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Re: Clarifying what a URL identifies (Four Uses of a URL)

From: Tim Berners-Lee <timbl@w3.org>
Date: Wed, 22 Jan 2003 22:02:22 -0500
Cc: www-tag@w3.org
To: Sandro Hawke <sandro@w3.org>
Message-Id: <17C21314-2E7F-11D7-B288-000393914268@w3.org>

> You said "URIs identify Resources".  I read that as claiming that
> there exists a special IETF-sanctioned function which maps each URI
> string to a "Resource", in the web-protocol world in general or at
> least HTTP in particular.  Are you claiming that?
>
> Even the most narrow HTTP-only form of that claim appears false, since
> TimBL and RoyF cowrote (with others) the relevant RFCs and yet don't
> agree on httpRange-14!  If they can't agree on the range of that
> function, that must mean their conceptualizations of the supposed
> function are quite different.  If the writers of the standard can't
> agree, I suggest that means the standard does not actually bear
> meaningfully on the issue.
>

Perhaps a little explanation is in order.

Roy and I agree on how HTTP works.  (Roy, forgive
me if I misrepresent you.)
HTTP relates URIs to representations which are returned.
While the spec mentions resources, the protocol itself
does not actually constrain what they actually are.

The issue only arises when, in the semantic web, we
we extend the formal system from network objects
and TCP streams to arbitrary concepts. Then,
in a formal system where one has to chose one,
we ask ourselves what exactly is the thing
we should say is identified by some http URI - the
picture of the car, or the car?  Either is consistent with HTTP.

We agree that with HTTP a number of different
representations of the thing identified by the URI.
I want to use the URI to identify the picture.
Roy has always felt it identifies the car.
Either system is self-consistent.
I use "representation" to refer to the relationship between
the picture and the bits. Roy uses it to refer to the
relationship between the car and the bits.
We are using the same english word for different
technical relations.

There are a number of reasons why I strongly prefer
the URI to identify the web page, and I have gone into
them elsewhere, for example in
http://www.w3.org/DesignIssues/HTTP-URI.html

This is the crux of the HTTP range issue 14.
(There are other different issues related to fragment identifiers
and content negotiation.)

One can't argue it by arguing about the meaning of english words.
"representation", "document".   One can't just argue it based on
appeal to the way humans use URIs to refer to things.
These aren't the formal system. They resolve ambiguities
all the time with great alacrity.
One *can* introduce a new system with a different design
and argue its merits. Sandro has designed an alternative
system http://www.w3.org/2002/12/rdf-identifiers/
which seems consistent and I haven't finished thinking
about - there are things I like about it and things I don't.
But it does address all the questions, I think.

Tim BL
Received on Wednesday, 22 January 2003 22:02:10 GMT

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