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RE: How does RDF/OWL formalism relate to meanings?

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 12 Apr 2004 11:05:46 -0500
Message-Id: <p06001f4dbca06a76b91d@[10.0.100.76]>
To: "John Black" <JohnBlack@deltek.com>
Cc: <public-sw-meaning@w3c.org>

>  > -----Original Message-----
>>  From: public-sw-meaning-request@w3.org
>>  [mailto:public-sw-meaning-request@w3.org]On Behalf Of Pat Hayes
>>  Sent: Saturday, April 10, 2004 12:20 AM
>>  To: Dan Connolly
>>  Cc: public-sw-meaning@w3c.org
>>  Subject: Re: How does RDF/OWL formalism relate to meanings?
>>
>>
>>
>>  >I think I get it now...
>>  >
>>  >On Fri, 2004-04-09 at 16:19, Pat Hayes wrote:
>>  >[...]
>>  >>
>>  >>  HttpRange-14 is obviously relevant to this issue, but it doenst
>>  >>  resolve it. OK, suppose URIs can indeed denote real red cars and
>>  >>  imaginary white whales. That resolves httpRange14.  Now,
>>  how does the
>>  >>  owner of a URI *say* that his URI shall be the name of
>>  *particular*
>>  >>  red car or white whale?
>>  >>
>>  >>  BTW, it would be perfectly fine to say that there was no
>>  general way
>>  >>  to do this, and that you just have to do your best to describe the
>>  >>  things as well as you can.
>>  >
>>  >OK, I suppose we can agree to that.
>>  >
>>  >>   At least then we would know where we stand. BUt then
>>  y'all ought to
>>  >>  take out all that mantra about resources being uniquely
>>  identified by
>>  >>  URIs.
>>  >
>>  >We have made some progress in that direction.
>>
>>  Great.  (Really !)
>>
>>  >I just scanned
>>  >the document, and nowhere does it say "each URI denotes a unique
>>  >resource"... at least not using the word "unique". I'll have
>>  >to re-read your message to public-webarch-comments, where
>>  >you excerpt lots of text that bothers you.
>>  >
>>  >I'm only aware of 1 at this point:
>>  >We still have "A URI must be assigned to a resource in order
>>  for agents
>>  >to be able to refer to the resource" which overstates the case since
>>  >you can refer to things using owl:InverseFunctionalProperty
>>  expressions
>>  >but without giving them URIs.
>>  >
>>
>
>In the RDF Semantics Recomendation it states:
>
>"1.2 URI references, Resources and Literals.
>This document does not take any position on the way that URI references
>may be composed from other expressions, e.g. from relative URIs or
>QNames; the semantics simply assumes that such lexical issues have been
>resolved in some way that is globally coherent, so that a single URI
>reference can be taken to have the same meaning wherever it occurs."
>- http://www.w3.org/TR/rdf-mt/#urisandlit
>
>What is the effect of the language, "...so that a single URI
>reference can be taken to have the same meaning wherever it occurs."?
>How important is this assumption to RDF semantics?

Very important, but it carries less weight than you (and others) 
might read into it. In hindsight I wish we had stated this more 
carefully and avoided the m-word. I was thinking of 'meaning' as a 
function from interpretations to referents; regrettably, many readers 
understand the 'meaning' of a URIref to be a single referent. My bad 
for not thinking of this potential misunderstanding.

All that is meant is that *whatever it means*, the URI is the same 
URI everywhere. This does not mean "The URI has a single unique 
referent". So if I communicate some RDF to you which uses a URI 
ex:uri and you also find some more RDF somewhere else that also uses 
ex:uri, then you are justified in teating these are really the same 
URI, and combining the information to draw conclusions. You don't 
have to think: Oh, this ex:uri over here might be understood in a 
different context from that ex:uri over there, so it might not really 
be the same thing. So it really is a single global constant, the same 
everywhere.  However, it can still be globally ambiguous what exactly 
it is referring to; it may well be that all the information available 
on the entire planet about ex:uri is not enough to tell you exactly 
what that URI is supposed to denote. So one should read the RDF as 
saying something like "ex:uri, whatever it is, has these 
properties...."  rather than " 'ex:uri' is the Name of a unique thing 
with these properties".

Another way to put it, if you are thinking along logical lines, is 
that bnodes are existential assertions with scope a single graph, 
while URIrefs are existentials with scope the entire Web. BUt they 
are logically very similar in everything but their scope.  Which 
emphasizes that the above text is intended to be making a syntactic 
point (about co-occurence of names and lack of contextuality) rather 
than a semantic point (about unique reference). But if this isnt 
helpful, ignore it.

Pat

>
>John Black


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Received on Monday, 12 April 2004 12:05:48 GMT

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