the categories of things that exist or may exist in some domain.

  • The product of such a study, called an ontology, is a catalog of the types of things that are assumed to exist in a domain of interest D from the perspective of a person who uses a language L for the purpose of talking about D.

  • The types in the ontology represent the predicates, word senses, or concept and relation types of the language L when used to discuss topics in the domain D.

  • By itself, logic says nothing about anything, but the combination of logic with an ontology provides a language that can express relationships about the entities in the domain of interest.



For the slides used in this talk, see

For more detail, see the guided tour of ontology:

All references are in the bibliography:

Copyright ©2001, John F. Sowa