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The OWL Web Ontology Language
OWL is a part of the Semantic Web, which is a vision for the
future
of the Web where applications can process information discovered on
the web 'intelligently', or at least not stupidly as most applications
are thought to process this information today. Web browsers, when
processing HTML, focus on presentation of text information and
accompanying images. Although browsers do make a start on
connecting web
documents using hyperlinks, which is one of the main things behind the
enormous success of the Web, this is generally unstructured data.
RDF goes further, for example, by enabling RSS browsers to
help user's find
recent news articles and interesting pod casts. However, RDF
doesn't go far
enough to make meaningful statements allowing reasoning about those
resources. Hendler, Berners-Lee, and Miller elaborate on
the Semantic Web in their paper Integrating Applications on the
Semantic Web14.
OWL is intended to support meaning to be
attached to objects and meaningful relationships to be defined between
objects. It has some of the elements of an object oriented
programming language but it is intended for description and discovery
rather than processing instructions and algorithms, as a programming
language is. OWL adds
more capability to RDF, including reasoning about properties and
classes,
disjointness and other relations between classes, cardinality of
relations (for example 'one-to-one')..
OWL is systematically described by the World Wide Web
Consortium
(W3C)
2004
OWL Web Ontology Language Reference3.
The OWL
Web Ontology Language Overview4
is a more informal
overview. The OWL Web Ontology Language Guide5 is a
demonstration of the technology. The formal description of OWL is
provided in OWL Web Ontology Language Semantics and Abstract Syntax6.
There are three flavors of OWL:
- OWL Lite is
intended
primarily for classification hierarchy and
simple constraints. It is appropriate for taxonomies.
- OWL DL (Description
Logics) includes the full OWL language with
certain restrictions to allow guarantees for computability
- OWL Full removes
the
restrictions on the use of language
constructs in OWL DL and looses the computational guarantees.
OWL Lite
A property (or relation) may be an inverse
of another property. If property R1 is the inverse of the property R2
and X is related to Y by R2, then Y is related to X by the R1. For
example, if hasWife is the inverse of hasHusband
and Fred hasWife Wilma then Wilma hasHusband
Fred. Some properties may be their own inverse. For
example, hasSpouse is its own inverse because if Fred hasSpouse
Wilma then that Wilma hasSpouse Fred.
A property P is transitive
if for every
instance Y related to X by P (X→Y) then if instance Z
related to Y by R (Y→Z) it implies that Z is related to X by R
(X→Z).
The property trusts is defined as transitive in many
security models. In that model if Fred trusts
Barney and
Barney trusts Joe then it is implied that Fred trusts
Joe.
A property P is symmetric
if for every Y related to X by P (X→Y), X is also related to Y by P
(Y→X). For example, if the property trusts is mutual
then it can be considered symmetric. In a mutual trust model if Fred trusts
Wilma then Wilma trusts Fred.
OWL Lite also has an intersectionOf constructor
with the usual
set
interpretation.
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