- From: Michael Schneider <schneid@fzi.de>
- Date: Mon, 5 Nov 2007 21:36:32 +0100
- To: "Danny Ayers" <danny.ayers@gmail.com>
- Cc: <public-owl-dev@w3.org>, "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Hi, Danny! In addition to Peter's answer perhaps a few words about the RDF side (as far as I have understood this myself, but if I am wrong in a point, others will certainly intervene immediatly :)). There is really a distinction between a "class resource itself", and its extension, i.e. the set of all the resources which are rdf:type this class. And analog for properties, for which their extension is a set of subject-object tuples. There is a nice (but somewhat confusing) figure demonstrating this "duality" (so to speak) for properties in the RDF semantics document. It's figure 1 in http://www.w3.org/TR/rdf-mt/#gddenot There are two resources (in "IR"), the numbers "1" and "2", and "1" is actually a property (in "IP"). Don't get confused: There are /two/ different URIs "a" and "b" denoting this same property "1". The extension of property "1" is the set IEXT(1) = {<1,2>, <2,1>} (This is kind of curious, btw., because property "1" itself occurs once as a subject, and once as an object in the tuples within the extension of "1". But that's allowed.) AFAICS, the distinction between a class resource C and its extension ICEXT(C) is actually perceivable in OWL-Full. Say you have some property :p assigned to some class :C (allowed in OWL-Full): :C a owl:Class . :p a owl:ObjectProperty . :x a owl:Thing . :C :p :x . In OWL-Full you can have another class :D, which equals :C, by asserting :D owl:sameAs :C and in this case you can entail :D :p :x . because by owl:sameAs you declare :C and :D really to be the same, sharing all their properties and extensions, so you also can entail: :D owl:equivalentClass :C . because this "only" means that the extensions of :D and :C are the same. But here's the interesting bit: If you just specify :D and :C to be /equivalentClass/ instead of sameAs, i.e. if you just demand that only the /extensions/ of :D and :C are equal, not the class resources denoted by :D and :C themself, then you will /not/ get all the properties of :C also for :D. I.e., asserting :D owl:equivalentClass :C does *not* entail :D :p :x . # not entailed! So the distinction between a class and its extension is not just play-on-words, it's kind of "real", though AFAIK it can only be perceived within OWL-Full [FIXME!], because only there you have the language features and the semantical conditions to exploit this distinction directly. Originally, I had a hard time to accept this distinction between a class resource and its extension. The reason for this separation given somewhere in the RDF semantics spec is, IIRC, that this allows a class C to be an element of itself. Well... :) But in the meanwhile I like this distinction, I find it very RDFish: It allows me to differentiate between comparing the /descriptions/ of two classes (using sameAs), and the extensions of two classes (using equivalentClass). I appreciate this, because two classes can happen to have the same extension, even if they were defined with completely different intentions in mind, i.e. described with different properties. Cheers, Michael >-----Original Message----- >From: Danny Ayers [mailto:danny.ayers@gmail.com] >Sent: Monday, November 05, 2007 10:49 AM >To: Peter F. Patel-Schneider >Cc: Michael Schneider; public-owl-dev@w3.org >Subject: Re: [OWLWG-COMMENT] Punning and the "properties for >classes" use case > >Hi Peter, > >I'd be grateful if you could help me with this one point - > >On 02/11/2007, Peter F. Patel-Schneider ><pfps@research.bell-labs.com> wrote: > >> Well, RDF overloads URIs already. A URI in RDF denotes a >entity in the >> domain of discourse as well as that entity's property >extension, which >> also carries the entity's class extension. > >I assumed the resource identification here was (very loosely) like >saying 'that bag of peanuts' - the peanuts inside are an implicit >characteristic of the thing identified, even though you might not be >able to see them. Perhaps you could contrast RDF's approach with >another (OWL 1.1's?) which doesn't overload? > >Cheers, >Danny. >-- > >http://dannyayers.com > -- Dipl.-Inform. Michael Schneider FZI Forschungszentrum Informatik Karlsruhe Abtl. Information Process Engineering (IPE) Tel : +49-721-9654-726 Fax : +49-721-9654-727 Email: Michael.Schneider@fzi.de Web : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555 FZI Forschungszentrum Informatik an der Universität Karlsruhe Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe Tel.: +49-721-9654-0, Fax: +49-721-9654-959 Stiftung des bürgerlichen Rechts Az: 14-0563.1 Regierungspräsidium Karlsruhe Vorstand: Rüdiger Dillmann, Michael Flor, Jivka Ovtcharova, Rudi Studer Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus
Received on Monday, 5 November 2007 20:36:50 UTC