OWL Terminology: Why do Individuals Have ObjectProperties (Instead of IndividualProperties)

Question

In the W3 OWL specification the properties of individuals are divided in to two groups: datattype properties and object properties. Object properties are defined (as one article I found put it):

"Object properties (owl:ObjectProperty) relates individuals (instances) of two OWL classes.

So in essence, object properties could also be called "individual properties", because they don't just point to generic objects of any sort, they point specifically to individuals.

Now, if this was just some random spec I would assume the authors simply chose their names poorly, but this is a W3 spec, and one specifically on the storage of knowledge no less; I have to assume people thought about the names of things!

Therefore, I'm hoping someone here can explain this seemingly strange naming choice. After all, you can call damn near anything in any spec an "ObjectFoo", because Object is a super-generic term, but normally people use the most specific term possible, not the least, when they name things.

Is there perhaps some other case where an ObjectProperty can refer to something other than an individual, or anything else I'm missing that might explain this?

Answer 1

The term "ObjectProperty" was (most probably) coined to distinguish it from "DatatypeProperty", in the sense that the latter can only have (datatyped) literal values, as opposed to full objects. And yes, it's not just individuals that can be the value of an ObjectProperty, classes can be values of them too - although if you do that, your ontology is no longer valid OWL DL and becomes OWL Full instead. But it's valid from a modeling perspective.

Answer 2

My answer is also a comment to Jeen. OWL DL and OWL Full are 2 distinct languages with a different (even disjoint) abstract syntax. OWL DL syntax is defined in terms of its structural specification. OWL Full syntax is RDF. The structural specification of OWL does not even contain any RDF triple.

Now, in OWL DL, it is invalid to relate two classes with an object property. Object properties can only relate instances of owl:Thing. They cannot relate literals, properties, ontologies, or datatypes either. If you call the notion IndividualProperty, you are creating an inconsistency in naming because DatatypeProperty does not mean a property that relates datatypes. It is a property that relates literals. So a better name would be ClassProperty. Or you would have to change both DatatypeProperty and ObjectProperty into LiteralProperty and IndividualProperty.

All in all, there are various ways to deal with this, the working group chose the one which gathered more votes. That's all how it always works in a standardisation group.