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Re: owl:sameAs - Is it used in a right way?

From: David Booth <david@dbooth.org>
Date: Sat, 16 Mar 2013 12:30:54 -0400
Message-ID: <51449E3E.5010609@dbooth.org>
To: Jim McCusker <mccusj@rpi.edu>
CC: Jeremy J Carroll <jjc@syapse.com>, Umutcan ŞİMŞEK <s.umutcan@gmail.com>, Kingsley Idehen <kidehen@openlinksw.com>, w3c semweb HCLS <public-semweb-lifesci@w3.org>
Hi Jim,

You are in good company in thinking that a URI always denotes the same 
resource, because that is a widespread misconception.  (I call it Myth 
#1 in http://dbooth.org/2010/ambiguity/paper.html .)  But it simply is 
not true in the RDF semantics.

The Architecture of the World Wide Web blithely states that "By design a 
URI identifies one resource":
http://www.w3.org/TR/webarch/#id-resources
But this is an architectural goal -- not the reality of RDF semantics. 
Although it is a good goal, and helpful as a guide to URI users, it 
turns out to be an oversimplification of reality.

The RDF Semantics is very clear that a URI denotes one resource only in 
*one* interpretation of a *given* RDF graph:
http://www.w3.org/TR/rdf-mt/#interp
But we do not have only one, giant, global RDF graph -- see Myth #2 -- 
we have *many* graphs.  And in general, a given RDF graph admits *many* 
satisfying interpretations.  The RDF semantics does not require that a 
URI denote the same resource in *different* graphs or in *different* 
interpretations of the same graph.

Thus, although as an architectural goal we would *like* a URI to always 
denote the same resource, the reality is that a URI can -- and often 
does -- denote *different* resources in different graphs, and this can 
cause inconsistencies when graphs are merged, as illustrated in Figure 26:
http://dbooth.org/2010/ambiguity/paper.html#inconsistent-merge

That is precisely why it is helpful to keep different perspectives in 
different graphs, as Jeremy suggested.

David

On 03/16/2013 01:08 AM, Jim McCusker wrote:
> David,
>
> The problem with this is that by definition, URIs ALWAYS denote the same
> resource. If there is doubt that you might be denoting something other
> than what a resource is, you should be defining your own resource.
>
> Jim
>
>
> On Sat, Mar 16, 2013 at 12:35 AM, David Booth <david@dbooth.org
> <mailto:david@dbooth.org>> wrote:
>
>     Hi Umutcan,
>
>     You have indeed stumbled on a deep question, and I think Jeremy's
>     suggestion is exactly right.  This paper on "Resource Identity and
>     Semantic Extensions:
>     Making Sense of Ambiguity" illustrates how owl:sameAs works in RDF
>     semantics:
>     http://dbooth.org/2010/__ambiguity/paper.html#sameAs
>     <http://dbooth.org/2010/ambiguity/paper.html#sameAs>
>
>     There are two keys to understanding owl:sameAs.  One is to answer
>     the question: what RDF graph are you considering?  The other is to
>     understand that the same URI may denote different things in
>     different RDF graphs.  It is only when RDF statements are in the
>     *same* graph that the RDF semantics requires the URI to denote the
>     same resource.  That is why the question of what graph you are
>     considering is crucial, and why Jeremy suggested keeping the
>     different perspectives in different graphs.
>
>     FYI, the above paper also explains how you can "split" the identity
>     of an RDF resource if you need to merge RDF graphs that use the same
>     URI in contradictory ways.
>
>     David
>
>
>
>     On 03/15/2013 02:29 PM, Jeremy J Carroll wrote:
>
>         I did not find this a rookie question at all.
>
>         This seems to get to the heart of some of the real difficult
>         issues in Semantic Web.
>
>         My perspective is different from yours, and a resource
>         description that I author is a description of the resource from
>         my perspective; a resource description that you author is a
>         description from your perspective.
>
>         If I have some detailed application that depends in some subtle
>         way on my description, I may want to ignore your version; on the
>         other hand, a third party might want to use both of our points
>         of view.
>
>         One way of tacking this problem is to have three graphs for this
>         case:
>
>         Gj, Gu, G=
>
>         Gj contains triples describing my point of view
>         Gu contains triples describing your point of view
>         G= contains the owl:sameAs triples
>
>         Then, in some application contexts, we use Gj, sometimes Gu, and
>         sometimes all three.
>
>         Jeremy
>
>
>
>
>         On Mar 15, 2013, at 11:02 AM, Umutcan ŞİMŞEK
>         <s.umutcan@gmail.com <mailto:s.umutcan@gmail.com>> wrote:
>
>             Thanks for the quick answer : )
>
>             So this issue is that subjective for contexts which allows
>             to use owl:sameAs to link resources  if they are not
>             semantically even a little bit related in real world?
>
>             Sorry if I'm asking too basic questions. I'm still a rookie
>             at this :D
>
>             Umutcan
>
>
>             On 15-03-2013 19:38, Kingsley Idehen wrote:
>
>                 On 3/15/13 1:05 PM, Umutcan ŞİMŞEK wrote:
>
>                     My question is, does LODD use owl:sameAs properly?
>                     For instance, are those two resources,
>                     dbpedia:Metamizole and drugbank:DB04817 (code for
>                     Metamizole), really identical? Or am I getting the
>                     word "property" in the paper wrong?
>
>                 The question is always about: do those URIs denote the
>                 same thing? Put differently, do the two URIs have a
>                 common referent?
>
>                 ## Turtle ##
>
>                 <#i> owl:sameAs <#you>.
>
>                 ## End ##
>
>                 That's a relation in the form of a 3-tuple based
>                 statement that carries entailment consequences for a
>                 reasoner that understand the relation semantics. Through
>                 some "context lenses" the statement above could be
>                 accurate, in others totally inaccurate.
>
>                 Conclusion, beauty lies eternally in the eyes of the
>                 beholder :-)
>
>
>
>
>
>
>
>
>
>
>
>
> --
> Jim McCusker
> Programmer Analyst
> Krauthammer Lab, Pathology Informatics
> Yale School of Medicine
> james.mccusker@yale.edu <mailto:james.mccusker@yale.edu> | (203) 785-4436
> http://krauthammerlab.med.yale.edu
>
> PhD Student
> Tetherless World Constellation
> Rensselaer Polytechnic Institute
> mccusj@cs.rpi.edu <mailto:mccusj@cs.rpi.edu>
> http://tw.rpi.edu
Received on Saturday, 16 March 2013 16:31:21 UTC

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