The built-in OWL property owl:sameAs links an individual to an individual. Such an owl:sameAs statement indicates that two URI references actually refer to the same thing: the individuals have the same "identity".

The ability to express equivalences using owl:sameAs can be used to state that seemingly different individuals are actually the same. Real class equality or property equality can only be expressed with the owl:sameAs construct. As this requires treating classes and properties as individuals, such axioms are only allowed in OWL Full.


    <rdf:Property rdf:ID="sameAs">


        <rdfs:domain rdf:resource="#Thing"/>

        <rdfs:range rdf:resource="#Thing"/>


Example listing




    <rdf:Description rdf:about="#William_Jefferson_Clinton">
         <owl:sameAs rdf:resource="#BillClinton"/>

Other information

Many languages have a so-called "unique names" assumption: different names refer to different things in the world. On the web, such an assumption is not possible. For example, the same person could be referred to in many different ways (i.e. with different URI references). For this reason OWL does not make this assumption. Unless an explicit statement is being made that two URI references refer to the same or to different individuals, OWL tools should in principle assume either situation is possible.

OWL provides three constructs for stating facts about the identity of individuals:

For individuals such as "people" this notion is relatively easy to understand. For example, we could state that the following two URI references actually refer to the same person:

    <rdf:Description rdf:about="#William_Jefferson_Clinton">
          <owl:sameAs rdf:resource="#BillClinton"/>

The owl:sameAs statements are often used in defining mappings between ontologies. It is unrealistic to assume everybody will use the same name to refer to individuals. That would require some grand design, which is contrary to the spirit of the web.

In OWL Full, where a class can be treated as instances of (meta)classes, we can use the owl:sameAs construct to define class equality, thus indicating that two concepts have the same intensional meaning. An example:

    <owl:Class rdf:ID="FootballTeam">

        <owl:sameAs rdf:resource="http://sports.org/US#SoccerTeam"/>


One could imagine this axiom to be part of a European sports ontology. The two classes are treated here as individuals, in this case as instances of the class owl:Class. This allows us to state that the class FootballTeam in some European sports ontology denotes the same concept as the class SoccerTeam in some American sports ontology. Note the difference with the statement:

    <footballTeam owl:equivalentClass us:soccerTeam />

which states that the two classes have the same class extension, but are not (necessarily) the same concepts.

NOTE: For details of comparison of URI references, see the section on RDF URI references in the RDF Concepts document [RDF Concepts].

Use in ISO 15926