New Version Available: OWL 2 (Document Status Update, 12 November 2009)
The OWL Working Group has produced a W3C Recommendation for a new version of OWL which adds features to this 2004 version, while remaining compatible. Please see OWL 2 Document Overview for an introduction to OWL 2 and a guide to the OWL 2 document set.
Please refer to the errata for this document, which may include some normative corrections.
See also translations.
Copyright © 2004 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
The Web Ontology Language OWL is a semantic markup language for publishing and sharing ontologies on the World Wide Web. OWL is developed as a vocabulary extension of RDF (the Resource Description Framework) and is derived from the DAML+OIL Web Ontology Language. This document contains a structured informal description of the full set of OWL language constructs and is meant to serve as a reference for OWL users who want to construct OWL ontologies.
This document has been reviewed by W3C Members and other interested parties, and it has been endorsed by the Director as a W3C Recommendation. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.
This is one of six parts of the W3C Recommendation for OWL, the Web Ontology Language. It has been developed by the Web Ontology Working Group as part of the W3C Semantic Web Activity (Activity Statement, Group Charter) for publication on 10 February 2004.
The design of OWL expressed in earlier versions of these documents has been widely reviewed and satisfies the Working Group's technical requirements. The Working Group has addressed all comments received, making changes as necessary. Changes to this document since the Proposed Recommendation version are detailed in the change log.
Comments are welcome at<EMAIL_ADDRESS>(archive) and general discussion of related technology is welcome at<EMAIL_ADDRESS>(archive).
A list of implementations is available.
The W3C maintains a list of any patent disclosures related to this work.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Parts of this document are derived from the DAML+OIL (March 2001) Reference Description [DAML+OIL] which was submitted as part of the DAML+OIL W3C Note . The sponsors of this document and its predecessor documents are gratefully acknowledged.
Jeremy Carroll, Jim Hendler, Brian McBride and Peter Patel-Schneider provided substantive reviews and contributed text to this document. Jeff Heflin contributed the section on deprecation. Jerome Euzenat contributed the example for an enumerated datatype.
This document is the result of extensive discussions within the Web Ontology Working Group as a whole. The participants in this Working Group included: Yasser alSafadi, Jean-François Baget, James Barnette, Sean Bechhofer, Jonathan Borden, Frederik Brysse, Stephen Buswell, Jeremy Carroll, Dan Connolly, Peter Crowther, Jonathan Dale, Jos De Roo, David De Roure, Mike Dean, Larry Eshelman, Jérôme Euzenat, Tim Finin, Nicholas Gibbins, Sandro Hawke, Patrick Hayes, Jeff Heflin, Ziv Hellman, James Hendler, Bernard Horan, Masahiro Hori, Ian Horrocks, Jane Hunter, Francesco Iannuzzelli, Rüdiger Klein, Natasha Kravtsova, Ora Lassila, Massimo Marchiori, Deborah McGuinness, Enrico Motta, Leo Obrst, Mehrdad Omidvari, Martin Pike, Marwan Sabbouh, Guus Schreiber, Noboru Shimizu, Michael Sintek, Michael K. Smith, John Stanton, Lynn Andrea Stein, Herman ter Horst, David Trastour, Frank van Harmelen, Bernard Vatant, Raphael Volz, Evan Wallace, Christopher Welty, Charles White, and John Yanosy.
This document gives a systematic, compact and informative description of all the modelling primitives of OWL, using the RDF/XML exchange syntax for OWL. We expect this document to serve as a reference guide for users of the OWL language.
This document is one component of the description of OWL, the Web Ontology Language, being produced by the W3C Web Ontology Working Group. The Document Roadmap section of the OWL Overview document describes each of the different parts and how they fit together. Readers unfamiliar with OWL may wish to first consult the OWL Overview document [OWL Overview], and subsequently the OWL Guide [OWL Guide] for a more narrative description and examples of the use of the language.
This document assumes the reader is familiar with the basic concepts of RDF [RDF Concepts] and has a working knowledge of the RDF/XML syntax [RDF/XML Syntax] and of RDF Schema [RDF Vocabulary].
The normative reference on the precise syntax of the OWL language constructs can be found in the OWL Semantics and Abstract Syntax document [OWL S&AS]. That document also contains a precise definition of the meaning of the language constructs in the form of a model-theoretic semantics. Notions such as consistency of OWL ontologies are discussed in that document.
Use cases and requirements for the OWL language are described in the OWL requirements document [OWL Requirements]. Test cases for OWL tools (e.g., entailment tests, consistency tests) are specified in the Test document [OWL Test Cases] .
As also discussed in the OWL Overview document [OWL Overview], and subsequently the OWL Guide [OWL Guide], the OWL language provides two specific subsets that we believe will be of use to implementors and language users. OWL Lite was designed for easy implementation and to provide users with a functional subset that will get them started in the use of OWL. OWL DL (where DL stands for "Description Logic") was designed to support the existing Description Logic business segment and to provide a language subset that has desirable computational properties for reasoning systems. The complete OWL language (called OWL Full to distinguish it from the subsets) relaxes some of the constraints on OWL DL so as to make available features which may be of use to many database and knowledge representation systems, but which violate the constraints of Description Logic reasoners.
NOTE: RDF documents will generally be in OWL Full, unless they are specifically constructed to be in OWL DL or Lite.
OWL Full and OWL DL support the same set of OWL language constructs. Their difference lies in restrictions on the use of some of those features and on the use of RDF features. OWL Full allows free mixing of OWL with RDF Schema and, like RDF Schema, does not enforce a strict separation of classes, properties, individuals and data values. OWL DL puts constraints on the mixing with RDF and requires disjointness of classes, properties, individuals and data values. The main reason for having the OWL DL sublanguage is that tool builders have developed powerful reasoning systems which support ontologies constrained by the restrictions required for OWL DL. For the formal definitions of the differences between OWL Full and OWL DL the reader is referred to the Semantics and Abstract Syntax document [OWL S&AS]. Sec. 8.2 "OWL DL" summarizes the differences between OWL Full and OWL DL.
OWL Lite is a sublanguage of OWL DL that supports only a subset of the OWL language constructs. OWL Lite is particularly targeted at tool builders, who want to support OWL, but want to start with a relatively simple basic set of language features. OWL Lite abides by the same semantic restrictions as OWL DL, allowing reasoning engines to guarantee certain desirable properties. A summary of the language constructs allowed in OWL Lite is given in Sec. 8.3. For a more formal description of the subset of OWL language constructs supported by OWL Lite the reader is referred to the Semantics and Abstract Syntax document [OWL S&AS].
NOTE: RDF users upgrading to OWL should be aware that OWL Lite is not simply an extension of RDF Schema. OWL Lite is a light version of OWL DL and puts constraints on the use of the RDF vocabulary (e.g., disjointness of classes, properties, etc.). OWL Full is designed for maximal RDF compatibility and is therefore the natural place to start for RDF users. When opting for either OWL DL or OWL Lite one should consider whether the advantages of OWL DL/Lite (e.g., reasoning support) outweigh the DL/Lite restrictions on the use of OWL and RDF constructs.
NOTE: OWL Lite is defined in this document as a number of additional restrictions on OWL DL. This mean that, OWL DL constructs are also part of OWL Lite, unless explicitly stated otherwise. Sec. 8.3.provides a summary of these additional OWL Lite restrictions.
An OWL ontology is an RDF graph [RDF Concepts], which is in turn a set of RDF triples. As with any RDF graph, an OWL ontology graph can be written in many different syntactic forms (as described in the RDF/XML Syntax Specification (Revised) [RDF/XML Syntax]). The current document uses some specific syntactic forms of RDF/XML for representing triples (as does the Guide document) . However, the meaning of an OWL ontology is solely determined by the RDF graph. Thus, it is allowable to use other syntactic RDF/XML forms, as long as these result in the same underlying set of RDF triples. Such other syntactic forms would then carry exactly the same meaning as the syntactic form used in this document.
As a simple example of an alternative syntactic form resulting in the same RDF triples, consider the following RDF/XML syntax:
<owl:Class rdf:ID="Continent"/>
The following RDF/XML syntax:
<rdf:Description rdf:about="#Continent"> <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#Class"/> </rdf:Description>
encodes the same set of RDF triples, and therefore would convey the same meaning.
OWL is a vocabulary extension of RDF [RDF Semantics]. Thus any RDF graph forms an OWL Full ontology. Further, the meaning given to an RDF graph by OWL includes the meaning given to the graph by RDF. OWL Full ontologies can thus include arbitrary RDF content, which is treated in a manner consistent with its treatment by RDF. OWL assigns an additional meaning to certain RDF triples. The OWL Semantics and Abstract Syntax document [OWL S&AS] specifies exactly which triples are assigned a specific meaning, and what this meaning is.
NOTE: As remarked before, OWL DL and OWL Lite extend the RDF vocabulary, but also put restrictions on the use of this vocabulary. Therefore, RDF documents will generally be in OWL Full, unless they are specifically constructed to be in OWL DL or Lite.
For readability purposes the examples in this document assume the
XML entities &rdf;
, &rdfs;
,
&owl;
and &xsd;
(for XML Schema datatypes)
are defined in the same way
as in Appendix B. The same assumption holds for the
corresponding namespaces rdf:
, rdfs:
,
owl:
and xsd:
.
The examples in this document are meant to serve as illustrations of the use of OWL language constructs. They do not form one consistent ontology. For an extended example the reader is referred to the Guide document [OWL Guide].
OWL's ability to express ontological information about individuals
appearing in multiple documents supports linking of data from diverse
sources in a principled way. The underlying semantics provides
support for inferences over this data that may yield unexpected
results. In particular, the ability to express equivalences using
owl:sameAs
can be used to state that seemingly
different individuals are actually the same.
Similarly,
owl:InverseFunctionalProperty
can be used to link
individuals together.
For example, if a property such as SocialSecurityNumber
is an
owl:InverseFunctionalProperty
, then two separate individuals
could be inferred to be identical based on having the same value of
that property. When individuals are determined to be the same by such
means, information about them from different sources can be
merged. This aggregation can be used to determine facts that
are not directly represented in any one source.
The ability of the Semantic Web to link information from multiple sources is a desirable and powerful feature that can be used in many applications. However, the capability to merge data from multiple sources, combined with the inferential power of OWL, does have potential for abuse. It may even be illegal to create or to process such linked information in countries with data protection laws, especially in the EU, without having a valid legal reason for such processing. Therefore, great care should be taken when using OWL with any kind of personal data that might be linked with other data sources or ontologies. Detailed security solutions were considered out of scope for the Working Group. Work currently underway elsewhere is expected to address these issues with a variety of security and preference solutions -- see for example SAML and P3P
This document has a set of appendices containing additional information.
Links in this document that are attached to definitions of language constructs provide access to the OWL Semantics and Abstract Syntax [OWL S&AS]. Appendix A contains a systematic set of links for each language construct to the corresponding sections in the Guide and the S&AS documents.
Appendix B
contains a RDF schema for the OWL language constructs.
This schema provides information about the OWL vocabulary that could
be a useful reference point for ontology builders and tool
developers. The restrictions provided by the schema on the OWL classes
and properties are informative and not complete. Also, this schema
does not make distinctions between OWL Full, OWL DL and OWL Lite.
Conventionally, classes
have a leading uppercase character; properties a leading
lowercase character. Thus, owl:Ontology
is a class, and
owl:imports
is a property.
NOTE: The RDF Schema file for OWL is not expected to be imported explicitly
(i.e., with owl:imports
) into an ontology. The schema
has an informative status and is
meant to provide the classes and properties to be used in the RDF/XML syntax.
People that do import this schema should expect the resulting ontology
to be an OWL Full ontology.
Appendix C gives a tabular overview of the OWL vocabulary in terms of the built-in OWL classes and properties (the latter with their domain and range).
For readers familiar with DAML+OIL, Appendix D lists many of the changes between DAML+OIL and OWL.
Finally, Appendix E provides a set of practical guidelines for specifying OWL DL ontologies in RDF.
An OWL document consists of optional ontology headers (generally at most one) plus any number of class axioms, property axioms, and facts about individuals. Please note that "axiom" is the formal term used in the S&AS document. These axioms are somewhat more informally called "definitions" in the Guide and Overview documents.
NOTE: OWL does not impose any order on OWL components. Humans writing ontologies are likely to use some sort of ordering, for example putting the ontology header in the beginning, but this has no impact on the meaning. Tools should not assume any order.
As with most RDF documents, the OWL code should be subelements of
a rdf:RDF
element.
This enclosing element generally holds XML namespace and base declarations.
Also, an OWL ontology document often starts with several entity declarations.
For a typical example of this sort of information, see the
example wine and food ontologies
discussed in the Guide document
[OWL Guide].
The built-in vocabulary for OWL all comes from the OWL namespace
http://www.w3.org/2002/07/owl#
conventionally associated with the namespace name owl
.
It is
recommended that ontologies not use names from this namespace except
for the built-in vocabulary. OWL tool builders should feel free to
signal a warning if other names from this namespace are used, but
should otherwise continue as normal.
The Web Ontology Working Group has not requested a separate MIME type for OWL
documents. Instead, we recommend to use the MIME type requested by the RDF Core
Working Group, namely application/rdf+xml
[RDF Concepts],
or alternatively the
XML MIME type application/xml
.
As file extension, we recommend to use either .rdf
or
.owl
.
Classes provide an abstraction mechanism for grouping resources with similar characteristics. Like RDF classes, every OWL class is associated with a set of individuals, called the class extension. The individuals in the class extension are called the instances of the class. A class has an intensional meaning (the underlying concept) which is related but not equal to its class extension. Thus, two classes may have the same class extension, but still be different classes.
When in this document we use wording such as "a class of individuals ..", this should be read as "a class with a class extension containing individuals ...".
NOTE: In OWL Lite and OWL DL an individual can never be at the same time a class: classes and individuals form disjoint domains (as do properties and data values). OWL Full allows the freedom of RDF Schema: a class may act as an instance of another (meta)class.
OWL classes are described through "class descriptions", which can be combined into "class axioms". We first describe class descriptions and subsequently turn to class axioms.
A class description is the term used in this document (and in the OWL Semantics and Abstract Syntax) for the basic building blocks of class axioms (informally called class definitions in the Overview and Guide documents). A class description describes an OWL class, either by a class name or by specifying the class extension of an unnamed anonymous class.
OWL distinguishes six types of class descriptions:
The first type is special in the sense that it describes a class through a class name (syntactically represented as a URI reference). The other five types of class descriptions describe an anonymous class by placing constraints on the class extension.
Class descriptions of type 2-6 describe, respectively, a class that contains exactly the enumerated individuals (2nd type), a class of all individuals which satisfy a particular property restriction (3rd type), or a class that satisfies boolean combinations of class descriptions (4th, 5th and 6th type). Intersection, union and complement can be respectively seen as the logical AND, OR and NOT operators. The four latter types of class descriptions lead to nested class descriptions and can thus in theory lead to arbitrarily complex class descriptions. In practice, the level of nesting is usually limited.
A type 1 class description is syntactically represented as an named
instance of
owl:Class
, a subclass of rdfs:Class
:
<owl:Class rdf:ID="Human"/>
This will assert the triple "ex:Human rdf:type owl:Class
."
, where ex:
is the namespace of the relevant
ontology .
NOTE: In OWL Lite and OWL DL, owl:Class
(or owl:Restriction
, see further)
must be used for all class descriptions.
NOTE:
owl:Class
is defined as a subclass of
rdfs:Class
. The rationale for having a separate OWL
class construct lies in the restrictions on OWL
DL (and thus also on OWL Lite), which imply that not all RDFS classes are legal
OWL DL classes. In OWL Full these restrictions do not exist and therefore
owl:Class
and rdfs:Class
are equivalent in
OWL Full.
The other five forms of class descriptions consist of a set of RDF triples
in which a blank node represents the class being described. That blank
node has an rdf:type
property whose value is
owl:Class
.
NOTE: If one provides an RDF identifier for class descriptions of the enumeration, intersection, union or complement type, this is not considered to be a class description, but a special kind of class axiom for complete classes. See Sec. 3.2.3 for details.
NOTE: In this document we sometimes use for readability purposes the shorthand "class description" to refer to "the class being described by the class description". Strictly speaking, these are different in the case of class descriptions of type 2-6: the class is represented by the corresponding blank node; the class description is represented by the triples that have this blank node as their subject.
Two OWL class identifiers are predefined, namely the classes owl:Thing
and owl:Nothing.
The class extension of owl:Thing
is the set of all
individuals.
The class extension of owl:Nothing
is the
empty set. Consequently, every OWL class is a
subclass of owl:Thing
and owl:Nothing
is a subclass
of every class (for the meaning of the subclass relation, see the section on rdfs:subClassOf
).
A class description of the
"enumeration" kind is defined with the
owl:oneOf
property. The value of this built-in OWL property must be a
list of individuals
that are the instances of the class.
This enables a class to be described by
exhaustively enumerating its instances.
The class extension of a class described with
owl:oneOf
contains exactly the enumerated
individuals, no more, no less. The list of individuals is typically
represented with the help of the RDF construct
rdf:parseType="Collection"
, which provides a convenient
shorthand for
writing down a set of list elements.
For example, the following RDF/XML syntax
defines a class of all continents:
<owl:Class> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Eurasia"/> <owl:Thing rdf:about="#Africa"/> <owl:Thing rdf:about="#NorthAmerica"/> <owl:Thing rdf:about="#SouthAmerica"/> <owl:Thing rdf:about="#Australia"/> <owl:Thing rdf:about="#Antarctica"/> </owl:oneOf> </owl:Class>
The RDF/XML syntax <owl:Thing rdf:about="..."/>
refers to some individual (remember:
all individuals are by definition instances of
owl:Thing
).
In the section on datatypes we will see another use of
the owl:oneOf
construct, namely to define an enumeration of data values.
NOTE: Enumeration is not part of OWL Lite
A property restriction is a special kind of class description. It describes an anonymous class, namely a class of all individuals that satisfy the restriction. OWL distinguishes two kinds of property restrictions: value constraints and cardinality constraints.
A value constraint puts constraints on the
range of the property when applied to this particular class
description.
For example, we might want to
refer to those individuals whose value of the property adjacentTo
should be some Region
,
and then use this within a class axiom, perhaps
even a class axiom for Region
itself.
Note that this is different from rdfs:range
, which is
applied to all situations in which the property is used.
A cardinality
constraint puts constraints on the number of values a property can take,
in the context of this particular class description.
For example, we might want to say that for a soccer team the
hasPlayer
property has 11 values. For a basketball team the same property
would have only 5 values.
OWL also supports a limited set of constructs for defining global property cardinality, namely owl:FunctionalProperty and owl:InverseFunctionalProperty (see the section on properties).
Property restrictions have the following general form:
<owl:Restriction> <owl:onProperty rdf:resource="(some property)" /> (precisely one value or cardinality constraint, see below) </owl:Restriction>
The class
owl:Restriction
is defined as a subclass of owl:Class
.
A restriction class should
have exactly one triple linking
the restriction to a particular property, using the owl:onProperty
property. The restriction class should also have exactly
one triple that represents the value constraint c.q. cardinality
constraint on the property under consideration, e.g., that the
cardinality of the property is exactly 1.
Property restrictions can be applied both to datatype properties (properties for which the value is a data literal) and object properties (properties for which the value is an individual). For more information about this distinction, see the section on properties.
The value constraint owl:allValuesFrom
is a built-in OWL
property that links a restriction class to either a class description or a data
range. A restriction containing an owl:allValuesFrom
constraint is used to describe a class of all individuals for which all
values of the property under consideration are either members of the class
extension of the class description or are data values within the specified data
range. In other words, it defines a class of individuals x for which holds
that if the pair (x,y) is an instance of P (the property concerned), then y
should be an instance of the class description or a value in the data range,
respectively.
A simple example:
<owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:allValuesFrom rdf:resource="#Human" /> </owl:Restriction>
This example describes an anonymous OWL class of all individuals for which
the hasParent
property only has values of class
Human
. Note that this class description does not state
that the property
always has values of this class; just that this is true for individuals
that belong to the class extension of the anonymous restriction class.
NOTE: In OWL Lite the only type of class description allowed as object of
owl:allValuesFrom
is a class name.
An owl:allValuesFrom
constraint is analogous to the universal (for-all) quantifier of
Predicate logic - for each instance of the class that is
being described, every value for P must fulfill the constraint. Also
notice that the correspondence of owl:allValuesFrom
with the
universal quantifier means that an owl:allValuesFrom
constraint for
a property P is trivially satisfied for an individual that has no
value for property P at all. To see why this is so, observe that the
owl:allValuesFrom
constraint demands that all values of P should be of type T, and if no
such values exist, the constraint is trivially true.
The value constraint owl:someValuesFrom
is a built-in OWL property that links a restriction class to a class description or a data range. A restriction containing
an owl:someValuesFrom
constraint
describes a class of all individuals for which at least one value of the
property concerned is an instance of the class description or a data value in
the data range. In other words, it defines a class of individuals x for
which there is at least one y (either an instance of the class description or
value of the data range) such that the pair (x,y) is an instance of P. This
does not exclude that there are other instances (x,y') of P for which y' does
not belong to the class description or data range.
The following example defines a class of individuals which have at least one parent who is a physician:
<owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:someValuesFrom rdf:resource="#Physician" /> </owl:Restriction>
The owl:someValuesFrom
constraint is analogous to the existential quantifier of Predicate logic - for
each instance of the class that is being defined, there exists at
least one value for P that fulfills the constraint.
NOTE: In OWL Lite the only type of class description allowed as object of
owl:someValuesFrom
is a class name.
The value constraint owl:hasValue
is a built-in OWL property that links a restriction class
to a value V, which can be either an
individual or a data value.
A restriction containing a owl:hasValue
constraint
describes a class of all individuals for
which the property concerned has at least one value semantically
equal to V (it may have other values as well).
NOTE: for datatypes "semantically equal" means that the lexical representation of the literals maps to the same value. For individuals it means that they either have the same URI reference or are defined as being the same individual (see owl:sameAs).
NOTE: the value constraint owl:hasValue
is not included in OWL Lite.
The following example describes the class of individuals who have the
individual referred to as Clinton
as their parent:
<owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:hasValue rdf:resource="#Clinton" /> </owl:Restriction>
In OWL, like in RDF, it is assumed that any instance of a class may have an arbitrary number (zero or more) of values for a particular property. To make a property required (at least one), to allow only a specific number of values for that property, or to insist that a property must not occur, cardinality constraints can be used. OWL provides three constructs for restricting the cardinality of properties locally within a class context.
NOTE: OWL Lite includes the use of all three types of cardinality constraints, but only when used with the values "0" or "1".
The cardinality constraint
owl:maxCardinality
is a built-in OWL property that
links a restriction class to a data value belonging to the value space of
the XML Schema datatype nonNegativeInteger
. A restriction
containing an owl:maxCardinality
constraint describes a class of
all individuals that have at most N semantically distinct values
(individuals or data
values) for the property concerned, where N is the value of the
cardinality constraint. Syntactically, the cardinality constraint is
represented
as an RDF property element with the corresponding rdf:datatype
attribute.
The following example describes a class of individuals that have at most two parents:
<owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:maxCardinality rdf:datatype="&xsd;nonNegativeInteger">2</owl:maxCardinality> </owl:Restriction>
RDF datatyping is discussed in more detail in Sec. 6.
The cardinality constraint
owl:minCardinality
is a built-in OWL property that
links a restriction class to a data value belonging to the value space of
the XML Schema datatype nonNegativeInteger
. A restriction
containing an owl:minCardinality
constraint describes a class of
all individuals that have at least N semantically distinct values
(individuals or data
values) for the property concerned, where N is the value of the
cardinality constraint. Syntactically, the cardinality constraint is represented
as an RDF property element with the corresponding rdf:datatype
attribute.
The following example describes a class of individuals that have at least two parents:
<owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:minCardinality rdf:datatype="&xsd;nonNegativeInteger">2</owl:minCardinality> </owl:Restriction>
Note that an owl:minCardinality
of one or more means that
all instances of the class must have a value for the property.
The cardinality constraint
owl:cardinality
is a built-in OWL property that
links a restriction class to a data value belonging to the range of
the XML Schema datatype nonNegativeInteger
. A restriction
containing an owl:cardinality
constraint describes a class of
all individuals that have exactly N semantically distinct values
(individuals or data
values) for the property concerned, where N is the value of the
cardinality constraint. Syntactically, the cardinality constraint is represented
as an RDF property element with the corresponding rdf:datatype
attribute.
This construct is in fact redundant as it
can always be replaced by a pair of matching
owl:minCardinality
and
owl:maxCardinality
constraints with the same value. It is included as a convenient shorthand for
the user.
The following example describes a class of individuals that have exactly two parents:
<owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:cardinality rdf:datatype="&xsd;nonNegativeInteger">2</owl:cardinality> </owl:Restriction>
The three types of class descriptions in this section represent the more advanced class constructors that are used in Description Logic. They can be viewed as representing the AND, OR and NOT operators on classes. The three operators get the standard set-operator names: intersection, union and complement. These language constructs also share the characteristic that they contain nested class descriptions, either one (complement) or more (union, intersection).
The owl:intersectionOf
property links a class to a list of
class descriptions.
An owl:intersectionOf
statement
describes a class for which the class extension contains
precisely those individuals
that are members of the class extension of
all class descriptions in the list.
An example:
<owl:Class> <owl:intersectionOf rdf:parseType="Collection"> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Tosca" /> <owl:Thing rdf:about="#Salome" /> </owl:oneOf> </owl:Class> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Turandot" /> <owl:Thing rdf:about="#Tosca" /> </owl:oneOf> </owl:Class> </owl:intersectionOf> </owl:Class>
In this example the value of owl:intersectionOf
is a list of two
class descriptions, namely two enumerations, both describing a class
with two individuals. The resulting intersection is
a class with one individual, namely
Tosca
. as this is the only individual that is
common to both enumerations.
NOTE: This assumes that the three individuals are all different. In fact, this is not by definition true in OWL. Different URI references may refer to the same individuals, because OWL does not have a "unique names" assumption. In Sec. 5 one can find OWL language constructs for making constraints about equality and difference of individuals.
NOTE: In this example we use enumerations to make clear what the meaning is of this language construct. See the OWL Guide [OWL Guide] for more typical examples.
NOTE: OWL Lite is restricted in its use of owl:intersectionOf
.
This is discussed later in this document, see Sec. 3.2.3
owl:intersectionOf
can be viewed as being analogous to logical
conjunction.
The owl:unionOf
property links a class to a list of
class descriptions.
An owl:unionOf
statement
describes an anonymous class for which the class extension contains
those individuals
that occur in at least one of the class extensions of the
class descriptions in the list.
An example:
<owl:Class> <owl:unionOf rdf:parseType="Collection"> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Tosca" /> <owl:Thing rdf:about="#Salome" /> </owl:oneOf> </owl:Class> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Turandot" /> <owl:Thing rdf:about="#Tosca" /> </owl:oneOf> </owl:Class> </owl:unionOf> </owl:Class>
This class description describes a class for which the class extension
contains three individuals, namely Tosca
, Salome
, and Turandot
(assuming they are all different).
NOTE: owl:unionOf
is not part of OWL Lite.
owl:unionOf
is analogous to logical disjunction.
An
owl:complementOf
property links a class to precisely one class
description.
An owl:complementOf
statement describes a class for which the class
extension contains exactly those individuals
that do not belong to the class extension of the
class description that is the object of the statement.
owl:complementOf
is
analogous to logical negation: the class extension consists of those
individuals that are NOT members of the class extension of the
complement class.
As an example of the use of complement, the expression "not meat" could be written as:
<owl:Class> <owl:complementOf> <owl:Class rdf:about="#Meat"/> </owl:complementOf> </owl:Class>
The extension of this class description contains all individuals
that do not belong to the class Meat
.
NOTE: owl:complementOf
is not part of OWL Lite.
Class descriptions form the building blocks for defining classes
through class axioms. The simplest form of a class axiom is a class description
of type 1, It just states
the existence of a class, using
owl:Class
with a class identifier.
For example, the following
class axiom declares the URI reference #Human
to be the
name of an OWL class:
<owl:Class rdf:ID="Human"/>
This is correct OWL, but does not tell us very much about the class
Human
. Class axioms typically contain additional components that
state necessary and/or sufficient characteristics of a class. OWL contains
three language constructs for combining class descriptions into class
axioms:
rdfs:subClassOf
allows one
to say that the class extension of a
class description is a subset of the class extension of another class
description.owl:equivalentClass
allows one
to say that a class description has exactly the
same class extension as another class description.owl:disjointWith
allows one
to say that the class extension of a class description has no members in
common with the class extension of another class description.Syntactically, these three language constructs are properties that have a class description as both domain and range. We discuss these properties in more detail in the following subsections.
In addition, OWL allows class axioms in which a class description
of the enumeration or the set-operator type is given a name. These
class axioms are semantically equivalent to class axioms with a
owl:equivalentClass
statement, so these will be discussed
right after that subsection (see Sec. 3.2.3 "Axioms for complete classes without
using owl:equivalentClass").
AXIOM SCHEMA:
class description rdfs:subClassOf
class
description
The rdfs:subClassOf
construct is
defined as part of RDF Schema
[RDF Vocabulary]. Its meaning in OWL
is exactly the same: if the class description C1 is defined as a subclass of
class description C2, then the set of individuals in the class extension of C1
should be a subset of the set of individuals in the class extension of C2.
A class is by definition a subclass of itself (as the subset may be
the complete set).
An example:
<owl:Class rdf:ID="Opera"> <rdfs:subClassOf rdf:resource="#MusicalWork" /> </owl:Class>
This class axiom declares a subclass relation between two OWL classes that
are described through their names (Opera
and
MusicalWork
).
Subclass relations provide necessary conditions for belonging to a class. In
this case, to be an opera the individual also needs to be a musical work.
NOTE: In OWL Lite the subject of an
rdfs:subClassOf
statement must be a class identifier. The
object must be either a class identifier or a property restriction.
For any class there may be any number of subClassOf axioms.
For example, we could add the following axiom about the class
Opera
:
<owl:Class rdf:about="#Opera"> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#hasLibrettist" /> <owl:minCardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:minCardinality> </owl:Restriction> </rdfs:subClassOf> </owl:Class>
This class axiom contains
a property restriction. The example states that Opera
is a subclass of an anonymous OWL class (remember: owl:Restriction
is a subclass of owl:Class
) that has as its class extension
the set of all individuals
for which the property hasLibrettist
has at least one
value. Thus, operas should have at least one librettist.
Class axioms can get more complex when class descriptions are nested.
For example, property restrictions with an
owl:allValuesFrom
or
owl:someValuesFrom
statement may point to any class description.
Consider the following example:
<owl:Class rdf:ID="TraditionalItalianOpera"> <rdfs:subClassOf rdf:resource="#Opera"/> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#hasOperaType"/> <owl:someValuesFrom> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#OperaSeria"/> <owl:Thing rdf:about="#OperaBuffa"/> </owl:oneOf> </owl:Class> </owl:someValuesFrom> </owl:Restriction> </rdfs:subClassOf> </owl:Class>
This example shows the use of the owl:oneOf
construct.
The class axiom defines traditional Italian opera as a subclass of
a class of operas that have as opera type either opera seria or
opera buffa (without an additional cardinality constraint, it
could actually have both values).
More examples can be found in the Guide document
[OWL Guide].
Subclass axioms provide us with partial definitions: they represent necessary
but not sufficient conditions for establishing class membership of an
individual. In the next subsection we will see that for defining
necessary and sufficient conditions OWL provides the
owl:equivalentClass
construct. As a stepping stone to such axioms,
consider the following example:
<owl:Class rdf:ID="Operetta"> <rdfs:subClassOf rdf:resource="#MusicalWork"/> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="#hasLibrettist" /> <owl:minCardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:minCardinality> </owl:Restriction> </rdfs:subClassOf> <rdfs:subClassOf> <owl:Class> <owl:complementOf rdf:resource="#Opera"/> </owl:Class> </rdfs:subClassOf> </owl:Class>
This class axiom states that an operetta is a musical work, that has a
at least one
librettist and is not an opera. The use of the subclass relation leaves open
the possibility that there are other musical works that have a librettist and
are
not operas. If we had wanted to say that operetta's are exactly
those musical works that have a librettist but are not operas, we
would need to use
the
owl:equivalentClass
construct.
AXIOM SCHEMA:
class description owl:equivalentClass
class
description
A class axiom may contain (multiple)
owl:equivalentClass
statements. owl:equivalentClass
is a built-in
property that links a class description
to another class description.
The meaning of such a class axiom is that the two class descriptions involved
have the same class extension
(i.e., both class extensions contain exactly the same set of individuals).
In its simplest form, an equivalentClass axiom states the equivalence (in terms of their class extension) of two named classes. An example:
<owl:Class rdf:about="#US_President"> <equivalentClass rdf:resource="#PrincipalResidentOfWhiteHouse"/> </owl:Class>
NOTE:
The use of owl:equivalentClass
does not imply class
equality. Class equality means that the classes have the same intensional
meaning (denote the same concept). In the example above, the concept of
"President of the US" is related to, but not equal to the concept of the
principal resident of a certain estate.
Real class equality can only be expressed with the owl:sameAs
construct.
As this requires treating classes as
individuals, class equality can only be expressed in OWL Full.
Axioms with owl:equivalentClass
can also be used to
define an enumerated class by linking a type 1 class description (a
class identifier) to a type 2 class description (an enumeration). An
example:
<owl:Class rdf:ID="DaPonteOperaOfMozart"> <owl:equivalentClass> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <Opera rdf:about="#Nozze_di_Figaro"/> <Opera rdf:about="#Don_Giovanni"/> <Opera rdf:about="#Cosi_fan_tutte"/> </owl:oneOf> </owl:Class> </owl:equivalentClass> </owl:Class>
This class axiom defines the class of operas that together represent the "Da Ponte operas of Mozart" (a popular subject in musicology). By using the equivalentClass construct we can state necessary and sufficient conditions for class membership, in this case consisting of an enumeration of three individuals, no less, no more.
NOTE: OWL DL does not put any constraints on the
types of class descriptions that can be used as subject and object
of an owl:equivalentClass
statement. In OWL Lite
the subject must be a class name and the object must be either a class
name or a property restriction.
NOTE: Although in principle different types of class descriptions are allowed as the subject of an equivalentClass statement, in practice it usually is some class identifier. This is also true for the examples in this section.
It is possible to have multiple equivalentClass axioms about the same class. However, this requires care. Both axioms must lead to the same outcome, i.e. exactly the same class extension. For example, an alternate equivalentClass axiom for Mozart's "Da Ponte operas" could be the following one:
<owl:Class rdf:about="#DaPonteOperaOfMozart"> <owl:equivalentClass> <owl:Class> <owl:intersectionOf rdf:parseType="Collection"> <owl:Restriction> <owl:onProperty rdf:resource="#hasComposer"/> <owl:hasValue rdf:resource="#Wolfgang_Amadeus_Mozart"/> </owl:Restriction> <owl:Restriction> <owl:onProperty rdf:resource="#hasLibrettist"/> <owl:hasValue rdf:resource="#Lorenzo_Da_Ponte"/> </owl:Restriction> </owl:intersectionOf> </owl:Class> </owl:equivalentClass> </owl:Class>
This states that the class extension of the Da Ponte operas of Mozart corresponds exactly to those operas which are composed by Mozart and for which the libretto is written by Da Ponte (note: intersection = "and"). This axiom indeed defines a class with exactly the same instances as the previous axiom.
NOTE: If we wanted to "upgrade" an axiom of the form "A subClassOf B" to "A equivalentClass B" (meaning that the class extension of A is not just any subset, but in fact the same set as the class extension of B), we could add a second subClassOf axiom of the form (B subClassOf A), which by definition makes the two class extensions equivalent (and thus has the same meaning as "A equivalentClass B"). Such subClassOf "cycles" are explicitly allowed. As OWL is usable in a distributed environment, this can be a useful feature.
AXIOM SCHEMA:
named class description of type 2 (with owl:oneOf
) or
type 4-6 (with owl:intersectionOf
, owl:unionOf
or owl:complementOf
OWL allows users to define class axioms by giving a name to class descriptions of the enumeration or set-operator type. Such a class axiom defines necessary and sufficient conditions for establishing class membership. An example:
<owl:Class rdf:ID="DaPonteOperaOfMozart"> <owl:oneOf rdf:parseType="Collection"> <owl:Thing rdf:about="#Nozze_di_Figaro"/> <owl:Thing rdf:about="#Don_Giovanni"/> <owl:Thing rdf:about="#Cosi_fan_tutte"/> </owl:oneOf> </owl:Class>
This class axiom should be interpreted as follows: the class extension of
the class DaPonteOperaOfMozart
is
exactly defined by the enumeration.
This class axiom is semantically equivalent to the
first opera example in the previous section, which included an additional
owl:equivalentClass
statement. Axioms of this type can also be
constructed with owl:intersectionOf
,
owl:unionOf
and owl:complementOf
.
An example with a union could be:
<owl:Class rdf:ID="LivingBeing"> <owl:unionOf rdf:parseType="Collection"> <owl:Class rdf:about="#Plant"/> <owl:Class rdf:about="#Animal"/> </owl:unionOf> </owl:Class>
This class axiom states that the class extension of
LivingBeing
exactly corresponds
to the union of the class extensions of Plant
and Animal
.
NOTE: OWL Lite only includes class axioms of this type which are constructed
with the owl:intersectionOf
property. The values of the
intersectionOf list must be class identifiers and/or property restrictions.
Thus, "complete class" axioms
using enumeration, complement and
union are not allowed in OWL Lite.
AXIOM SCHEMA:
class description owl:disjointWith
class
description
A class axiom may also contain (multiple)
owl:disjointWith
statements.
owl:disjointWith
is a
built-in OWL property with a class
description as domain and range.
Each owl:disjointWith
statement asserts that
the class extensions of the two class descriptions
involved have no individuals in common. Like axioms with
rdfs:subClassOf
, declaring two classes to be disjoint is a partial
definition: it imposes a necessary but not sufficient condition on the
class.
This is a popular example of class disjointness:
<owl:Class rdf:about="#Man"> <owl:disjointWith rdf:resource="#Woman"/> </owl:Class>
Whether this is actually true is a matter for biologists to decide. The following example shows a common use of class disjointness in subclass hierarchies:
<owl:Class rdf:about="#MusicDrama"> <owl:equivalentClass> <owl:Class> <owl:unionOf rdf:parseType="Collection"> <owl:Class rdf:about="#Opera"/> <owl:Class rdf:about="#Operetta"/> <owl:Class rdf:about="#Musical"/> </owl:unionOf> </owl:Class> </owl:equivalentClass> </owl:Class> <owl:Class rdf:about="#Opera"> <rdfs:subClassOf rdf:resource="#MusicDrama"/> </owl:Class> <owl:Class rdf:about="#Operetta"> <rdfs:subClassOf rdf:resource="#MusicDrama"/> <owl:disjointWith rdf:resource="#Opera"/> </owl:Class> <owl:Class rdf:about="#Musical"> <rdfs:subClassOf rdf:resource="#MusicDrama"/> <owl:disjointWith rdf:resource="#Opera"/> <owl:disjointWith rdf:resource="#Operetta"/> </owl:Class>
Here, owl:disjointWith
statements are used together with
owl:unionOf
in order to define a set of mutually disjoint and complete subclasses of a
superclass. In natural language: every MusicDrama
is either an
opera, an Operetta
, or a Musical
(the subclass
partitioning is complete) and individuals belonging to one subclass,
e.g.,
Opera
, cannot belong to another subclass, e.g.,
Musical
(disjoint or non-overlapping subclasses). This is a common
modelling notion used in many data-modelling notations.
NOTE: OWL Lite does not allow the use of owl:disjointWith
.
OWL distinguishes between two main categories of properties that an ontology builder may want to define:
NOTE: OWL also has the notion of annotation properties
(owl:AnnotationProperty
) and ontology properties
(owl:OntologyProperty
). These are needed in OWL DL for semantic
reasons. See Sec. 7
and the OWL Semantics and Abstract Syntax document
[OWL S&AS].
An object property is defined as an instance of the built-in OWL class
owl:ObjectProperty.
A datatype property is defined as an instance of the built-in
OWL class owl:DatatypeProperty.
Both owl:ObjectProperty
and
owl:DatatypeProperty
are subclasses of the RDF class
rdf:Property
(see Appendix B).
NOTE: In OWL Full, object properties and datatype properties are
not disjoint. Because data values can be treated as individuals, datatype
properties are effectively subclasses of object properties.
In OWL Full owl:ObjectProperty
is equivalent to
rdf:Property
In practice, this
mainly has consequences for the use of
owl:InverseFunctionalProperty
.
See also the OWL Full characterization in Sec. 8.1.
A property axiom defines characteristics of a property. In its simplest form, a property axiom just defines the existence of a property. For example:
<owl:ObjectProperty rdf:ID="hasParent"/>
This defines a property with the restriction that its values should be individuals.
Often, property axioms define additional characteristics of properties. OWL supports the following constructs for property axioms:
rdfs:subPropertyOf
,
rdfs:domain
and rdfs:range
owl:equivalentProperty
and
owl:inverseOf
owl:FunctionalProperty
and
owl:InverseFunctionalProperty
owl:SymmetricProperty
and
owl:TransitiveProperty
In the next subsections, the various types of property axioms are discussed in more detail.
NOTE: In this section we use the term "property extension" in a similar fashion to "class extension". The property extension is the set of instances that is associated with the property. Instances of properties are not single elements, but subject-object pairs of property statements. In relational database terms, property instances would be called "tuples" of a binary relation (the property).
The constructs in this section are discussed in detail in the RDF Schema document [RDF Vocabulary]. The description in this section provides a synopsis of these constructs and provides some OWL-specific aspects and examples.
A rdfs:subPropertyOf
axiom defines that the property
is a subproperty of some other property.
Formally this means that if P1 is a subproperty of P2, then the property
extension of P1 (a set of pairs) should be a subset of the property extension
of P2 (also a set of pairs).
An example:
<owl:ObjectProperty rdf:ID="hasMother"> <rdfs:subPropertyOf rdf:resource="#hasParent"/> </owl:ObjectProperty>
This states that all instances (pairs) contained in the property extension of the property "hasMother" are also members of the property extension of the property "hasParent".
Subproperty axioms can be applied to both datatype properties and object properties.
NOTE: In OWL DL the subject and object of a subproperty statement must be either both datatype properties or both object properties.
For a property one can define (multiple)
rdfs:domain
axioms.
Syntactically, rdfs:domain
is a built-in
property that links a property
(some instance of the class rdf:Property
)
to a class
description.
An rdfs:domain
axiom asserts that the subjects of such
property statements must belong to the class extension of
the indicated class description.
Multiple rdfs:domain
axioms are allowed and should be interpreted as a conjunction: these
restrict the domain of the property to those individuals that belong to the
intersection of the class descriptions. If one would want to say
that multiple classes can act as domain, one should use a class description
of the owl:unionOf
form.
For example, if we want to say that the domain of the property
hasBankAccount
can be either a Person
or a Corporation
, we would need to
say something like this:
<owl:ObjectProperty rdf:ID="hasBankAccount"> <rdfs:domain> <owl:Class> <owl:unionOf rdf:parseType="Collection"> <owl:Class rdf:about="#Person"/> <owl:Class rdf:about="#Corporation"/> </owl:unionOf> </owl:Class> </rdfs:domain> </owl:ObjectProperty>
NOTE: In OWL Lite the value of rdfs:domain
must be a class identifier.
For a property one can define (multiple)
rdfs:range
axioms.
Syntactically, rdfs:range
is a built-in
property that links a property
(some instance of the class rdf:Property
) to
to either a class
description or a data range.
An rdfs:range
axiom asserts that the values of this
property must belong to the class extension of
the class description or to data values in the specified data range.
Multiple range
restrictions are interpreted as stating that the range of the property is the
intersection of all ranges (i.e.,
the intersection of the class extension of the class descriptions
c.q. the intersection of the data ranges). Similar to
rdfs:domain
,
multiple alternative
ranges can be specified by using a class description of the
owl:unionOf
form (see the previous
subsection).
Note that, unlike any of the value
constraints described in the section on class descriptions,
rdfs:range
restrictions are
global. Value constraints such as owl:allValuesFrom
are used in a class description and are only
enforced on the property when applied to that class. In contrast,
rdfs:range
restrictions apply to the property irrespective of the
class to which it is applied.
Thus, rdfs:range
should be
used with care.
NOTE: In OWL Lite the only type of class descriptions allowed as objects of
rdfs:range
are class names.
The owl:equivalentProperty
construct can be used to state that two properties have the same property
extension. Syntactically, owl:equivalentProperty
is a
built-in OWL property
with rdf:Property
as both its domain and range.
NOTE: Property equivalence is not the same as property
equality. Equivalent properties have the same "values" (i.e.,
the same property extension),
but may have different intensional meaning (i.e., denote different concepts).
Property equality should be expressed with the owl:sameAs
construct. As this
requires that properties are treated as individuals, such axioms are only
allowed in OWL Full.
Properties have a direction, from domain to range. In practice, people often find it useful to define relations in both directions: persons own cars, cars are owned by persons. The owl:inverseOf construct can be used to define such an inverse relation between properties.
Syntactically, owl:inverseOf
is a built-in OWL
property with owl:ObjectProperty
as its domain and
range.
An axiom of the
form P1 owl:inverseOf P2
asserts
that for every pair
(x,y) in the property extension of P1, there is a pair (y,x) in the property
extension of P2, and vice versa. Thus, owl:inverseOf
is a
symmetric property.
An example:
<owl:ObjectProperty rdf:ID="hasChild"> <owl:inverseOf rdf:resource="#hasParent"/> </owl:ObjectProperty>
A functional property is a property that can have only one
(unique) value y for each instance x, i.e. there
cannot be two distinct values y1 and y2 such that the pairs (x,y1) and
(x,y2) are both instances of this property.
Both object properties and datatype properties can be declared as
"functional". For this purpose,
OWL defines the built-in class owl:FunctionalProperty
as a special subclass of the RDF class
rdf:Property
.
The following axiom states that the husband
property is
functional, i.e., a woman can have at most one husband (a good
example of culture dependence of ontologies):
<owl:ObjectProperty rdf:ID="husband"> <rdf:type rdf:resource="&owl;FunctionalProperty" /> <rdfs:domain rdf:resource="#Woman" /> <rdfs:range rdf:resource="#Man" /> </owl:ObjectProperty>
As always, there are syntactic variations. The example above is semantically equivalent to the one below:
<owl:ObjectProperty rdf:ID="husband"> <rdfs:domain rdf:resource="#Woman" /> <rdfs:range rdf:resource="#Man" /> </owl:ObjectProperty> <owl:FunctionalProperty rdf:about="#husband" />
If a
property is declared to be inverse-functional, then the object of a
property statement uniquely
determines the subject (some individual). More formally, if we state that
P is an owl:InverseFunctionalProperty
, then this asserts that a
value y can only be the value of P for a single instance x, i.e. there
cannot be two distinct instances x1 and x2 such that both pairs (x1,y) and
(x2,y) are instances of P.
Syntactically, an inverse-functional property axiom is specified by
declaring the property to be an instance of the built-in OWL class
owl:InverseFunctionalProperty,
which is a subclass of the OWL class owl:ObjectProperty
.
NOTE: Because in OWL Full datatype properties are a subclass of object properties, an inverse-functional property can be defined for datatype properties. In OWL DL object properties and datatype properties are disjoint, so an inverse-functional property cannot be defined for datatype properties. See also Sec. 8.1 and Sec. 8.2.
A typical example of an inverse-functional property:
<owl:InverseFunctionalProperty rdf:ID="biologicalMotherOf"> <rdfs:domain rdf:resource="#Woman"/> <rdfs:range rdf:resource="#Human"/> </owl:InverseFunctionalProperty>
This example states that for each object
of biologicalMotherOf
statements (some human) one should be able
to uniquely identify a subject (some woman). Inverse-functional
properties resemble the notion of a key in databases.
One difference with functional properties is that for inverse-functional properties no additional object-property or datatype-property axiom is required: inverse-functional properties are by definition object properties.
Notice that owl:FunctionalProperty
and owl:InverseFunctionalProperty
specify global cardinality constraints. That is, no matter which class the
property is applied to, the cardinality constraints must hold. This is
different from the cardinality constraints contained in property restrictions. The latter are class
descriptions and are only enforced on the property when applied to that
class.
When one defines a property P to be a transitive property, this means that if a pair (x,y) is an instance of P, and the pair (y,z) is also instance of P, then we can infer the the pair (x,z) is also an instance of P.
Syntactically, a property is defined as being transitive by making it an
instance of the the built-in OWL class
owl:TransitiveProperty,
which is defined as a subclass of owl:ObjectProperty
.
Typical examples of transitive properties are properties
representing certain part-whole relations.
For example, we might want to say that the
subRegionOf
property between regions is transitive:
<owl:TransitiveProperty rdf:ID="subRegionOf"> <rdfs:domain rdf:resource="#Region"/> <rdfs:range rdf:resource="#Region"/> </owl:TransitiveProperty>
From this an OWL reasoner should be able to derive that if
ChiantiClassico
,
Tuscany
and Italy
are regions, and
ChiantiClassico
is a subregion of Tuscany
, and
Tuscany
is a subregion of Italy
, then
ChiantiClassico
is also a subregion of Italy
.
Note that because owl:TransitiveProperty
is a subclass of
owl:ObjectProperty
, the following syntactic variant is equivalent
to the example above:
<owl:ObjectProperty rdf:ID="subRegionOf"> <rdf:type rdf:resource="&owl;TransitiveProperty"/> <rdfs:domain rdf:resource="#Region"/> <rdfs:range rdf:resource="#Region"/> </owl:ObjectProperty>
NOTE: OWL DL requires that for a transitive property no local or global cardinality constraints should be declared on the property itself or its superproperties, nor on the inverse of the property or its superproperties.
A symmetric property is a property for which holds that
if the pair (x,y) is an instance
of P, then the pair (y,x) is also
an instance of P.
Syntactically, a property is defined as symmetric by making it an
instance of the built-in OWL class
owl:SymmetricProperty,
a subclass of owl:ObjectProperty
.
A popular example of a symmetric property is the friendOf
relation:
<owl:SymmetricProperty rdf:ID="friendOf"> <rdfs:domain rdf:resource="#Human"/> <rdfs:range rdf:resource="#Human"/> </owl:SymmetricProperty>
The domain and range of a symmetric property are the same.
Individuals are defined with individual axioms (also called "facts"). We discuss two types of facts:
Many facts typically are statements indicating class membership
of individuals and property values of individuals.
As an
example, consider the following set of statements about an instance of the
class Opera
:
<Opera rdf:ID="Tosca"> <hasComposer rdf:resource="#Giacomo_Puccini"/> <hasLibrettist rdf:resource="#Victorien_Sardou"/> <hasLibrettist rdf:resource="#Giuseppe_Giacosa"/> <hasLibrettist rdf:resource="#Luigi_Illica"/> <premiereDate rdf:datatype="&xsd;date">1900-01-14</premiereDate> <premierePlace rdf:resource="#Roma"/> <numberOfActs rdf:datatype="&xsd;positiveInteger">3</numberOfActs> </Opera>
This example includes a number of facts
about the individual
Tosca
, an instance of the class Opera
.
Tosca
is
composed by Giacomo Puccini.
The opera has three libretto writers. The property
premiereDate
links an opera to a typed literal
with the XML Schema datatype
date
. The XML schema document on datatypes
[XML Schema Datatypes]
contains the relevant information about syntax and semantics of
this datatype.
Individual axioms need not necessarily be about named individuals: they can
also refer to anonymous individuals. As an example, consider the piece of
RDF/XML below. The example defines some facts about an anonymous instance of
the class
Measurement
, a quantitative observation for which facts such as
the subject under observation, the observed phenomenon, the observed value, and
the observation time are listed:
<Measurement> <observedSubject rdf:resource="#JaneDoe"/> <observedPhenomenon rdf:resource="#Weight"/> <observedValue> <Quantity> <quantityValue rdf:datatype="&xsd;float">59.5</quantityValue> <quantityUnit rdf:resource="#Kilogram"/> </Quantity> </observedValue> <timeStamp rdf:datatype="&xsd;dateTime">2003-01-24T09:00:08+01:00</timeStamp> </Measurement>
This individual axiom contains two anonymous individuals, namely some
Measurement
and some Quantity
. In natural language,
for
the subject Jane Doe the measured value of the phenomenon
Weight
is some
quantity, which has a value of 59.5 using the unit of kilogram. The time of
measurement is January 24, 2003, eight seconds past nine in the morning, in the
time zone UTC+1 (winter time in Amsterdam, Berlin, Paris). As before,
float
and dateTime
are XML Schema datatypes, the
syntactic and semantic details of which can be found in the relevant XML Schema
documentation
[XML Schema Datatypes].
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:
owl:sameAs
is used to state that two URI references refer to
the same individual. owl:differentFrom
is used to state that two URI references
refer to different individualsowl:AllDifferent
provides an idiom for stating that a list
of individuals are all different.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".
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"/> </rdf:Description>
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"/> </owl:Class>
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].
The built-in OWL owl:differentFrom
property links an individual to an individual.
An owl:differentFrom
statement indicates that two URI references
refer to different individuals.
An example:
<Opera rdf:ID="Don_Giovanni"/> <Opera rdf:ID="Nozze_di_Figaro"> <owl:differentFrom rdf:resource="#Don_Giovanni"/> </Opera> <Opera rdf:ID="Cosi_fan_tutte"> <owl:differentFrom rdf:resource="#Don_Giovanni"/> <owl:differentFrom rdf:resource="#Nozze_di_Figaro"/> </Opera>This states that there are three different operas.
For ontologies in which the unique-names assumption holds, the use
of owl:differentFrom
is likely to lead to a large number
of statements, as all individuals have to be declared pairwise
disjoint. For such situations OWL provides a special idiom in the form
of the construct owl:AllDifferent.
owl:AllDifferent
is a special built-in OWL class, for
which the property owl:distinctMembers
is
defined, which links an instance of owl:AllDifferent
to a
list of individuals. The intended meaning of such a statement is that
all individuals in the list are all different from each other.
An example:
<owl:AllDifferent> <owl:distinctMembers rdf:parseType="Collection"> <Opera rdf:about="#Don_Giovanni"/> <Opera rdf:about="#Nozze_di_Figaro"/> <Opera rdf:about="#Cosi_fan_tutte"/> <Opera rdf:about="#Tosca"/> <Opera rdf:about="#Turandot"/> <Opera rdf:about="#Salome"/> </owl:distinctMembers> </owl:AllDifferent>
This states that these six URI references all point to different operas.
NOTE: owl:distinctMembers
is a special syntactical construct added
for convenience and should always be used with an owl:AllDifferent
individual as its subject.
In a number of places in this document we have seen the notion of a data range for specifying a range of data values. OWL allows three types of data range specifications:
owl:oneOf
construct.The minimal level of tool support for datatypes is discussed in Sec. 6.3.
OWL makes use of the RDF datatyping scheme, which provides a mechanism for referring to XML Schema datatypes [XML Schema Datatypes]. For a detailed description the reader is referred to the RDF documents, e.g., [RDF Concepts]. For the convenience of the reader, we provide here a synopsis of the use of RDF datatypes.
Data values are instances of the RDF Schema class
rdfs:Literal
. Literals can be either
plain (no datatype) or typed. Datatypes are instances of the class
rdfs:Datatype
.
In RDF/XML, the type of a literal is specified by an rdf:datatype
attribute of which
the value is recommended to be one of the following:
http://www.w3.org/2001/XMLSchema#NAMEwhere "NAME" should be the name of a simple XML Schema built-in datatype, as defined in Section 3 of [XML Schema Datatypes], with the provisos specified below.
rdf:XMLLiteral
.
This datatype is used to
include XML content into an RDF/OWL document. The URI reference of this
datatype is:
http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteralFor details about this datatype, see the RDF Concepts document [RDF Concepts].
The RDF Semantics document [RDF Semantics, Section 5]. recommends use of the following simple built-in XML Schema datatypes.
xsd:string
,
plus the following datatypes derived from xsd:string:
xsd:normalizedString
,
xsd:token
,
xsd:language
,
xsd:NMTOKEN
,
xsd:Name
,and
xsd:NCName
.xsd:boolean
.xsd:decimal
,
xsd:float
, and
xsd:double
,
plus all derived types of xsd:decimal
(
xsd:integer
,
xsd:positiveInteger
.
xsd:nonPositiveInteger
,
xsd:negativeInteger
,
xsd:nonNegativeInteger
,
xsd:long
,
xsd:int
,
xsd:short
,
xsd:byte
,
xsd:unsignedLong
,
xsd:unsignedInt
,
xsd:unsignedShort
,
xsd:unsignedByte
)xsd:dateTime
,
xsd:time
,
xsd:date
,
xsd:gYearMonth
,
xsd:gYear
,
xsd:gMonthDay
,
xsd:gDay
, and
xsd:gMonth
.xsd:hexBinary
,
xsd:base64Binary
, and
xsd:anyURI
.
NOTE: It is not illegal, although not recommended, for applications to define
their own datatypes by defining an instance of rdfs:Datatype
. Such
datatypes are "unrecognized", but are treated in a similar fashion as
"unsupported datatypes" (see Sec. 6.3
for details about how these should be treated by OWL tools).
When using datatypes, please note that even if a property is defined
to have a range of a certain datatype, RDF/XML still requires that
the datatype be specified each time the property is used.
An example could be the declaration of a property
that we used earlier in the Measurement
example:
<owl:DatatypeProperty rdf:about="#timeStamp"> <rdfs:domain rdf:resource="#Measurement"/> <rdf:range rdf:resource="&xsd;dateTime"/> </owl:DatatypeProperty> <Measurement> <timeStamp rdf:datatype="&xsd;dateTime">2003-01-24T09:00:08+01:00</timeStamp> </Measurement>
In addition to the RDF datatypes, OWL provides one additional construct for
defining a range of data values, namely an enumerated datatype. This datatype
format makes use of the owl:oneOf
construct, that is also used for
describing an enumerated class. In the case of
an enumerated datatype, the subject of owl:oneOf
is a blank
node of class
owl:DataRange
and the object is a list of
literals. Unfortunately, we cannot use the
rdf:parseType="Collection"
idiom for specifying the literal list,
because RDF requires the collection to be a list of RDF node
elements. Therefore we have to specify the list of data values with the basic
list constructs rdf:first
, rdf:rest
and
rdf:nil
.
NOTE: Enumerated datatypes are not part of OWL Lite.
The example below specifies the range of the property
tennisGameScore
to be the list of integer values {0, 15,
30, 40}:.
<owl:DatatypeProperty rdf:ID="tennisGameScore"> <rdfs:range> <owl:DataRange> <owl:oneOf> <rdf:List> <rdf:first rdf:datatype="&xsd;integer">0</rdf:first> <rdf:rest> <rdf:List> <rdf:first rdf:datatype="&xsd;integer">15</rdf:first> <rdf:rest> <rdf:List> <rdf:first rdf:datatype="&xsd;integer">30</rdf:first> <rdf:rest> <rdf:List> <rdf:first rdf:datatype="&xsd;integer">40</rdf:first> <rdf:rest rdf:resource="&rdf;nil" /> </rdf:List> </rdf:rest> </rdf:List> </rdf:rest> </rdf:List> </rdf:rest> </rdf:List> </owl:oneOf> </owl:DataRange> </rdfs:range> </owl:DatatypeProperty>
Tools may vary in terms of support for datatype reasoning. As a minimum,
tools must support datatype reasoning for the XML Schema datatypes
xsd:string
and xsd:integer
.
OWL Full tools must also support rdf:XMLLiteral
.
For unsupported datatypes, lexically identical
literals should be considered equal, whereas lexically different literals
would not be known to be either equal or unequal.
Unrecognized datatypes should be treated in the same way as
unsupported datatypes.
OWL Full does not put any constraints on annotations in an ontology. OWL DL allows annotations on classes, properties, individuals and ontology headers, but only under the following conditions:
dc:creator
cannot be at the same time a datatype property and
an annotation property.AnnotationPropertyID rdf:type owl:AnnotationProperty .
Five annotation properties are predefined by OWL, namely:
owl:versionInfo
rdfs:label
rdfs:comment
rdfs:seeAlso
rdfs:isDefinedBy
Here is an example of legal use of an annotation property in OWL DL:
<owl:AnnotationProperty rdf:about="&dc;creator"/> <owl:Class rdf:about="#MusicalWork"> <rdfs:label>Musical work</rdfs:label> <dc:creator>N.N.</dc:creator> </owl:Class>
The example assumes &dc;
and dc:
point
respectively to the Dublin Core URI and namespace. Thus, using
Dublin Core properties as annotation properties
in OWL DL requires an explicit typing triple.
This ensures annotations are handled in a semantically correct fashion by OWL
DL reasoners
(see the OWL Semantics and Abstract Syntax document [OWL S&AS] for details).
Once we define dc:creator
as an annotation property, OWL DL does
NOT allow property axioms such as the range constraint below:
<-- This is illegal in OWL DL --> <owl:AnnotationProperty rdf:about="&dc;creator"> <rdfs:range rdf:resource="&xsd;string"/> </owl:AnnotationProperty>
Note that one can still specify the value type of a literal in an annotation-property statement:
<Opera rdf:about="#Tosca"> <dc:creator rdf:datatype="&xsd;string">Giacomo Puccini</dc:creator> </Opera>
A document describing an ontology typically contains information about the ontology itself. An ontology is a resource, so it may be described using properties from the OWL and other namespaces, e.g.:
<owl:Ontology rdf:about=""> <owl:versionInfo> ... </owl:versionInfo> <rdfs:comment>...</rdfs:comment> <owl:imports rdf:resource="..."/> </owl:Ontology>
This is commonly called the ontology header and is typically found near the beginning of the RDF/XML document. The line
<owl:Ontology rdf:about="">
states that this block describes the current ontology. More
precisely, it states the current base URI identifies an instance of
the class owl:Ontology
.
It is recommended that the
base URI be defined using an xml:base
attribute in the
<rdf:RDF>
element at the beginning of the document.
A sample ontology header could look like this:
<owl:Ontology rdf:about=""> <owl:versionInfo>v 1.17 2003/02/26 12:56:51 mdean</owl:versionInfo> <rdfs:comment>An example ontology</rdfs:comment> <owl:imports rdf:resource="http://www.example.org/foo"/> </owl:Ontology>
The following sections describe the various types of statements that are typically used within the header.
NOTE: The ontology-import construct owl:imports
and the ontology-versioning constructs
owl:priorVersion,
owl:backwardCompatibleWith
and
owl:incompatibleWith
are defined in the OWL vocabulary as instances of the OWL built-in
class owl:OntologyProperty
.
Instances of owl:OntologyProperty
must have the class
owl:Ontology
as their domain and range.
It is permitted to define other instances of
owl:OntologyProperty
. In OWL DL for
ontology properties the same constraints hold as those specified for
annotation properties in Sec. 7.1.
An owl:imports
statement references another OWL ontology containing definitions,
whose meaning
is considered to be part of the meaning of
the importing ontology. Each reference consists of a URI
specifying from where the ontology is to be imported.
Syntactically, owl:imports
is a property with the
class owl:Ontology
as its domain and range.
The owl:imports
statements are transitive, that is, if
ontology A imports B, and B imports C, then A imports both B and C.
Note that whether or not an OWL tool must load an imported ontology depends on the purpose of the tool. If the tool is a complete reasoner (including complete consistency checkers) then it must load all of the imported ontologies. Other tools, such as simple editors and incomplete reasoners, may choose to load only some or even none of the imported ontologies.
Although owl:imports and namespace declarations may appear redundant, they actually serve different purposes. Namespace declarations simply set up a shorthand for referring to identifiers. They do not implicitly include the meaning of documents located at the URI. On the other hand, owl:imports does not provide any shorthand notation for referring to the identifiers from the imported document. Therefore, it is common to have a corresponding namespace declaration for any ontology that is imported.
NOTE: owl:imports
is an instance of owl:OntologyProperty
.
An
owl:versionInfo
statement generally has as its object a string giving information about this
version, for example RCS/CVS keywords. This statement does not
contribute to the logical meaning of the ontology other than that
given by the RDF(S) model theory.
Although this property is typically used to make statements about
ontologies, it may be applied to any OWL construct. For example, one could
attach a owl:versionInfo
statement to an OWL class.
NOTE: owl:versionInfo
is an instance of owl:AnnotationProperty
.
An owl:priorVersion
statement contains a reference to another ontology. This identifies
the specified ontology as a prior version of the containing ontology.
This has no meaning in the model-theoretic semantics other than that
given by the RDF(S) model theory. However, it may be used by software
to organize ontologies by versions.
owl:priorVersion
is a built-in OWL property with the class
owl:Ontology
as its domain and range.
NOTE: owl:priorVersion
is an instance of owl:OntologyProperty
.
An owl:backwardCompatibleWith
statement contains a reference to another ontology. This identifies
the specified ontology as a prior version of the containing ontology,
and further indicates that it is backward compatible with it. In
particular, this indicates that all identifiers from the previous
version have the same intended interpretations in the new version.
Thus, it is a hint to document authors that they can safely change
their documents to commit to the new version (by simply updating
namespace declarations and owl:imports
statements to refer to the URL of
the new version). If owl:backwardCompatibleWith
is not
declared for two versions, then compatibility should not be assumed.
owl:backwardCompatibleWith
has no meaning in the model theoretic semantics other than that given by the
RDF(S) model theory.
owl:backwardCompatibleWith
is a built-in OWL
property with the class owl:Ontology
as its domain and range.
NOTE:
owl:backwardCompatibleWith
is an instance of owl:OntologyProperty
.
An owl:incompatibleWith
statement contains a reference to another ontology. This indicates that the
containing ontology is a later version of the referenced ontology, but is not
backward compatible with it. Essentially, this is for use by ontology authors
who want to be explicit that documents cannot upgrade to use the new version
without checking whether changes are required.
owl:incompatibleWith
has
no meaning in the model theoretic semantics other than that given by the
RDF(S) model theory.
owl:incompatibleWith
is a built-in OWL property with
the class owl:Ontology
as its domain and range.
NOTE:
owl:backwardCompatibleWith
is an instance of owl:OntologyProperty
.
Deprecation is a feature commonly used in versioning software (for
example, see the Java programming language) to indicate that a
particular feature is preserved for backward-compatibility purposes,
but may be phased out in the future. Here, a specific identifier is
said to be of type owl:DeprecatedClass
or owl:DeprecatedProperty,
where owl:DeprecatedClass
is a subclass of
rdfs:Class
and owl:DeprecatedProperty
is a
subclass of rdf:Property
. By deprecating a term, it
means that the term should not be used in new documents that commit to
the ontology. This allows an ontology to maintain
backward-compatibility while phasing out an old vocabulary (thus, it
only makes sense to use deprecation in combination with backward
compatibility). As a result, it it easier for old data and
applications to migrate to a new version, and thus can increase the
level of adoption of the new version. This has no meaning in the
model theoretic semantics other than that given by the RDF(S) model
theory. However, authoring tools may use it to warn users when
checking OWL markup.
An example of deprecation is:
<owl:Ontology rdf:about=""> <rdfs:comment>Vehicle Ontology, v. 1.1</rdfs:comment> <owl:backwardCompatibleWith rdf:resource="http://www.example.org/vehicle-1.0"/> <owl:priorVersion rdf:resource="http://www.example.org/vehicle-1.0"/> </owl:Ontology> <owl:DeprecatedClass rdf:ID="Car"> <rdfs:comment>Automobile is now preferred</rdfs:comment> <owl:equivalentClass rdf:resource="#Automobile"/> <!-- note that equivalentClass only means that the classes have the same extension, so this DOES NOT lead to the entailment that Automobile is of type DeprecatedClass too --> </owl:DeprecatedClass> <owl:Class rdf:ID="Automobile" /> <owl:DeprecatedProperty rdf:ID="hasDriver"> <rdfs:comment>inverse property drives is now preferred</rdfs:comment> <owl:inverseOf rdf:resource="#drives" /> </owl:DeprecatedProperty> <owl:ObjectProperty rdf:ID="drives" />
In the introduction we briefly discussed the three sublanguages of OWL. In this section an informative specification is given of the differences between the three "species" of OWL. A formal account of the differences is given in the Semantics and Abstract Syntax document [OWL S&AS].
OWL Full is not actually a sublanguage. OWL Full contains all the OWL
language constructs and provides free, unconstrained use of RDF constructs.
In OWL Full the resource owl:Class
is equivalent to
rdfs:Class
. This is different from OWL DL
and OWL Lite, where owl:Class
is a proper subclass of
rdfs:Class
(meaning that not all RDF classes are OWL
classes in OWL DL and OWL Lite).
OWL Full also allows classes to be treated as individuals. For
example, it is perfectly legal in OWL Full to have a "Fokker-100" identifier
which acts both as a class name (denoting the set of Fokker-100 airplanes
flying around the world) and as an individual name (e.g.,
an instance of the
class AirplaneType
).
In OWL Full all data values are considered also to be part of the individual
domain. In fact, in OWL Full the universe of individuals consists of
all resources (owl:Thing
is equivalent to rdfs:Resource
).
This means that object properties and datatype properties are not
disjoint. In OWL Full owl:ObjectProperty
is equivalent to
rdf:Property
. The consequence is
that datatype properties are effectively
a subclass of object properties. (Note: the fact that
owl:ObjectProperty
and owl:DatatypeProperty
are both
subclasses of rdf:Property
is not inconsistent with this).
OWL Full will typically be useful for people who want to combine the expressivity of OWL with the flexibility and metamodelling features of RDF. However, use of the OWL Full features means that one loses some guarantees (see below) that OWL DL and OWL Lite can provide for reasoning systems.
NOTE: RDF documents will generally be in OWL Full, unless they are specifically constructed to be in OWL DL or Lite.
NOTE: Thus, in OWL Full
owl:Thing
is equivalent to
rdfs:Resource
,
owl:Class
is equivalent to
rdfs:Class
, and
owl:ObjectProperty
is equivalent to
rdf:Property
,
OWL DL is a sublanguage of OWL which places a number of constraints on the use of the OWL language constructs. Briefly, these constraints are the following:
The last constraint implies that all classes and properties that one refers to are explicitly typed as OWL classes or properties, respectively. For example, if the ontology contains the following component:
<owl:Class rdf:ID="C1"> <rdfs:subClassOf rdf:resource="#C2" /> </owl:Class>
then the ontology (or an ontology imported into this ontology)
should contain a owl:Class
triple for
C2
.
These constraints of OWL DL may seem like an arbitrary set, but in fact they are not. The constraints are based on work in the area of reasoners for Description Logic, which require these restrictions to provide the ontology builder or user with reasoning support. In particular, the OWL DL restrictions allow the maximal subset of OWL Full against which current research can assure that a decidable reasoning procedure can exist for an OWL reasoner.
NOTE: Appendix E provides a set of practical guidelines for specifying OWL DL ontologies in RDF.
OWL Lite abides by all the restrictions OWL DL puts on the use of the OWL language constructs. In addition, OWL Lite forbids the use of:
OWL Lite also requires that:
owl:equivalentClass
triples be class names
and the object of owl:equivalentClass
triples be class names
or restrictions.rdfs:subClassOf
triples be class names
and the object of rdfs:subClassOf
triples be class names or
restrictions;owl:intersectionOf
be used only on lists of length greater
than one that contain only class names and restrictions;
NOTE: This is a prototypical example of legal use of
owl:intersectionOf
in OWL Lite:
<owl:Class rdf:ID="Woman"> <owl:intersectionOf rdf:parseType="Collection"> <owl:Class rdf:about="#Female"/> <owl:Class rdf:about="#Human"/> </owl:intersectionOf> </owl:Class/>
owl:allValuesFrom
and
owl:someValuesFrom
triples be class
names or datatype names;rdf:type
triples be class names or
restrictions;rdfs:domain
triples be class names;
andrdfs:range
triples be class names or
datatype names.
The idea behind the OWL Lite expressivity limitations is that they provide a minimal useful subset of language features, that are relatively straightforward for tool developers to support. The language constructs of OWL Lite provide the basics for subclass hierarchy construction: subclasses and property restrictions. In addition, OWL Lite allows properties to be made optional or required. The limitations on OWL Lite place it in a lower complexity class than OWL DL. This can have a positive impact on the efficiency of complete reasoners for OWL Lite.
Implementations that support only the OWL Lite vocabulary, but otherwise relax the restrictions of OWL DL, cannot make certain computational claims with respect to consistency and complexity. However, such implementations may be useful in providing interoperability of OWL systems with RDFS models, databases, markup tools, or other non-reasoning tools. The Web Ontology Working Group has not provided a name for this potentially useful subset.
NOTE: This appendix only contains the OWL-specific constructs. For the RDF/RDFS constructs see the relevant RDF documentation, in particular the RDF Schema document [RDF Vocabulary].
See Sec. 1.7 for a description of the purpose of this appendix. The RDF/XML version of this appendix can be found at http://www.w3.org/2002/07/owl
<?xml version="1.0"?> <!DOCTYPE rdf:RDF [ <!ENTITY rdf "http://www.w3.org/1999/02/22-rdf-syntax-ns#" > <!ENTITY rdfs "http://www.w3.org/2000/01/rdf-schema#" > <!ENTITY xsd "http://www.w3.org/2001/XMLSchema#" > <!ENTITY owl "http://www.w3.org/2002/07/owl#" > ]> <rdf:RDF xmlns ="&owl;" xmlns:owl ="&owl;" xml:base ="http://www.w3.org/2002/07/owl" xmlns:rdf ="&rdf;" xmlns:rdfs="&rdfs;" > <Ontology rdf:about=""> <imports rdf:resource="http://www.w3.org/2000/01/rdf-schema"/> <rdfs:isDefinedBy rdf:resource="http://www.w3.org/TR/2004/REC-owl-semantics-20040210/" /> <rdfs:isDefinedBy rdf:resource="http://www.w3.org/TR/2004/REC-owl-test-20040210/" /> <rdfs:isDefinedBy rdf:resource="http://www.w3.org/TR/2004/REC-owl-features-20040210/" /> <rdfs:comment>This file specifies in RDF Schema format the built-in classes and properties that together form the basis of the RDF/XML syntax of OWL Full, OWL DL and OWL Lite. We do not expect people to import this file explicitly into their ontology. People that do import this file should expect their ontology to be an OWL Full ontology. </rdfs:comment> <versionInfo>10 February 2004</versionInfo> </Ontology> <rdfs:Class rdf:ID="Class"> <rdfs:label>Class</rdfs:label> <rdfs:subClassOf rdf:resource="&rdfs;Class"/> </rdfs:Class> <Class rdf:ID="Thing"> <rdfs:label>Thing</rdfs:label> <unionOf rdf:parseType="Collection"> <Class rdf:about="#Nothing"/> <Class> <complementOf rdf:resource="#Nothing"/> </Class> </unionOf> </Class> <Class rdf:ID="Nothing"> <rdfs:label>Nothing</rdfs:label> <complementOf rdf:resource="#Thing"/> </Class> <rdf:Property rdf:ID="equivalentClass"> <rdfs:label>equivalentClass</rdfs:label> <rdfs:subPropertyOf rdf:resource="&rdfs;subClassOf"/> <rdfs:domain rdf:resource="#Class"/> <rdfs:range rdf:resource="#Class"/> </rdf:Property> <rdf:Property rdf:ID="disjointWith"> <rdfs:label>disjointWith</rdfs:label> <rdfs:domain rdf:resource="#Class"/> <rdfs:range rdf:resource="#Class"/> </rdf:Property> <rdf:Property rdf:ID="equivalentProperty"> <rdfs:label>equivalentProperty</rdfs:label> <rdfs:subPropertyOf rdf:resource="&rdfs;subPropertyOf"/> </rdf:Property> <rdf:Property rdf:ID="sameAs"> <rdfs:label>sameAs</rdfs:label> <rdfs:domain rdf:resource="#Thing"/> <rdfs:range rdf:resource="#Thing"/> </rdf:Property> <rdf:Property rdf:ID="differentFrom"> <rdfs:label>differentFrom</rdfs:label> <rdfs:domain rdf:resource="#Thing"/> <rdfs:range rdf:resource="#Thing"/> </rdf:Property> <rdfs:Class rdf:ID="AllDifferent"> <rdfs:label>AllDifferent</rdfs:label> </rdfs:Class> <rdf:Property rdf:ID="distinctMembers"> <rdfs:label>distinctMembers</rdfs:label> <rdfs:domain rdf:resource="#AllDifferent"/> <rdfs:range rdf:resource="&rdf;List"/> </rdf:Property> <rdf:Property rdf:ID="unionOf"> <rdfs:label>unionOf</rdfs:label> <rdfs:domain rdf:resource="#Class"/> <rdfs:range rdf:resource="&rdf;List"/> </rdf:Property> <rdf:Property rdf:ID="intersectionOf"> <rdfs:label>intersectionOf</rdfs:label> <rdfs:domain rdf:resource="#Class"/> <rdfs:range rdf:resource="&rdf;List"/> </rdf:Property> <rdf:Property rdf:ID="complementOf"> <rdfs:label>complementOf</rdfs:label> <rdfs:domain rdf:resource="#Class"/> <rdfs:range rdf:resource="#Class"/> </rdf:Property> <rdf:Property rdf:ID="oneOf"> <rdfs:label>oneOf</rdfs:label> <rdfs:domain rdf:resource="&rdfs;Class"/> <rdfs:range rdf:resource="&rdf;List"/> </rdf:Property> <rdfs:Class rdf:ID="Restriction"> <rdfs:label>Restriction</rdfs:label> <rdfs:subClassOf rdf:resource="#Class"/> </rdfs:Class> <rdf:Property rdf:ID="onProperty"> <rdfs:label>onProperty</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> <rdfs:range rdf:resource="&rdf;Property"/> </rdf:Property> <rdf:Property rdf:ID="allValuesFrom"> <rdfs:label>allValuesFrom</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> <rdfs:range rdf:resource="&rdfs;Class"/> </rdf:Property> <rdf:Property rdf:ID="hasValue"> <rdfs:label>hasValue</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> </rdf:Property> <rdf:Property rdf:ID="someValuesFrom"> <rdfs:label>someValuesFrom</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> <rdfs:range rdf:resource="&rdfs;Class"/> </rdf:Property> <rdf:Property rdf:ID="minCardinality"> <rdfs:label>minCardinality</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> <rdfs:range rdf:resource="&xsd;nonNegativeInteger"/> </rdf:Property> <rdf:Property rdf:ID="maxCardinality"> <rdfs:label>maxCardinality</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> <rdfs:range rdf:resource="&xsd;nonNegativeInteger"/> </rdf:Property> <rdf:Property rdf:ID="cardinality"> <rdfs:label>cardinality</rdfs:label> <rdfs:domain rdf:resource="#Restriction"/> <rdfs:range rdf:resource="&xsd;nonNegativeInteger"/> </rdf:Property> <rdfs:Class rdf:ID="ObjectProperty"> <rdfs:label>ObjectProperty</rdfs:label> <rdfs:subClassOf rdf:resource="&rdf;Property"/> </rdfs:Class> <rdfs:Class rdf:ID="DatatypeProperty"> <rdfs:label>DatatypeProperty</rdfs:label> <rdfs:subClassOf rdf:resource="&rdf;Property"/> </rdfs:Class> <rdf:Property rdf:ID="inverseOf"> <rdfs:label>inverseOf</rdfs:label> <rdfs:domain rdf:resource="#ObjectProperty"/> <rdfs:range rdf:resource="#ObjectProperty"/> </rdf:Property> <rdfs:Class rdf:ID="TransitiveProperty"> <rdfs:label>TransitiveProperty</rdfs:label> <rdfs:subClassOf rdf:resource="#ObjectProperty"/> </rdfs:Class> <rdfs:Class rdf:ID="SymmetricProperty"> <rdfs:label>SymmetricProperty</rdfs:label> <rdfs:subClassOf rdf:resource="#ObjectProperty"/> </rdfs:Class> <rdfs:Class rdf:ID="FunctionalProperty"> <rdfs:label>FunctionalProperty</rdfs:label> <rdfs:subClassOf rdf:resource="&rdf;Property"/> </rdfs:Class> <rdfs:Class rdf:ID="InverseFunctionalProperty"> <rdfs:label>InverseFunctionalProperty</rdfs:label> <rdfs:subClassOf rdf:resource="&owl;ObjectProperty"/> </rdfs:Class> <rdfs:Class rdf:ID="AnnotationProperty"> <rdfs:subClassOf rdf:resource="&rdf;Property"/> </rdfs:Class> <AnnotationProperty rdf:about="&rdfs;label"/> <AnnotationProperty rdf:about="&rdfs;comment"/> <AnnotationProperty rdf:about="&rdfs;seeAlso"/> <AnnotationProperty rdf:about="&rdfs;isDefinedBy"/> <rdfs:Class rdf:ID="Ontology"> <rdfs:label>Ontology</rdfs:label> </rdfs:Class> <rdfs:Class rdf:ID="OntologyProperty"> <rdfs:subClassOf rdf:resource="&rdf;Property"/> </rdfs:Class> <rdf:Property rdf:ID="imports"> <rdfs:label>imports</rdfs:label> <rdf:type rdf:resource="#OntologyProperty"/> <rdfs:domain rdf:resource="#Ontology"/> <rdfs:range rdf:resource="#Ontology"/> </rdf:Property> <rdf:Property rdf:ID="versionInfo"> <rdfs:label>versionInfo</rdfs:label> <rdf:type rdf:resource="#AnnotationProperty"/> </rdf:Property> <rdf:Property rdf:ID="priorVersion"> <rdfs:label>priorVersion</rdfs:label> <rdf:type rdf:resource="#OntologyProperty"/> <rdfs:domain rdf:resource="#Ontology"/> <rdfs:range rdf:resource="#Ontology"/> </rdf:Property> <rdf:Property rdf:ID="backwardCompatibleWith"> <rdfs:label>backwardCompatibleWitesh</rdfs:label> <rdf:type rdf:resource="#OntologyProperty"/> <rdfs:domain rdf:resource="#Ontology"/> <rdfs:range rdf:resource="#Ontology"/> </rdf:Property> <rdf:Property rdf:ID="incompatibleWith"> <rdfs:label>incompatibleWith</rdfs:label> <rdf:type rdf:resource="#OntologyProperty"/> <rdfs:domain rdf:resource="#Ontology"/> <rdfs:range rdf:resource="#Ontology"/> </rdf:Property> <rdfs:Class rdf:ID="DeprecatedClass"> <rdfs:label>DeprecatedClass</rdfs:label> <rdfs:subClassOf rdf:resource="&rdfs;Class"/> </rdfs:Class> <rdfs:Class rdf:ID="DeprecatedProperty"> <rdfs:label>DeprecatedProperty</rdfs:label> <rdfs:subClassOf rdf:resource="&rdf;Property"/> </rdfs:Class> <rdfs:Class rdf:ID="DataRange"> <rdfs:label>DataRange</rdfs:label> </rdfs:Class> </rdf:RDF>
Classes in the OWL vocabulary:
Properties in the OWL vocabulary:
| rdf:Property | rdfs:domain | rdfs:range |
|---|---|---|
| owl:allValuesFrom | owl:Restriction | rdfs:Class |
| owl:backwardCompatibleWith | owl:Ontology | owl:Ontology |
| owl:cardinality | owl:Restriction | xsd:nonNegativeInteger |
| owl:complementOf | owl:Class | owl:Class |
| owl:differentFrom | owl:Thing | owl:Thing |
| owl:disjointWith | owl:Class | owl:Class |
| owl:distinctMembers | owl:AllDifferent | rdf:List |
| owl:equivalentClass | owl:Class | owl:Class |
| owl:equivalentProperty | rdf:Property | rdf:Property |
| owl:hasValue | owl:Restriction | |
| owl:imports | owl:Ontology | owl:Ontology |
| owl:incompatibleWith | owl:Ontology | owl:Ontology |
| owl:intersectionOf | owl:Class | rdf:List |
| owl:inverseOf | owl:ObjectProperty | owl:ObjectProperty |
| owl:maxCardinality | owl:Restriction | xsd:nonNegativeInteger |
| owl:minCardinality | owl:Restriction | xsd:nonNegativeInteger |
| owl:oneOf | owl:Class | rdf:List |
| owl:onProperty | owl:Restriction | rdf:Property |
| owl:priorVersion | owl:Ontology | owl:Ontology |
| owl:sameAs | owl:Thing | owl:Thing |
| owl:someValuesFrom | owl:Restriction | rdfs:Class |
| owl:unionOf | owl:Class | rdf:List |
| owl:versionInfo |
This section summarizes the changes from DAML+OIL [DAML+OIL] to OWL.
rdfs:domain
and
rdfs:range
properties are handled as intersection<size> <xsd:integer rdf:value="10"/> </size>Instead use
<size rdf:datatype="http://www.w3.org/2001/XMLSchema#integer">10</size>
daml:List
construct used to represent closed collections
was largely incorporated into RDF
rdf:parseType="daml:collection"
rdf:List
, rdf:first
, rdf:rest
and rdf:nil
replace
daml:List
, daml:first
, daml:rest
,
and daml:nil
daml:item
is not supported.
As this feature was typically used to create
typed lists, we include here an example of creating such a list
without using daml:item
:
<rdfs:Class rdf:ID="OperaList"> <rdfs:subClassOf rdf:resource="&rdf;List"/> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="&rdf;first"/> <owl:allValuesFrom rdf:resource="#Opera"/> </owl:Restriction> </rdfs:subClassOf> <rdfs:subClassOf> <owl:Restriction> <owl:onProperty rdf:resource="&rdf;rest"/> <owl:allValuesFrom rdf:resource="#OperaList"/> </owl:Restriction> </rdfs:subClassOf> </rdfs:Class>This example defines a list of which the elements most be operas. This is achieved by two restrictions, one on the
rdf:first
value (representing the type of the list element) and a second
restriction on the rdf:rest
value (which should the name
of the list being defined).
daml:cardinalityQ
daml:hasClassQ
daml:maxCardinalityQ
daml:minCardinalityQ
| DAML+OIL | OWL |
|---|---|
daml:differentIndividualFrom |
owl:differentFrom |
daml:equivalentTo |
owl:sameAs |
daml:sameClassAs |
owl:equivalentClass |
daml:samePropertyAs |
owl:equivalentProperty |
daml:hasClass |
owl:someValuesFrom |
daml:toClass |
owl:allValuesFrom |
daml:UnambiguousProperty |
owl:InverseFunctionalProperty |
daml:UniqueProperty |
owl:FunctionalProperty |
owl:SymmetricProperty
was added.
owl:AnnotationProperty
,
owl:OntologyProperty
and
owl:DataRange
.
owl:DatatypeProperty
may be an
owl:InverseFunctionalProperty
in OWL Full.
daml:comment
daml:domain
daml:label
daml:isDefinedBy
daml:Literal
daml:Property
daml:range
daml:seeAlso
daml:subClassOf
daml:subPropertyOf
daml:type
daml:value
daml:disjointUnionOf
was removed from the language,
since it can be effected using
owl:unionOf
or
rdfs:subClassOf
and
owl:disjointWith
.
daml:equivalentTo
was renamed to
owl:sameAs
,
and is no longer a superproperty of
owl:equivalentClass
and
owl:equivalentProperty
,
The OWL Abstract Syntax and Semantics document [OWL S&AS] provides a characterization of OWL ontologies in terms of an abstract syntax, along with a mapping to RDF triples.
The following rules give an informal characterization of the conditions for an RDF graph to be a DL ontology. This is not intended to replace the characterization given in S&AS, but instead gives some general pointers — the idea is that if you stick to these guidelines, you're more likely to produce OWL DL ontologies. Nor is this intended to tell you how to turn the triple representation into something closer to the abstract syntax.
The built-in properties and classes should not be redefined. In general this means that things in the OWL, RDF or RDFS namespaces should not appear as subjects of triples.
Everything should have a type1. If any
URI reference x
is used where a class is expected, the
graph should contain a triple stating that
x rdf:type owl:Class
Similarly, if a property p
is used where an object
property is expected then there should be a triple2
p rdf:type owl:ObjectProperty
If a property q
is used where a data property is
expected then there should be a triple
q rdf:type owl:DatatypeProperty
If a property o
is used where an ontology property is
expected then it should either be one of the built in ontology properties
(owl:imports
, owl:priorVersion
,
owl:backwardCompatibleWith
, and
owl:incompatibleWith
) or there should be a triple:
o rdf:type owl:OntologyProperty
If a property a
is used where an annotation property
is expected then it should either be one of the built in annotation
properties (owl:versionInfo
, rdfs:label
,
rdfs:comment
, rdfs:seeAlso
, and
rdfs:isDefinedBy
) or there should be a triple:
a rdf:type owl:AnnotationProperty
Any individuals that occur in the ontology should have at least one
type specified, i.e. for an individual i
, there must be
a triple:
i rdf:type c
where c
is an owl:Class
or
owl:Restriction
.
URI references for classes, properties (object, datatype, ontology and annotation) and individuals should be disjoint. Thus we cannot have things like:
x rdf:type owl:Class x rdf:type owl:ObjectProperty
In particular, this means that we cannot use classes as instances, i.e.
x rdf:type owl:Class y rdf:type owl:Class x rdf:type y
is not valid OWL DL. A general rule here is that if there is a node
x
in the graph with a triple:
x rdf:type owl:Class
then x
should not appear as the subject of any other
triple with predicate rdf:type
.3
If a node x
has rdf:type
owl:Restriction
then the following should be the case:
rdf:type
3.owl:onProperty
, with the object of that triple being an
owl:ObjectProperty
or
owl:DatatypeProperty
.owl:someValuesFrom
. In this
case, the type of the property which is the object of the
owl:onProperty
triple should be appropriate. By this we
mean that if the object of this triple is a datatype, the property
should be an
owl:DatatypeProperty
. If the object is a class
description, the property should be an
owl:ObjectProperty
. This typing information should be
present (due to the restrictions outlined above).owl:allValuesFrom
. Similar
restrictions hold as for owl:someValuesFrom
owl:hasValue
. If the type of the
property involved in the owl:onProperty
triple is
owl:ObjectProperty
then the object of this triple should
be an individual. If the type of the
property involved in the owl:onProperty
triple is
owl:DatatypeProperty
the the object of this triple
should be a data literal. owl:minCardinality
. The object
of this triple should be a data literal representing a non negative
integer.owl:maxCardinality
. Restriction
as for owl:minCardinality
. owl:cardinality
. Restriction as
for owl:minCardinality
. x
is the subject should
have predicate owl:equivalentClass
or
owl:disjointWith
.For any triples with predicate rdfs:subClassOf
or
owl:equivalentClass
or owl:disjointWith
,
both the subject and object of the triples should be an
owl:Class
or owl:Restriction
, i.e. if we
have:
x rdfs:subClassOf y
then the graph must contain one of:
x rdf:type owl:Class
or
x rdf:type owl:Restriction.
and one of
y rdf:type owl:Class
or
y rdf:type owl:Restriction.
For any triples with predicate rdfs:subPropertyOf
or
owl:equivalentProperty
, both the subject and object of the
triples should have the same type which should be one of
owl:ObjectProperty
or owl:DatatypeProperty
,
i.e. if we have:
p owl:equivalentProperty q
then the graph must contain either:
p rdf:type owl:ObjectProperty q rdf:type owl:ObjectProperty.
or
p rdf:type owl:DatatypeProperty q rdf:type owl:DatatypeProperty.
Triples with predicate rdfs:domain
should have as their
subject an owl:ObjectProperty
or
owl:DatatypeProperty
and as their object an
owl:Class
or owl:Restriction
.
Triples with predicate rdfs:range
should have as their subject
either an owl:ObjectProperty
or an
owl:DatatypeProperty
. In the case of the former, the
object of the triple should then be an owl:Class
or
owl:Restriction
, in the case of the latter, the object
should be either an XML Schema datatype, rdfs:Literal
,
or an owl:oneOf
specifying a data range with type owl:DataRange
.
Both the subject and object of an owl:inverseOf
triple
must have type owl:ObjectProperty
.
For any triples with predicate
owl:sameAs
or
owl:differentFrom
, the subject and object must be individuals.
Note that relating two classes via
owl:sameAs
is a very different thing to
relating them via owl:equivalentClass
. The former says
that the two objects are in fact the same, is actually an example of
class as instance, and thus pushes the ontology out of OWL
DL. The latter is an assertion that the extension (e.g. the collection
of members) of the classes is equivalent.
Similarly, relating classes via owl:differentFrom
is not
the same as relating them via owl:disjointWith
(and is
again an example of an OWL Full construct). Two classes may be
different objects but still share the same extension.
If
a node x
has rdf:type
owl:AllDifferent
, then the following should be the
case:
owl:distinctMembers
, the object of which should be a
(well-formed) rdf:List
, all of whose elements are
individuals.Boolean Operators (and, or, not) are represented in OWL using
owl:intersectionOf
, owl:unionOf
and
owl:complementOf
.
The subject of an owl:complementOf
triple must be an
owl:Class
, the object must be either an
owl:Class
or owl:Restriction
.
The subject of an owl:unionOf
or
owl:intersectionOf
triple must be an
owl:Class
, the object must be a (well-formed)
rdf:List
, all of whose elements are either
owl:Class
or
owl:Restriction
. These could either be represented
explicitly using expanded rdf:Lists
, or if RDF-XML is
being used, an rdf:parseType="Collection"
attribute.
<owl:Class> <owl:intersectionOf rdf:parseType="Collection"> <owl:Class rdf:about="x"/> <owl:Class rdf:about="y"/> </owl:intersectionOf> </owl:Class>
If the owl:Class
is a blank node (i.e. the class is
unnamed), then it can only be the subject of at most one triple with
predicate
owl:intersectionOf
,
owl:unionOf
or owl:complementOf
. If the
class is named, any number of such triples are allowed.
The subject of any triple with predicate owl:oneOf
must be
either an owl:Class
or an owl:DataRange
. In
the case of the former, the object must be a (well-formed)
rdf:List
, all of whose elements are individuals. In the case of the latter, the object must be a
(well-formed) rdf:List
, all of whose elements are data
literals. Again, as with the boolean operators,
rdf:parseType="Collection"
can be used.
The subject and object of any triple with an ontology predicate
should be an ontology, e.g. a node x
such that there is a
triple:
x rdf:type owl:Ontology
The subject of any triple with an annotation predicate should be a named (i.e. non-bnode) class, a property, an individual or an ontology. The object of a triple with an annotation predicate should be an individual, a data literal or an arbitrary URI reference.
With the exception of predicates within the OWL, RDF and RDFS vocabularies, annotation properties are the only predicates that should appear in triples with a class or property as the subject.
Annotation and ontology properties themselves should be
typed, and should not appear as the
subject or object of triples other than as the subject of a triple
with predicate rdf:type
or an annotation property.
In general, the S&AS description of OWL does not permit structure sharing in the RDF representation. This effectively means that an anonymous node in the RDF graph representing a particular description should only occur once (as the object of a triple). Thus things like:
x1 rdf:type owl:Class x1 rdfs:subClassOf _:y x2 rdf:type owl:Class x2 rdfs:subClassOf _:y _:y rdf:type owl:Class _:y owl:complementOf z
should be avoided. There are some tricky corner cases where this is permitted. In general, however, graphs should use distinct blank nodes whenever a class description is used in more than one place.
In general, blank nodes occurring in the graph either represent unnamed individuals, or should be exactly one of the following:
rdfs:subClassOf
,
owl:equivalentClass
, owl:disjointWith
,
owl:someValuesFrom
, owl:allValuesFrom
or
rdf:type
triple.rdf:type
triple with object
owl:AllDifferent
.rdf:List
.Orphan blank nodes, i.e. those which are not the object of a triple are,
in general, not allowed (other than the owl:AllDifferent
case described above).
Ontologies may contain assertions of ground facts (e.g. triples
that assert the properties of individuals). The properties used in
these assertions must be an
owl:ObjectProperty
or
owl:DatatypeProperty
. The subject of any such triple must
be an individual (which should be typed). The object can either be a reference
to an individual (if the property is an
owl:ObjectProperty
) or a data literal (if the property is
an owl:DatatypeProperty
).
OWL Lite documents should follow the same rules as OWL DL documents, with a number of extra restrictions, primarily concerning the vocabulary allowed. An OWL Lite document should not use any of the following vocabulary:
owl:unionOf
owl:complementOf
owl:oneOf
owl:hasValue
owl:disjointWith
Any objects which are the object or subject of a triple with
predicate owl:equivalentClass
should not be b-nodes.
The object of any triples with predicate
owl:minCardinality
, owl:maxCardinality
or
owl:cardinality
should be a data literal representing the
integer 0 or 1.
The situation regarding the use of owl:intersectionOf
in OWL Lite is a little more complex. The predicate should
not be used to form arbitrary expressions, but is
needed in order to represent complete class definitions. The
restrictions above tell us that the subject of any triple with predicate
owl:intersectionOf
should be a owl:Class
. In
OWL Lite, we have the further restriction that this class should be
named, i.e. the subject should not be a bnode.
Be careful of the use of owl:Thing
. For example, the
following OWL-RDF fragment:
<owl:Class rdf:about="#A"> <rdfs:subClassOf> <owl:Thing/> </rdfs:subClassOf> </owl:Class>
does not describe a class A
that is a
subclass of owl:Thing
, but in fact describes a class
A
that is a subclass of some anonymous instance of
owl:Thing
. This is thus a use of class as instance and is
outside OWL DL. The desired effect of a subclass of
owl:Thing
is obtained through:
<owl:Class rdf:about="#A"> <rdfs:subClassOf> <owl:Class rdf:about="http://www.w3.org/2002/07/owl#Thing"/> </rdfs:subClassOf> </owl:Class>
Be careful not to confuse owl:Class
and
rdfs:Class
. The following is not in DL
due to the fact that c
is not given an appropriate
type.
c rdf:type rdfs:Class
[1] Of course the necessity to type everything does not apply to things from the OWL, RDF or RDFS namespaces.
[2] Strictly speaking, if the property is defined as
being an owl:TransitiveProperty
,
owl:SymmetricProperty
or
owl:InverseFunctionalProperty
then this is not
necessary.
[3] An exception here is that we can have:
x rdf:type rdfs:Class x rdf:type owl:Class
p rdf:type rdf:Property p rdf:type owl:ObjectProperty
or
q rdf:type rdf:Property q rdf:type owl:DatatypeProperty
In addition, for restrictions, we can have:
x rdf:type owl:Restriction x rdf:type rdfs:Class x rdf:type owl:Class
http://www.w3.org/TR/daml+oil-reference
.