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

Hi David 

On Mar 16, 2013, at 11:26 PM, David Booth wrote:

> Hi Alan,
> 
> On 03/16/2013 01:49 PM, Alan Ruttenberg wrote:
>> David's assertion that a uri can mean different things in different
>> graphs is an opinion
> 
> An opinion?  It is direct consequence of standard RDF Semantics!  

No, it isn't. 

> Read the spec:
> http://www.w3.org/TR/rdf-mt/

Indeed. Section 1.2, first paragraph: "... the semantics simply assumes that ... a single URI reference can be taken to have the same meaning wherever it occurs. Similarly, the semantics has no special provision for tracking temporal changes. It assumes, implicitly, that URI references have the same meaning whenever they occur."

> The RDF semantics is only defined for a *given* RDF graph.  It does not constrain a URI's resource identity across *different* graphs.

No, it does so constrain what URIs mean. It presumes that a given URI denotes some single thing, wherever it occurs. The interpretation mappings are defined as mappings upon URIs. not upon occurences of a URI token in a particular graph; and the semantics does not mention contexts or any other mechanism which would allow a URI in one place to denote differently from the same URI in another place (or at another time, cf the above quote.)

Entailment, for example, is defined upon *sets* of RDF graphs, and the definition of merging makes sense only if the URIs in these various graphs all denote in the same way. 

>  And here is a trivial existence proof that demonstrates that a URI does *not* necessarily denote the same resource in different graphs.
> 
> Graph 1 (assuming standard owl: prefix):
> 
>  <http://example/h> a <http://example/WhiteHorse> .
>  <http://example/WhiteHorse>
>       owl:disjointWith <http://example/BlackHorse> .
> 
> Graph 2:
> 
>  <http://example/h> a <http://example/BlackHorse> .
>  <http://example/WhiteHorse>
>       owl:disjointWith <http://example/BlackHorse> .
> 
> Each graph (by itself) has satisfying interpretations per standard RDF (and OWL) semantics.  And <http://example/h> denotes a resource in each graph.  But clearly it denotes a *different* resource in each graph.

The conclusion you should draw here is that these graphs cannot both be true. And indeed, if you merge those graphs into one, then that merged graph is owl-inconsistent. But that does not mean that the URIs denote differently in the two graphs. What it does reflect is the fact that no interpretation of names will make a contradiction true. 

>> that does not concur with either the
>> web specifications
> 
> Correct.  As I pointed out, the AWWW's statement that "a URI identifies one resource" is a good goal, but it does not concur with standard RDF semantics.

It is quite consistent with, indeed presupposed by, RDF semantics. As to whether it is really the case, that is a whole more complicated question. But note that when AWWW says "identify", it apparently does not always mean what RDF semantics means by "denote". There is no assumption in RDF theory or practice that a URI must denote what it 'dereferences' to using http. It might well be the case that awww-identification is unique even while denotation is not. 

> 
> nor the goals they were built to satisfy. Caveat emptor.
> 
> Not true!  As I said before, I *agree* with the goal stated in the AWWW, that a URI should denote one resource!  But that does not change the reality: that a URI does *not* necessarily denote only one resource.

You are probably right, but to the extent that they do not, then meaningful communication fails. So communication usually presumes that they do, until circumstances force this assumption to be revisited. 

Pat

> I also think world peace is a good goal, but it is *not* the reality.
> 
> If we're going to make the semantic web work, we need to keep the goals in mind while *also* recognizing the reality.  Facing reality should not be construed as dismissing the goals.  We cannot simply wish the reality away.  We need to do the engineering to make it work.
> 
> David
> 
> 

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Received on Monday, 18 March 2013 02:02:31 UTC