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URIs: resources and contradictions was: Re: httpRange proposed text

From: Jonathan Borden <jonathan@openhealth.org>
Date: Tue, 30 Jul 2002 12:22:21 -0400
Message-ID: <07a701c237e5$4b79ff00$0a2e249b@nemc.org>
To: "Tim Bray" <tbray@textuality.com>, "Tim Berners-Lee" <timbl@w3.org>
Cc: <www-tag@w3.org>

Tim Bray wrote:
>
> Tim Berners-Lee wrote:
> > On Monday, July 29, 2002, at 06:17 PM, Tim Bray wrote:
> >> Joshua Allen wrote:
> >>> "If two people independently use the same URI as an identifier, they
> >>> should be able to have a reasonable degree of confidence that they are
> >>> identifying the same resource.  People should not be required to
> >>> parse, dereference, or otherwise
> >>> acquire any *additional* disambiguating information to provide this
> >>> basic guarantee.  ...

>
> >> The intent seems good, but how on earth do you build this confidence?
> >> By relying on the human-language semantics of the opaque part of the
URI?
>
> > Absolutely not.  Joshua didn't mean that you knew what each URI meant by
> > just looking at it -- he meant (I think/hope!) that you know from the
> > architecture that the two occurrences of the URI will identify the same
> > thing, whatever that is.  There is no ambiguity built into the
> > architecture itself.  This is a core principle fo the Web which we seem
> > to be in danger of forgetting.
>
> Indeed, I mis-parsed Joshua's point, but I come back with the same
> question:  How do you get this kind of confidence?  Joshua suggests that
> the answer has to do with how you go about naming resources.  I don't
> get it; further explanation please? -Tim
>

I would say what Joshua said even more strongly (this *is* a core
principle):

"
When two people use the same URI to identify a resource _they are
identifying the same resource_
"

This is true by definition (indeed an axiom)

Considering this raises some interesting questions e.g. how can we ensure
that they are referring to the same resource?

Let's use a short example where two people both use the same URI
http://example.org/car but assert different and conflicting properties about
the resource. I am going to demonstrate some RDF statements which make these
assertions and use some OWL classes and properties. The question is: Are
"A" and "B" referring to the same or different resources?

A:

<http://example.org/car> rdf:type timbl:Document .
<http://example.org/car> ex:length "1024" .


and

B:
<http://example.org/car> rdf:type rf:Car.
<http://example.org/car> ex:length "102" .

rf:Car owl:subClassOf owl:Class .
rf:Car owl:disjointWith timbl:Document .

now let us assume that "ex:length" is a functional property e.g. that a
resource may have only one value for this property.
e.g.

ex:length owl:subPropertyOf owl:FunctionalProperty .

What do we have?

Well we have a few contradictions that's all, we are trying to say that:

"1024" = "102" which is patently false
and we are also trying to say both that http://example.org/car is both a
timbl:Document and an rf:Car which is a contradiction if we also say that
rf:Car owl:disjointWith timbl:Document .

To summarize, a URI always identifies a single resource, regardless of who
is using the URI and regardless of which claims are made about the resource.
Indeed people might make conflicting claims, which are simply
contradictions, but that happens all the time and we deal with it.

Jonathan
Received on Tuesday, 30 July 2002 12:28:47 GMT

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