Using Classes As Property Values

Natasha Noy, Stanford University
Last revised: May 3d, 2004

[Note: This document will be a part of a larger document that will provide an introduction and overview of all ontology design patterns produced by the Semantic Web Best Practices and Deployment Working Group]

Comments, suggestions and modifications suggested by Pat Hayes shown in yellow. Comments are in monospace type, suggested modifications to text in normal font. Comments written May 14 2004

General issue

When and how to use classes as values for properties? Why only values? Why not as having properties applied to them i.e. more generally as individuals? What are advantages and drawbacks? What are different approaches and workarounds? Which solutions are in OWL DL (and hence OWL Lite) and which solutions are in OWL Full but not in OWL DL require us to move outside OWL DL?

Use case example

Suppose we have a set of books about animals and want to annotate each book with its subject, which is a particular species class? That is, does it have to be a species? of animal (or animals) that it talks about. This emphasis on books is unfortunate since it gets the issue mixed up with issues of symbol vs. symbolised, class vs. concept, etc.. Further, we want to be able to say infer? that a book about african lions is also a book about lions (For example, when retrieving all books about lions from a repository, we want books that are annotated as books about african lions to be included in the results).

More specifically, consider two book examples: (1) "Lions: Life in the Pride", which is a book that "presents an introduction to lions describing their physical characteristics, habitat, young, food, predators, and relationship to people"; and (2) "The African Lion," which "describes the physical characteristics, habitat, and behavior of the" african lions. We would like to specify that the first book described the animal species of Lions, and the second describes a species of African Lion. However, we also want to retrieve the second book when a query is about Lions, not just African Lions.

We consider animals classes of animals? to be subjects of the books and would like to use the Dublin Core property dc:subject for this annotation.

Other use case scenarios

This issue arises in general when we have a hierarchy of concepts and would like to use it as a terminology to annotate other classes or individuals. Terminology?? This reads oddly to me, and seems on a tangent. The classes as individuals issue arises in many cases which have nothing particularly to do with terminology. Consider using a hierarchy of different genre genres? to annotate music CDs, or linking classes or individuals in an ontology to the corresponding concepts in a standard reference terminology (e.g., UMLS is such standard reference terminology for many medical applications).


In all the figures below, ovals represent classes and rectangles represent individuals. The orange color signifies classes or individuals that are specific to a particular approach.


Approach 1: using classes directly as values

In the first approach, we can simply use classes from the subject hierarchy as values for properties (in our example, as values for the dc:subject property). We can define a class BookAboutAnimals of all books about animals. Suggest omit this as it plays no role in the example. For simplicity, we omit classes for other animals, such as mammals and felines.

Here is a definition of an individual (a specific book that we are annotating) that is an instance of the BookAboutAnimals class with the corresponding subject (for simplicity, we assume that each book discusses only one class of animals):

a :BookAboutAnimals ;
rdfs:seeAlso <> ;
:bookTitle "Lions: Life in the Pride" ;
dc:subject :Lion .

The book "The African Lion" will be represented as:

a :BookAboutAnimals ;
rdfs:seeAlso <> ;
:bookTitle "The African Lion" ;
dc:subject :AfricanLion .
And the class AfricanLion is a subclass of the class Lion:
a owl:Class;
rdfs:subClassOf :Lion .

Considerations when choosing approach 1:

OWL code for approach 1

[N3] [RDF/XML abbrev] [Abstract syntax]

Summary for approach 1

This approach is a good one to use if you care about simplicity, do not have to be in OWL DL, and either do not need to limit the range of the dc:subject values or do not care that you need to also use classes as subjects in order have subjects as instances of a subclass of owl:Class to implement this restriction.

Approach 2: Creating a hierarchy of subjects and a parallel set of subject individuals

We can treat the hierarchy of animal species as a hierarchy of subjects, create individuals corresponding to all the subjects and use these individuals as values for the dc:subject property. Thus, we will have, for example, an individual LionSubject that will be an instance of the Lion class.

Why did you use rdf:type as the link? On the face of it, that seems clearly broken, and does not seem necessary. There is a perfectly rational approach which would be like this but using a different property to connect to the classes, say 'ex:referenceClass', thereby avoiding the need to include the subject in the class (and making a clear distinction between this case and your case 4, which currently there is no distinction as far as I can see.) Of course this would not be in DL, but it still makes perfect sense as an option in Full.

As this would help to make clear, many of the problems arise because in DL, the ONLY property that can have a class as a value is rdf:type. (It might be worth saying this explicitly somewhere.) If this restriction is lifted, many options are opened up.

Suggestion: make this modification to this example, and say clearly what the merits of it are but that it is not in DL. Then treat your example 4 as being what you get when you 'squeeze' this style into DL, and so force the subjects to be actual lions. That has the merit of making the disticntion clear and also showing clearly the reason for having to introduce "prototypical lions".

We can then use the LionSubject as the value of the property dc:subject for the LionsLifeInThePrideBook individual:

a :Lion .

In this case, the definition of the LionsLifeInThePrideBook refers to the LionSubject individual

a :BookAboutAnimals ;
rdfs:seeAlso <> ;
:bookTitle "Lions: Life in the Pride" ;
dc:subject :LionSubject . 

Considerations when choosing approach 2:

OWL code for approach 2

[N3] [RDF/XML abbrev] [Abstract syntax]

Summary of approach 2

This approach results in an OWL DL ontology and may be a good one to use if staying in OWL DL is important. The approach has a potential disadvantage of having actual subject values be unrelated to one another and hence not allowing a general-purpose reasoner to relate books with a subject "lion" to books with a subject "african lions", for example. You need to maintain consistency between the set of classes representing subjects and the set of corresponding individuals. If the subject hierarchy is not solely a terminology (e.g., you need to represent specific animals), you will need to create a separate class hierarchy for that.

Approach 3: using a property other than rdfs:subclassOf to organize the subject hierarchy

There is something weird about this, since it makes no reference to classes as values at all. So why are you even talking about it?

I'd suggest making this the last 'appraoch', almost as an afterthought, along the lines of :of course, you can just ignore the class heirarchy altogether and describe your own heirarchy. That is ALWAYS an option, after all, and it is one very reliable strategy for staying in the DL space, at an obvious cost.

We can create a single class Subject and make all the subjects to be individuals that are instances of this class Subject:

a :Subject ;
:parentSubject :AnimalSubject .

We can then create explicit relations between different subjects, which will re-create the hierarchy for animals that we have in mind. While we create our own property parentSubject, we can also use the corresponding properties from the SKOS-Core 1.0 schema, which is an RDF schema for representing thesauri and similar types of knowledge organization systems. So, for example parentSubject is similar to skos:broader. The SKOS schema provides a rich vocabulary for handling subject hierarchy, with additional properties such as skos:narrower, skos:related, and so on.

a owl:TransitiveProperty , owl:ObjectProperty ;
rdfs:domain :Subject ;
rdfs:range :Subject ;
rdfs:seeAlso <> .
a :Subject ;
:parentSubject :LionSubject .

Considerations when choosing Approach 3

OWL code for approach 3

[N3] [RDF/XML abbrev][Abstract syntax]

Summary of Approach 3

This approach may be a good one to use if staying within OWL DL is important. It also allows you to use a DL reasoner to infer transitive relationships between subjects. It does carry the penalty of having two parallel "hierarchies."

Approach 4: using members of a class as values for the property

This seems to me to be identical to # 2.

We can approximate the interpretation that we used in the previous approaches by using unspecified members of a class rather than the class itself as property values. We can define the class BookAboutAnimals as a class of books where the subject is some animal. Correspondingly, a BookAboutLions class will be a class of books where a subject is some (unidentified) lion:

For example, we can define the class BookAboutLions as follows:

a owl:Class ;
[ a owl:Class ;
owl:intersectionOf ([ a owl:Restriction ;
owl:onProperty dc:subject ;
owl:someValuesFrom :Lion
] :Book)
] .

A specific instance of this class LionsLifeInThePrideBook would then be defined as follows:

a :BookAboutLions ;
rdfs:seeAlso <> ;
:bookTitle "Lions: Life in the Pride" .

Alternatively, MyLionBook can be defined as

a :Book; [ a owl:Restriction ;
owl:onProperty dc:subject ;
owl:someValuesFrom :Lion
A DL classifier will be able to classify MyLionBook as an instance of the class BookAboutLions

Considerations when choosing Approach 3

OWL code for approach 4

[N3] [RDF/XML abbrev][Abstract syntax]

Summary of Approach 4

This approach can make the most use of DL classifiers. It represents a different interpretation of the subject as being a prototypical instance of a class rather than a whole class of things or a specific subject corresponding to that class. With the understanding that you are using a different interpretation, this approach may be a good one to use if using DL reasoners to classify individuals based on their subjects (or another property used in its place) is important.

Approach 5: using classes as values for annotation properties

This is not an 'approach'; it is a refusal to solve the problem. Another way to stay in OWL DL is to use classes as values for annotation properties:

a owl:AnnotationProperty .

a :BookAboutAnimals ;
rdfs:seeAlso <> ;
:bookTitle "Lions: Life In The Pride" ;
dc:subject :Lion .

Considerations when choosing Approach 5

OWL code for approach 5

[N3] [RDF/XML abbrev] [Abstract syntax]

Summary of Approach 5

This approach allows you to use classes directly as property values while staying in OWL DL. No, it doesn't in any real sense.  However, the properties that will have classes as values will have to be defined as annotations and therefore cannot have any additional restrictions defined on them (and should not be declared as object or datatype properties elsewhere). DL reasoners will not use values of annotation properties.


[1] Using allValuesFrom restriction for the book's subject implies that instances of the class BookAboutAnimals can have only animals as values for the dc:subject property. If we want to allow other terms to be listed as subjects, a someValuesFrom restriction will be appropriate.