Ontology vs Taxonomy

This is confusing, because a taxonomy is a thing that can be implemented in an ontology.

• An onotology is the description of the structure of entities and their relationships
• A taxonomy is grouping or organization of entities that are somehow related

For example, if I was going to catalog all the people I knew, this is how the two concept will differ:

• The ontology would define the entity (class) of a “person” and specify what information I tracked, like first name, last name, date of birth, etc. It would also define relationships between entities, such as “Person A” is the parent of “Person B.” Another entity might be “Organization” with relationships of “is a member of” or “founded.”

• The taxonomy would group entities. I could group a bunch of people into “Friends,” “Co-workers,” “Enemies,” etc. I could group organizations into “Civic,” “Religious,” etc. It might also group groups into a hierarchy – “Friends” might subdivide into “Childhood” and “Adult.”

Incidentally, “ontology” can be a tricky word to research because it has a significant meaning in philosophy. Wikipedia has two pages on it:

• Ontology: Ontology is the branch of philosophy that studies concepts such as existence, being, becoming, and reality.
• Onotology (information science): In computer science and information science, an ontology encompasses a representation, formal naming, and definition of the categories, properties, and relations between the concepts, data, and entities that substantiate one, many, or all domains of discourse.

One usage difference is saying “an ontology,” rather than just “ontology.” For former implies a single, specific thing, which would be a description of a domain of knowledge.

Schema.org is an example of an onotology, but they explicitly address the applicability of such on their site:

The type hierarchy presented on this site is not intended to be a ‘global ontology’ of the world. When founded in 2011 it was strictly focused around the types of entities for which the project’s founders (Microsoft, Yahoo!, Google and Yandex), could reasonably expect to provide some special treatment for via search engines. As the project has evolved, introducing more community collaboration and extension mechanisms, its scope has expanded gradually. However it is still the case that schema.org is not intended as a universal ontology.

What I feel gets confusing is when a taxonomy is modeled via an ontology. For example, is a category just an entity? And is an entity’s existence – even another category entity – in a category just a relationship? Therefore, can’t we represent an taxonomy as part of the ontology?

I asked this question on Twitter. I’ll update here when and if I get a consensus.

To make it more complicated, the first response to my above tweet said this:

I think I would describe it more like this: quite often, we use taxonomies to create what are more accurately described as ontological relationships.

So, sometimes we try to model ontologies as taxonomies.

Additionally, the most basic taxonomic category is simply an ontological class. If my ontology includes classes for Person, Organization, Location, and Role, then those naturally overlay onto at least one facet of taxonomic categorization.