At this year’s Taxonomy Boot Camp conference, I was invited to present on the panel giving 5-minute “Pecha Kucha” lightning talks, for which this year’s theme was ontology. Just as there are different understandings and usages of “taxonomy,” so are there different understandings and usages of “ontology.” You can come to if from different angles. If you come to ontologies from the experience of taxonomies and the field of information management, then, most simply, an ontology is a more complex type of taxonomy that contains richer information.
In my brief presentation, “From Accidental Taxonomist to Accidental Ontologist,” I summed up the differences between taxonomies and ontologies as follows:
- Relationships: Taxonomies have hierarchical and sometimes a simple “related term” associative, but ontologies have semantic relationships, which are custom-created.
- Term Attributes: Taxonomies generally don’t have term attributes, but ontologies do.
- Term Classes: Taxonomies generally don’t have classes for terms, unless you consider facets as classes, but ontologies do.
- Guidelines/Standards: Taxonomies should follow the ANSI/NISO Z39.19 (2005) or ISO 25964, whereas ontologies are expected to follow the Web Ontology Language (OWL) guidelines and make use of the Resource Description Framework (RDF).
- Purposes: Taxonomies support indexing/tagging, categorization, and/or classification of content, and in turn information findability and retrieval. The primary purpose of an ontology is to describe a domain of knowledge, and support of indexing/tagging, categorization, classification, findability, and retrieval can be secondary.
- Tools: Some software supports the creation of only taxonomies, some software is for ontologies, and some software can do both quite well. Additionally, some taxonomy/thesaurus software can support most, if not all, features of ontologies.
Coming at ontologies from taxonomies, the biggest distinguishing feature of ontologies is the semantic nature of the relationships.
In a taxonomy or thesaurus, you may have generic relationships, such as:
Automobile industry RT (related term) Cars, and
Cars RT (related term) Automobile industry
Ford Motor Company NT (narrower term) Lincoln Division, and
Lincoln Division BT (broader term) Ford Motor Company
In an ontology, you may have customized, semantic relationships, such as:
Automobile industry MAN (manufactures) Cars, and
Cars IND (manufactured by the industry) Automobile industry
Ford Motor Company SUB (has subsidiary or division) Lincoln Division, and
Lincoln Division PAR (has parent) Ford Motor Company
If you can customize the relationships, does this change a taxonomy into a ontology? No. Customized relationships are just one feature of an ontology, although perhaps the most important feature. In my online course on taxonomies, although I don’t teach how to create ontologies, I do provide a lesson on customized/semantic relationships. It is often desirable to create a more complex taxonomy without necessarily meeting all the requirements of an ontology.
Furthermore, a customized relationship might not be fully semantic. In the example above, the second set of relationships are customized, because they are designated by the ontologist for the particular case. The relationships are also “semantic” because they contain specific meaning. (Semantic means “has meaning.”) It is possible to customize relationships while still not making them fully semantic. You may decide to simply rename the standard relationships for your particular application and audience. For example, you might rename broader term (BT)/narrower term (NT) as “parent/child,” or rename Related Term as “see also.” If your taxonomy/thesaurus software is more sophisticated, it will allow you to specify any number of customized relationships, and thus you can add more nuances of meaning.
A key component of truly semantic relationships as expected in ontologies is the ability to create directional relationships that are distinct in each direction, with reciprocity. Most of these semantic relationships will be variants of “related term” (RT), rather than variants of the hierarchical relationship. The generic RT relationship, however, is singularly bidirectional. If you simply customized it by renaming it, it would have to be the same in both directions, such has “has partner.” To create a semantic relationship pair, such as MAN (manufactures) and IND (manufactured by the industry), you need a tool that supports ontological relationships and not just “customized” relationships.
If your tool supports customized relationships but not the ability to create distinct pairs of directional relationships that are associative rather than hierarchical, the results cans still be very useful. You may have a “near ontology” if not a strictly defined ontology. For example, you could rename the singular “related term” (RT) as “Manufacturer-Product” with an abbreviation such as MAN-PRO (Credit to Alice Redmond-Neal of Access Innovations, Inc. for the example). Thus, the relationship is the same in either direction:
Automobile industry MAN-PRO Cars, and
Cars MAN-PRO Automobile industry
It is not completely semantic, with the directional details missing, but this may be good enough for your purposes. After all, it should be obvious which is the manufacturer and which is the product. Therefore, taxonomy/thesaurus software that provides most, if not all, features of an ontology may be sufficient, too.
What matters is serving your needs. Rather than calling it an “ontology” when it does not meet all the definitions of an ontology (and causing confusion or disagreement), it may be safer to say your sophisticated taxonomy “has features of an ontology.”