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RDF Concepts - IRIs do *not* always denote the same resource

From: David Booth <david@dbooth.org>
Date: Wed, 02 Oct 2013 01:05:18 -0400
Message-ID: <524BA98E.7020203@dbooth.org>
To: public-rdf-comments <public-rdf-comments@w3.org>
In https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html
I see this statement:

   "IRIs have global scope: Two different appearances of an IRI
   denote the same resource."

This is wrong.  If it were true then there could never be a URI Collision
http://www.w3.org/TR/webarch/#URI-collision
and there would be no point in the AWWW discussing it or admonishing 
against it.

An IRI can and often does denote different resources in different 
*interpretations*.  And this, in practice, means that an IRI often 
denotes different resources in different *graphs*, because any graph has 
a set of satisfying interpretations, and different graphs may have 
different sets of satisfying interpretations.  For example, suppose 
graphs g1 and g2 have sets of satisfying interpretations s1 and s2, 
respectively, and those sets may be disjoint.  Then colloquially (and 
technically) we can say that an IRI may map to one resource in g1 (i.e., 
in some interpretation in s1) and a different resource in g2 (i.e., in 
some interpretation in s2).

This requires thinking about graphs in terms of sets of satisfying 
interpretations -- an important and valid perspective -- rather than 
assuming that one looks at them only through the lens of a single 
interpretation.

As a simple example of how a URI can denote different things in 
different graphs, suppose Alice sends this graph G1 from her smart phone 
to her home computer to turn *on* her porch light (assuming the usual 
URI prefix definitions):

G1: {  @prefix db: <http://dbooth.org/>
        ex:alicePorchLight rdf:value db:x .
        db:x owl:sameAs ex:on .
        ex:on owl:differentFrom ex:off . }

and her light turns on.

In contrast, Bob sends this graph G2 from his smart phone to his home 
computer to turn *off* his oven:

G2: {  ex:bobOven rdf:value db:x .
        db:x owl:sameAs ex:off .
        ex:on owl:differentFrom ex:off . }

and his oven turns off.

It is perfectly reasonable and natural to ask "What resource does db:x 
denote in G1?", and it is reasonable and natural to ask the same of G2. 
  The RDF Semantics (along with OWL) tells us that in G1 db:x denotes 
whatever ex:on denotes, whereas in G2 db:x denotes whatever ex:off 
denotes.   That is useful!  Furthermore, the semantics tells us that if 
we merge those graphs then we have a contradiction -- there are no 
satisfying interpretations for the merge -- and that is useful to know 
also, because it means that Alice and Bob's graphs **cannot be used 
together**.

Furthermore, the RDF Semantics notion of an interpretation maps well to 
real life applications: in effect, an application chooses a particular 
interpretation when it processes RDF data.  This is a very useful aspect 
of the model theoretic style of the semantics.  In this example, Alice's 
home control app interpreted db:x to denote "on" and Bob's home control 
app interpreted it to denote "off".  And *both* were correct (in 
isolation): they both did The Right Thing.

In short, I think the above statement needs to be qualified somehow, 
such as:

   "IRIs are *intended* to have global scope: Two different
   appearances of an IRI are *intended* to denote the same resource."
   (However, the RDF Semantics explains how an IRI may denote
   different resources in different interpretations.)

David
Received on Wednesday, 2 October 2013 05:05:48 UTC

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