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Subject: RE: Could owl:sameAs reference non-OWL resources

Title: Re: Could owl:sameAs reference non-OWL resources
Dear Pat
Your response is clear and satisfactory, and sums up clearly a few exchanges I had about it since the question was set, through which, I hope, my understanding of the debate about URIs as names vs URIRefs linked to a particular addressing protocol has clarified a bit. It figures interaction between OWL and Subject Indicators might turn out to be less obvious than what I imagined a few months ago. But I still hope to come out with some proposal about it.
Thanks for your time

Bernard Vatant
Senior Consultant
Knowledge Engineering
Mondeca - www.mondeca.com

 -----Message d'origine-----
De : public-webont-comments-request@w3.org [mailto:public-webont-comments-request@w3.org]De la part de pat hayes
Envoyé : jeudi 3 juillet 2003 00:23
À : bernard.vatant@mondeca.com
Cc : public-webont-comments@w3.org
Objet : Re: Could owl:sameAs reference non-OWL resources

Dear Bernard

This is in response to your comment/query

The short answer is 'yes'; that is, OWL can, in principle, interoperate with 'non-OWL resources'. In fact there is not really a well-defined notion of an "OWL resource" and hence not of a non-OWL resource: like RDF, OWL simply uses URIrefs to denote things; and 'things' here can literally be *any* thing at all; so the use of owl:sameAs to indicate that two things are the same (more exactly, that two different URIrefs refer to the same thing) is not restricted in any way: in particular, it is not restricted to "OWL resources". (There is a restriction in OWL DL in that owl:sameAs cannot be used there to refer to identity of OWL classes or properties: this is to keep the OWL DL inference process within manageable complexity bounds. But this restriction is private to OWL, as it were: it doesn't prevent OWL from spreading its referential net widely across the universe of Web resources.)

In the example you cite, the URIref "http//sports.org/US#SoccerTeam" is supposed to indicate a class, probably the class of members of the team; so this is indeed an externally defined resource. The OWL text asserts that this is being regarded as an OWL class, which is merely OWL's way of saying that it claims the right, as it were, to perform class reasoning about it, eg think about the number of things in it, use it to help define other classes, and so on.  But OWL is a general-purpose class reasoner, and it can reason about virtually any class of virtually any kind of membership. It is not restricted to a special category of "OWL classes" distinct from other kinds of class or groupings. (OWL DL is somewhat restricted, eg it cannot deal with classes containing other OWL classes, or classes of OWL properties. Again, this is deliberate design decision which trades some expressive power for a gain in processing speed in a reasoning engine.  But any class that fits the OWL DL standards - which will be many of the cases one meets with in practice - will be a suitable OWL-DL class.)

I am not sufficiently familiar with XTM subject indicators to answer that part of your question definitively, but certainly the OWL use of URIrefs does not restrict them to those that retrieve a document which use any particular format. Note however that your useage of the phrase 'the resource at "http//sports.org/US#SoccerTeam" ' suggests that you think of this URI as indicating the document 'at' the URL. OWL does *not* make his assumption: it uses URIrefs simply as names, and assumes that they refer to something.  Exactly how these entities - the referents of the URIrefs - are defined is not discussed by the OWL semantics. To this extent, OWL (and RDF and DAML) are somewhat disconnected from the RFC 2396 picture of the use of URIrefs within the REST architecture of an idealized Web. They do not deny this architecture, of course, but (with a few exceptions, most notably the use of owl:imports and the associated terminology) they largely ignore it. Thus, there is nothing in the OWL spec that says how, if at all, one can *determine* what class "http//sports.org/US#SoccerTeam" actually denotes: the OWL stance on that question might be summed up as 'whatever class it is, the following conclusions must be true of it, given what I know about it...'; and OWL's task is simply to ensure that the process of drawing those conclusions is error-free and as complete as possible. The same observation applies to RDF and DAML; to this extent, these languages are similar in the way they approach the meanings of URIrefs.

Please let me know (with a cc: public-webont-comments@w3.org) whether this reply adequately responds to your comment.


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