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Re: ISSUE: Classes as instances

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Thu, 18 Jul 2002 14:54:45 +0100
Message-ID: <15670.51365.837439.480251@merlin.oaklands.net>
To: "Raphael Volz" <volz@aifb.uni-karlsruhe.de>
Cc: "Webont" <www-webont-wg@w3.org>

On July 11, Raphael Volz writes:
> 
> TITLE: Classes as instances
> DESCRIPTION:
> In certain cases it is necessary to represent "classes as instances"
> <p>
> <b>Scenario 1:</b>
> Representing thesauri in OWL.
> Thesauri are build on terms and have a set of predefined relations
> to establish links between terms. One can distinguish two kinds
> of approaches to represent thesauri for RDF(S):
> <ol>
> <li>Syntactic representation, such as done in
> http://www.semanticweb.org/library/,
> does not use classes to represent terms (or synsets in WordNet).
> OWL could be
> used to represent all terms as instances of a class <i>Term</i>.
> Additionally
> the set of relations can be tranlated to properties having this class as
> domain
> and range. Eventually additional features of these properties, such as
> transitivity may
> be specified, e.g. for the hyperonym relation.
> <li>&quot;Semantic representation&quot;. Such as work carried out at the
> university
> of Karlsruhe. Here terms are converted to OWL classes and the hyperonym
> relation is converted
> to subclassof properties. All other thesaurus properties are difficult to
> translate since
> they are used to relate classes. However, in OWL properties do only relate
> instances which are members of
> the classes specified in domain and range constraints. The semantically
> correct representation would
> be to extend the metamodel of the ontology language, leading to information
> that cannot be processed
> by OWL aware agents. <p>
> Another possibility is to treat classes as instances allowing to related
> classes using properties
> other than subclassof.

This was covered by Peter's reply to Guus - you can get what you want
simply by declaring the hyponym relation to be transitive. Converting
hyponym to (a subProperty of) subClass seems to be a hack that is
being used in order to get the behaviour you want from RDFS.


> </ol>
> <b>Scenario 2: Ontology Interoperability</b>
> The representation of an entity as an instance or a class may depend on the
> context and perspective of the user.
> For example,  in a biological ontology, the class Orangutan may have
> individual animals as its instances. However, the class Orangutan may itself
> be an instance of the class Species. Note, that Orangutan is not a subclass
> of Species, because then that would say that each instance of Orangutan (an
> animal) is an instance of Species.
> (example taken from R14 in http://www.w3.org/TR/webont-req/)

As has often been pointed out, you can state that Orangutan is an
instance of Species and that Orangutan is a subClass of Animal. Of
course the Orangutan instance has no connection with the Orangutan
class. This typically doesn't cause any problem, and it isn't obvious
that it would do in this case. Can you think of any problems?


> <p>
> Since this decision may be context dependent, the issue of making classes
> equivalent to instances arises in
> ontology interoperability and mapping scenarios. For example, a boing 777
> may be represented as an instance
> of airplane in a general aviation ontology. However, in the ontology of a
> particular aerospace company
> boing 777 may be a class that has several instances. If both ontologies must
> be aligned the appropriate mapping
> must be able to bridge the distinct set of instances and classes.

This is a tricky and long-standing KR problem that comes back to the
different choices that can be made when modelling a domain. It isn't
at all clear that allowing classes as instances solves the problem. If
in one ontology Boeing 777 is said to be a subClass of aeroplane, and
in another it is said to be an instance of aeroplane, then the two
ontologies represent different and (perhaps) fundamentally
irreconcilable views of the world. How are you proposing to align the
two ontologies in this case?

Ian

> 
> RAISED BY: Raphael Volz, email of 7/11/02.
> 
Received on Thursday, 18 July 2002 09:57:49 GMT

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