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.
Status of this document
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/.
Publication as a Candidate Recommendation does not imply
endorsement by the W3C Membership. This is a draft document and may be
updated, replaced or obsoleted by other documents at any time. It is
inappropriate to cite this document as other than "work in
progress".
This draft is one of six
parts of the Candidate
Recommendation (CR) 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 18 August 2003.
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 Last Call
Working Draft are detailed in the change log.
The Working Group now hopes to gather experience from the growing
number of OWL
implementations in order to increase confidence in the language
and meet specific exit criteria.
This CR period will extend until at least . After
that date, when and if the exit criteria are met, the group intends to
request
Proposed Recommendation status.
Please send reports of implementation experience to public-webont-comments@w3.org
(archive).
Reports of any success or difficulty with the Test Cases
are encouraged, and reports received by 20 September 2003 will be
particularly helpful. General discussion of related technology is
welcome at www-rdf-logic@w3.org (archive).
Although OWL is essentially stable, later versions of these
documents are expected to contain minor improvements. The test site is likely to
include new, clarifying tests, even during this CR period.
Additionally, the design of OWL depends in part on the design of RDF,
and at this time the relevant RDF specifications are only Working
Drafts. It is therefore possible that unanticipated changes in RDF
may require changes to OWL. In
particular, the reference to the list of the recommended datatypes in Sec. 6 of
this document will need to be updated once a new version of the RDF Semantics
document [RDF Semantics] is
published.
The W3C maintains a list of any
patent disclosures related to this work.
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 members of this group working group included:
Yasser al Safadi, Jean-Francois Baget, James Barnette, Sean
Bechhofer, Jonathan Borden, Frederik Brysse, Stephen Buswell, Peter
Crowther, Jos De Roo, David De Roure, Mike Dean, Larry Eshelman,
Jerome Euzenat, Dieter Fensel, Tim Finin, Nicholas Gibbins, Pat
Hayes, Jeff Heflin, Ziv Hellman, James Hendler, Bernard Horan,
Masahiro Hori, Ian Horrocks, Francesco Iannuzzelli, Mario Jeckle,
Ruediger Klein, Ora Lassila, Alexander Maedche, Massimo Marchiori,
Deborah McGuinness, Libby Miller, Enrico Motta, Leo Obrst, Laurent
Olivry , Peter Patel-Schneider, Martin Pike, Marwan Sabbouh, Guus
Schreiber, Shimizu Noboru, Michael Sintek, Michael Smith, Ned
Smith, John Stanton, Lynn Andrea Stein, Herman ter Horst, Lynne R.
Thompson, David Trastour, Frank van Harmelen, Raphael Volz, Evan
Wallace, Christopher Welty, Charles White, and John Yanosy.
1. Introduction
1.1 Purpose of this document
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 Schema].
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 Use Cases and Requirements].
Test cases for OWL tools (e.g., entailment tests, consistency tests)
are specified in the Test document
[OWL Test Cases] .
1.2 OWL Full/DL/Lite
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].
Sect. 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
Section 8.3 "OWL Lite constructs".
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.
1.3 OWL syntax
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.
1.4 OWL and RDF semantics
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.
2. OWL document
Information in OWL is gathered into ontologies, which can then be
stored as documents in the World Wide Web. One aspect of OWL, the
importing of ontologies, depends on this ability to store OWL
ontologies in the Web.
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 any RDF document, the OWL code should be subelements of
an 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 am 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:
- a class identifier (a URI reference)
- an exhaustive enumeration of individuals
that together form the instances of a class
- a property restriction
- the intersection of two or more class
descriptions
- the union of two or more class
descriptions
- the complement of a class
description
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:Restrictions, 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
he type 2-6, this is not considered to be a class description, but a
special kind of class axiom for complete classes.
See Section 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. This 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
Section "6. Datatypes".
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. 6 "Individuals"
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 Section 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 Section 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 Schema]. Its meaning in OWL
is exactly the same: if the class description C1 is defined as a subclass of
class description C2, than 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
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
only class identifiers and property restrictions are allowed as
subject and object.
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. The 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 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
intersectionOf list should 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:
- Object properties link individuals to individuals.
- Datatype properties link individuals to data values.
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 are 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 Section 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:
- RDF Schema constructs:
rdfs:subPropertyOf,
rdfs:domain and rdfs:range
- relations to other properties:
owl:equivalentProperty and
owl:inverseOf
- global cardinality constraints:
owl:FunctionalProperty and
owl:InverseFunctionalProperty
- logical property characteristics:
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 Schema]. 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 description allowed as object of
rdfs:range is a class name.
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 person 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 Sections 8.1 and 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
subproperties, nor on the inverse of the property or its subproperties.
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:
- Facts about class membership and property values of individuals
- Facts about individual identity
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 as datatype 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.
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.
6. Datatypes
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:
The minimal level of tool support for datatypes is discussed in
Sec. 6.3.
6.1 RDF Datatypes
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:
The RDF Semantics document
[RDF Semantics].
recommends use of the following
simple built-in XML Schema datatypes.
- The primitive datatype
xsd:string,
plus the following datatypes derived from xsd:string:
xsd:normalizedString,
xsd:token,
xsd:language,
xsd:NMTOKEN,
xsd:Name,and
xsd:NCName.
- The primitive datatype
xsd:boolean.
- The primitive numerical datatypes
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)
- The primitive time-related datatypes:
xsd:dateTime,
xsd:time,
xsd:date,
xsd:gYearMonth,
xsd:gYear,
xsd:gMonthDay,
xsd:gDay, and
xsd:gMonth.
- The primitive datatypes
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 Sect. 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>
6.2 Enumerated datatype
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>
The following is a somewhat more readable
syntactic variation of the example, using the RDF
notion of nodeID (see the RDF Concepts document
[RDF Concepts] for
details):
<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:nodeID="l2"/>
</rdf:List>
<rdf:List rdf:nodeID="l2">
<rdf:first rdf:datatype="&xsd;integer">15</rdf:first>
<rdf:rest rdf:nodeID="l3"/>
</rdf:List>
<rdf:List rdf:nodeID="l3">
<rdf:first rdf:datatype="&xsd;integer">30</rdf:first>
<rdf:rest rdf:nodeID="l4"/>
</rdf:List>
<rdf:List rdf:nodeID="l4">
<rdf:first rdf:datatype="&xsd;integer">40</rdf:first>
<rdf:rest rdf:resource="&rdf;nil"/>
</rdf:List>
</owl:oneOf>
</owl:DataRange>
</rdfs:range>
</owl:DatatypeProperty>
6.3 Support for datatype reasoning
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. 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.
7.1 Annotations
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:
Five annotation properties are predefined by OWL, namely:
owl:versionInfordfs:labelrdfs:commentrdfs:seeAlsordfs: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</rdf: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 annotations 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).
NOTE: The use of owl:AnnotationProperty is not needed in
OWL Full.
A document describing an ontology typically contains information
about the ontology itself. An ontology is a resource, so 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.
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.
Importing an ontology into itself is considered a null action, so if
ontology A imports B and B imports A, then they are considered to be
equivalent.
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 rdf: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
rdf: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:
- OWL DL requires a pairwise separation between classes, datatypes,
datatype properties, object properties, annotation properties, ontology
properties (i.e., the import and versioning stuff),
individuals, data values and the built-in vocabulary. This means that, for
example, a class cannot be at the same time an individual.
- In OWL DL the set of object properties and datatype properties are
disjoint. This implies that the following four property characteristics:
can never be specified for datatype
properties
- OWL DL requires that no cardinality constraints (local nor global)
can be placed on transitive properties or their inverses or any of their
superproperties.
- Annotations are allowed only under certain conditions. See Section 7.1 for details.
- Most RDF(S) vocabulary cannot be used within OWL DL. See the OWL
Semantics and Abstract Syntax document [OWL S&AS] for details.
- All axioms must be well-formed, with no missing or extra components, and
must form a tree-like structure.
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:about="#C2" />
</owl:Class>
then the ontology (or an ontology imported into this ontology)
should contain a owl:Class triple for
C2.
- Axioms (facts) about individual equality and difference must be about
named individuals.
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:
- the subject of
owl:equivalentClass triples be named classes
and the object of owl:equivalentClass triples be named classes
or restrictions.
- the subject of
rdfs:subClassOf triples be named classes
and the object of rdfs:subClassOf triples be named classes or
restrictions;
owl:intersectionOf be used only on lists of length greater
than one that contain only named classes 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/>
- the object of
owl:allValuesFrom and
owl:someValuesFrom triples be named
classes or named datatypes;
- the object of
rdf:type triples be named classes or
restrictions;
- the object of
rdfs:domain triples be named classes;
and
- the object of
rdfs:range triples be named classes or
datatypes.
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 Schema].
Appendix B. RDF Schema of OWL
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;"
xmlns:dc ="http://purl.org/dc/elements/1.1/"
>
<Ontology rdf:about="">
<imports rdf:resource="http://www.w3.org/2000/01/rdf-schema"/>
<rdfs:seeAlso>http://www.w3.org/2001/sw/WebOnt/</rdfs:seeAlso>
<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>
</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>
<rdf:Description rdf:about="&rdfs;label">
<rdf:type rdf:resource="#AnnotationProperty"/>
</rdf:Description>
<rdf:Description rdf:about="&rdfs;comment">
<rdf:type rdf:resource="#AnnotationProperty"/>
</rdf:Description>
<rdf:Description rdf:about="&rdfs;seeAlso">
<rdf:type rdf:resource="#AnnotationProperty"/>
</rdf:Description>
<rdf:Description rdf:about="&rdfs;isDefinedBy">
<rdf:type rdf:resource="#AnnotationProperty"/>
</rdf:Description>
<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>backwardCompatibleWith</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>
Appendix C.
OWL Quick Reference
Classes in the OWL vocabulary:
Properties in the OWL vocabulary:
Appendix D.
Changes from DAML+OIL
This section summarizes the changes from
DAML+OIL
[DAML+OIL]
to OWL.
- The semantics have changed significantly. With respect to the three
sublanguages, the DAML+OIL semantics are closest to the OWL DL semantics.
- The namespace was changed to
http://www.w3.org/2002/07/owl.
- Various updates to RDF and RDF Schema from the
RDF Core Working Group
were incorporated,
including
- cyclic subclasses are now allowed
- multiple
rdfs:domain
and
rdfs:range
properties are handled as intersection
- RDF Semantics [RDF Semantics]
- datatypes
- the
daml:List construct used to represent closed collections
was largely incorporated into RDF
- Qualified restrictions were removed from the language,
resulting in the removal of the following properties:
daml:cardinalityQdaml:hasClassQdaml:maxCardinalityQdaml:minCardinalityQ
- Various properties and classes were renamed,
as shown in the following table:
-
owl:SymmetricProperty
was added.
- An
owl:DatatypeProperty
may be an
owl:InverseFunctionalProperty
in OWL Full.
-
Synonyms for RDF and RDF Schema classes and properties were removed from the language,
resulting in the removal of:
daml:commentdaml:domaindaml:labeldaml:isDefinedBydaml:Literaldaml:Propertydaml:rangedaml:seeAlsodaml:subClassOfdaml:subPropertyOfdaml:typedaml: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 following properties and classes were added to support versioning:
- owl:AllDifferent
and
owl:distinctMembers
were added to address the
Unique Names Assumption.
Appendix E.
Rules of Thumb for OWL DL ontologies
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.
Don't mess with the vocabulary
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.
Provide Explicit Typing
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.
Keep names separate
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
Restrictions
If a node x has rdf:type
owl:Restriction then the following should be the case:
- It is a blank node (i.e. unnamed).
- It is not the subject of any other triple with predicate
rdf:type3.
- It is the subject of exactly one triple with predicate
owl:onProperty, with the object of that triple being an
owl:ObjectProperty or
owl:DatatypeProperty.
- It is the subject of exactly one of:
- A triple with predicate
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).
- A triple with predicate
owl:allValuesFrom. Similar
restrictions hold as for owl:someValuesFrom
- A triple with predicate
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.
- A triple with predicate
owl:minCardinality. The object
of this triple should be a data literal representing a non negative
integer.
- A triple with predicate
owl:maxCardinality. Restriction
as for owl:minCardinality.
- A triple with predicate
owl:cardinality. Restriction as
for owl:minCardinality.
- Any other triples in which
x is the subject should
have predicate owl:equivalentClass or
owl:disjointWith.
Class axioms
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.
Property axioms
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.
Individual axioms
For any triples with predicate
owl:sameAs4 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:
- It is a blank node (i.e. unnamed).
- It is the subject of a triple with predicate
owl:distinctMembers, the object of which should be a
(well-formed) rdf:List, all of whose elements are
individuals.
- It is not the subject (or object) of any other triples.
Boolean class expressions
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:unionOfor
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.
Enumerations
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.
Ontology and Annotation Properties
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.
Avoid structure sharing
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.
Avoid orphan blank nodes
In general, blank nodes occurring in the graph either represent unnamed
individuals, or should be exactly
one of the following:
- The object of an
rdfs:subClassOf,
owl:equivalentClass, owl:disjointWith,
owl:someValuesFrom, owl:allValuesFrom or
rdf:type triple.
- The subject of an
rdf:type triple with object
owl:AllDifferent.
- An element in an
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).
Ground facts
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
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:unionOfowl:complementOfowl:oneOfowl:hasValueowl: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.
Miscellaneous
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.
x rdf:type rdfs:Class
Notes
[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
[4] Or owl:sameAs.
Appendix F.
Change Log
This section summarizes changes to this document since the
Last Call
version.
It will be removed in the final version.
- change to Sec. 8.3 after comment from
Qu
- changes to Sec. 1, Sec. 7 and the appendices after several
comments from Patel-Schneider:
24 April
24 April
24 April
24 April
- owl:Nothing is now in OWL Lite,
per
Minutes telecon May 1, 2003
- changes to Appendix D suggested in Language Ref Appendix D editorial comments
- "For historical reasons" deleted in Sec. 5.2, owl:sameAs after
comment
on Guide.
- Added forward reference to datatype support in intro Sec. 6 plus
changed "should" to "must" in Sec. 6.3 after
comment
from Connolly.
- Changed superclass of InverseFunctionalProperty to
owl:ObjectProperty after
comment
from Finin.
- Updated RDF/XML version of Appendix B as separate owl.owl file and
included link to this file after comment from
Dave Beckett
- Several changes in response to comments by
Lacy Lee
- Several changes in response to comments by the
RDFCore Working Group
- Included Appendix E "Rules of Thumb for OWL DL ontologies" in response to
comments about a readable description of OWL DL as RDF graphs. .
- Several small changes after comments from
Brian McBride
- Revised description of purpose of Appendix B after comments from
Dave Beckett
- Changed link in Sec. 6 for RDF datatype details to point to RDF
Semantics document after a comment from
Brian McBride
- Revised Sec. 7.3 on imports after comments from
Jennifer Golbeck
and Dave Beckett
- Revised Sec. 1.6 on privacy after comments from
Rigo
Wenning
- Revised Sec. 7.2 on ontology documents after comments from
the RDFCore WG
- Added paragraph in the Introduction about the set of OWL documents
after in response to the review by the
QA WG
- Changes reflecting the
new resolution on issue 4.6
to use owl:sameAs as the only term
for equality of individuals.
- Revised Appendix D (daml:item) after comments from
Bijan Parsia.
- [DAML+OIL]
-
DAML+OIL (March 2001) Reference Description.
Dan Connolly, Frank van Harmelen, Ian Horrocks,
Deborah L. McGuinness, Peter F. Patel-Schneider, and Lynn Andrea Stein.
W3C Note 18 December 2001.
Latest version
is available at
http://www.w3.org/TR/daml+oil-reference.
- [OWL Use Cases and Requirements]
-
OWL
Web Ontology Language Use Cases and Requirements.
Jeff Heflin, ed.
W3C Candidate Recommendation 18 August 2003.
Latest version is available at
http://www.w3.org/TR/webont-req/.
- [OWL Overview]
-
OWL Web Ontology
Language Overview.
Deborah L. McGuinness and Frank van Harmelen, eds.
W3C Candidate Recommendation 18 August 2003.
Latest version is available at
http://www.w3.org/TR/owl-features/.
- [OWL Guide]
-
OWL
Web Ontology Language Guide.
Michael K. Smith, Christopher Welty, Raphael Volz, and Deborah McGuinness.
W3C Candidate Recommendation 18 August 2003.
Latest version is available at
http://www.w3.org/TR/2003/CR-owl-guide-20030818/.
- [OWL S&AS]
-
OWL Web Ontology
Language Semantics and Abstract Syntax.
Peter F. Patel-Schneider, Patrick Hayes, and Ian Horrocks, eds.
W3C Candidate Recommendation 18 August 2003.
Latest version is available at
http://www.w3.org/TR/2003/CR-owl-semantics-20030818/.
- [OWL Test Cases]
-
OWL Web Ontology
Language Test Cases.
Jeremy J. Carroll and Jos De Roo, eds.
W3C Candidate Recommendation 18 August 2003.
Latest version is available at
http://www.w3.org/TR/owl-test/.
- [RDF Concepts]
-
Resource Description Framework (RDF): Concepts and Abstract Syntax.
Graham Klyne and Jeremy J. Carroll, eds.
W3C Working Draft 23 January 2003.
Latest version
is available at
http://www.w3.org/TR/rdf-concepts/.
- [RDF Schema]
-
RDF Vocabulary Description Language 1.0: RDF Schema.
Dan Brickley and R.V. Guha, eds.
W3C Working Draft 23 January 2003.
Latest version
is available at
http://www.w3.org/TR/rdf-schema/.
- [RDF Semantics]
-
RDF Semantics.
Patrick Hayes, ed.
W3C Working Draft 23 January 2003.
Latest version
is available at
http://www.w3.org/TR/rdf-mt/.
- [RDF/XML Syntax]
-
RDF/XML Syntax Specification (Revised).
Dave Beckett, ed.
W3C Working Draft 23 January 2003.
Latest version
is available at
http://www.w3.org/TR/rdf-syntax-grammar/.
- [XML-SCHEMA2]
- XML Schema
Part 2: Datatypes - W3C Recommendation, World Wide Web
Consortium, 2 May 2001.