Is a Taxonomy an Ontology?

At last month’s Knowledge Graph Conference, in addition to knowledge graphs and graph databases, there is a growing interest in ontologies, but the role of taxonomies does not seem so well understood. For example, in one presentation I attended, it was said that you get synonyms/alternative labels into a knowledge graph via ontologies, rather than mentioning taxonomies. More than one person asked me: isn’t a taxonomy a kind of ontology?  

The fact that, technically, SKOS (the data model for interoperability used for taxonomies) has been designed as upper ontology, can lead to the conclusion that all taxonomies modeled on SKOS are then domain ontologies, as they are instances of the SKOS upper ontology. However, that is a more theoretical way, than a practical way, to look at taxonomies.

When I write or speak about taxonomies, I aim to be practical. While theoretically a taxonomy is a kind of ontology, in practice it is not, and maintaining a distinction helps clarify how each a taxonomy and an ontology can improve on each when they are combined.

If you are an ontologist and see everything through the lens of ontologies, then you probably consider that a taxonomy is a simple type of ontology that merely does not utilize all the features of a full ontology. If an ontology is simply defined as a knowledge model that has classes (things), relationships between the things, and attributes as properties of the things, then, yes, a taxonomy is a kind of ontology. It has concepts, hierarchical relationships, and often other attributes for concepts, that typically merely definitions, scope notes, or other notes.

The problem of calling any taxonomy an ontology is that the benefits of semantically enriching a taxonomy with an added ontology or extending an ontology with a taxonomy might not be well understood. We add an ontology to a taxonomy in order to provide customized semantic relationships and attributes of all kinds. Additionally, basing the added ontology on OWL (Web Ontology Language) enables capabilities of inferencing and reasoning.

Furthermore, saying that a taxonomy is an ontology could lead to less than sufficient attention to the taxonomy features that ontologies alone lack. These features include alternative labels and hidden labels that match variants in both tagging and user searching, equivalent foreign language labels for concepts, concept schemes that can be implemented as search facets, distinct fields for definitions and different kinds of notes that are standardized for interoperability

If following the Semantic Web’s stack of data model recommendations, then a taxonomy can be defined as what is built on SKOS (Simple KnowledgeOrganization System), and an ontology is defined as what is built on RDFS(RDF-Schema) and OWL (Web Ontology Language). I find that a very clear explanation of the difference between taxonomies and ontologies to those who are familiar with ontologies. These different data models may be integrated within the same knowledge model, and that’s how we get taxonomies extended with ontologies or ontologies extended with taxonomies.

We might call taxonomy-ontology combinations “knowledge models” or “semantic models.” If the model has mostly taxonomy (SKOS-based) data, such as a large taxonomy with a little ontology added, it is best called a taxonomy, and if the model has mostly ontology (RDFS and OWL-based) data, such as a large ontology with some taxonomy  data, it is best called an ontology.

The organizers of the Knowledge Graph Conference understood the distinct role of taxonomies in knowledge graphs and thus welcomed me again to present a tutorial specifically on taxonomies.