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Re: owl:sameAs use/misuse/abuse Re: homonym URIs

From: Richard Cyganiak <richard@cyganiak.de>
Date: Mon, 25 Jun 2007 12:06:43 +0200
Message-Id: <B8E985E5-1C44-402C-A376-7086224C3FC6@cyganiak.de>
Cc: Bernard Vatant <bernard.vatant@mondeca.com>, Tim Berners-Lee <timbl@w3.org>, semantic-web@w3.org
To: Jacek Kopecky <jacek.kopecky@deri.org>

On 25 Jun 2007, at 01:01, Jacek Kopecky wrote:

> Wow. It's a nice description of the difference between the symbol and
> its referent. But I guess I should be able to state that
> @prefix ex <http://example.org/>
> ex:A http:redirects303To <http://example.org/A/doc>
> ex:B http:redirects303To <http://example.org/B/doc>
> ex:A owl:sameAs ex:B
> (which I think was Bernard's point)
> Or is it wrong to create http:redirects303To as a statement about the
> HTTP resource? If so, why?

It's wrong indeed. Because RDF statements always are about the  
referents, and never about the identifier. The redirection is a  
property of the identifier system (the URI), and not of the  
identified thing. If I say:

<http://dbpedia.org/resource/Berlin> http:redirectsTo <http:// 
dbpedia.org/page/Berlin> .

Then I have said “the city of Berlin redirects to a web page about  
the city of Berlin.” Which is nonsense.

Same with things like:

<http://dbpedia.org/resource/Berlin> str:numOfCharacters 33 .

“The city of Berlin is 33 characters long.” Nonsense again.

You cannot talk directly about URIs in RDF. The statement will always  
be about the referent.

If you want to talk about the URI, then you would have to use  
something like this:

"http://dbpedia.org/resource/Berlin"^^xsd:anyURI str:numOfCharacters  
33 .

But of course literals are not allowed in the subject position ...

> Or does it perhaps show that while so very
> convenient, http: URIs (without fragment ID) already introduce a pun
> that makes owl:sameAs too strong?

I don't think so. It shows that you have to keep in mind that RDF  
doesn't talk about URIs, but about the resources identified by the URIs.

> I've always been uneasy about the 303 approach to having http: URIs
> denote non-information resources; I guess I'd be in the 'hash' camp.
> Basically, my feeling is that 303 does not fully solve the issue,  
> so it
> should be a softer recommendation than a W3C Recommendation MUST.

It isn't a MUST, and I've never seen anyone suggest that it should be.

Hash URIs and 303 URIs are both perfectly fine as identifiers for non- 
information resources, both with their pros and cons (discussed at  
length in e.g. [1], [2] and [3]).


[1] http://www.w3.org/TR/swbp-vocab-pub/
[2] http://www.dfki.uni-kl.de/~sauermann/2006/11/cooluris/
[3] http://www.w3.org/2001/tag/doc/httpRange-14/2007-05-31/HttpRange-14

> I'm
> off-line at the moment though so I can't verify whether the WebArch
> document makes a MUST-level statement about information resources and
> 200 vs. 303 responses.
> Having endless fun with the subtleties of SemWeb,
> Jacek Kopecky
> On Fri, 2007-06-15 at 16:23 +0200, Richard Cyganiak wrote:
>> Bernard,
>> On 15 Jun 2007, at 10:30, Bernard Vatant wrote:
>>> But is not 'A owl:sameAs B' intended to mean, by OWL definition,
>>> that in any context using the semantics of URIs, those semantics
>>> are the same for A and B, so whatever assertion is true of  A is
>>> true of B, so A and B can be used indifferently.
>> Huh? No. A and B are *symbols*. They are two different symbols. They
>> cannot be used indifferently.
>> The symbols A and B denote two *referents* A' and B' (glossing over
>> the question how exactly they denote those referents). A owl:sameAs B
>> means that anything that is true for A' is true for B' and vice
>> versa. You can use A' and B' indifferently (because they are the
>> same, if you choose to believe the owl:sameAs claim), but not A  
>> and B.
>> We communicate using symbols. In natural language, in RDF, and in
>> OWL. Changing the symbols changes what is communicated, even if the
>> symbols denote the same thing.
>>> But if through http protocol  you retrieve "what the owner of A
>>> declares is true of A" and "what the owner of B declares is true of
>>> B" (read : RDF descriptions of A and B), with no certitude
>>> whatsoever if those descriptions are consistent or not, that means
>>> http protocol is not a context where A and B have the same  
>>> semantics.
>> “A and B have the same semantics” is a very murky statement best
>> avoided.
>> HTTP deals only with symbols (URIs), descriptions of referents (where
>> the referent is a non-information resource), and representations of
>> referents (where the referent is an information resource). It never
>> deals with the referents themselves.
>> Two symbols having different descriptions doesn't preclude me from
>> interpreting them as having the same referent. The same-ness of
>> referents never enters the picture at the HTTP level.
>>> I can live with that, but it seems at least hard to understand and
>>> harder to explain, if one judge by the everthread about it.
>>> OTOH, if one uses owl:sameAs for URIs identifying resources which
>>> are *information resources*, then they should actually redirect to
>>> the same document. Yes?
>> That would be a good idea, though technically I don't believe you
>> *have to*.
>> Richard
Received on Monday, 25 June 2007 10:07:01 UTC

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