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Re: My action item on Moby Dec, issue 14, etc

From: Tim Berners-Lee <timbl@w3.org>
Date: Tue, 24 Sep 2002 12:24:51 -0400
Cc: WWW-Tag <www-tag@w3.org>
To: Tim Bray <tbray@textuality.com>
Message-Id: <26FA5706-CFDA-11D6-A31E-000393914268@w3.org>


On Thursday, September 19, 2002, at 12:38 AM, Tim Bray wrote:

>
> When I say "Webarch" I mean 
> http://www.w3.org/TR/2002/WD-webarch-20020830/
>
> Following on a recent TAG telecon, I took an action item to chip away 
> at the coalface exposed by excavating around our issue HttpRange-14, in 
> which area  the TAG is more or less at an impasse.  Although I say 
> nothing about the range of HTTP URIs, I nonetheless claim this 
> discharges my action item.
>
> I'm going to focus on the Webarch Principle that reads "Absolute URI 
> references are unambiguous: Each absolute URI reference unambiguously 
> identifies one resource."
>
> Upon consideration, I think that this principle:
>
> (a) is probably false, and
> (b) there is no way to test, observationally, whether this principle is 
> true or false, thus it falls outside the scope of science, and
> (c) has little bearing on Web architecture from the point of view of 
> people who implement real software, and
> (d) Webarch has no need to say anything about what resources are, aside 
> from a reference to the lightweight definition found in RFC2396 
> ("anything that has identity"), so
> (e) This principle should simply be removed from Webarch forthwith.
>
> In fact, unless a new line of argument comes up to make me change my 
> mind, I'm probably going to become a hard-core advocate (as in do this 
> or take my name off the document) of losing this counter-productive 
> alleged "principle".
>
> To take these up in order:
>
> (a) I could easily synthesize a URI that when dereferenced, alternately 
> returned representations of the current weather in Oaxaca and pictures 
> of my adorable cat Bodoni.  I could, whenever the whimsy struck me, 
> change one of the alternate representation generators to whatever 
> struck my whimsy that day. This URI simply does not in any practical 
> sense identify a single anything.
>
> (b) Notwithstanding the above, I could construct an argument (which 
> would be pretty strained and sophistry-packed) that the URI above does 
> represent a resource whose definition involves T. Bray's whimsy and so 
> on.  Man people would not be convinced.  It doesn't matter; since a URI 
> is just a URI, there is no machine-testable way to ascertain whether 
> the resource(s) whose representation(s) is(are) retrieved are one or 
> many. Thus this so-called principle is non-testable, outside the domain 
> of science, and can be (at best) an assertion of a particular 
> world-view. I will freely grant that we normally assume URIs are bound 
> unambiguously to resources that have some material or conceptual 
> solidity, but this is such a truism that I'd feel silly spelling it out 
> in this document.
>
> (c) From the software builder's point of view, a URI is a character 
> string whose syntax is governed by the appropriate RFCs upon which a 
> small number of operations can be performed; the only one you can count 
> on is comparison with other URIs.  Some URIs can also be dereferenced 
> to yield a representation.  The workings of none of these operations 
> are affected in the slightest by what a resource "is", granted only 
> that the implementor bears in mind that what they're getting is a 
> representation, with the limitations that implies.
>
> URIs are also useful (as in RDF) in representing knowledge and building 
> frameworks of assertions with the aim of performing inference.  
> Clearly, chains of inference break messily if a URI is used to refer to 
> different things.  For example, if there is ambiguity about whether a 
> particular URI refers to Moby Dick, the work, or Moby Dick, the 
> particular printed volume on the shelves of some library, or the 
> electronic catalogue record describing that particular paper artifact, 
> all sorts of problems arise: is "size" to be measured in words of the 
> novel, in pages of one printing, or in bytes consumed by a catalog 
> record?



> Unfortunately, in the real world, this kind of confusion will arise - 
> no conceivable application of computer technology or social 
> organization can prevent it - and knowledge representation systems need 
> to be able to deal with the resulting dissonance.  The smooth operation 
> of any system on which inference is to be based had better have a type 
> system, and for any one object, agreement on the type of the object, 
> before inference can proceed. In fact, detection of the case where 
> ambiguity exists, and a robust strategy for operating when it is 
> detected (even if it's only the KR equivalent of "404 not found") seems 
> like a <i>sine qua non</i> for the progress of the Semantic Web.
>
> An assertion in Webarch that such confusion cannot occur is as futile 
> as the medieval church's insistence that Copernicus was wrong, or the 
> classical hypertext theorists' assertion that links must not be allowed 
> to break.
>

What you say, Tim, holds about natural language and I am sure always 
will.
Human confusion will occur when trying to understand specs, too.
However, if a spec cannot be clear about what something means the 
semantic web
is impossible and will have to have a completely separate architecture.
S, while people can be sloppy in their speech let's not remove the
fundamental axiom  (x=x) of URIs as identifiers.


> (d) The discussion about what a resource is is a well-known rathole 
> which has consumed endless cycles and time among some of the world's 
> smartest and most experienced practitioners.  Once we've noted what 
> RFC2396 says on the subject, there are a bunch of other useful things 
> we can say (all the other principles in section 2.1 of Webarch), none 
> of which are in the slightest weakened or perceptibly affected by what 
> a resource may or may not be. Fortunately, we need not go there.
>
> (e) Let's get rid of this thing.  We will free up time to work on the 
> important material in Section 3, we will remove content that has to do 
> more with metaphysics than engineering, and we will not remove the 
> tiniest atom of value from the document.  -Tim

If someone could produce a version of the diagram   
http://www.w3.org/DesignIssues/diagrams/http-uri-1.{ai,png}
which replaces the names for concepts, and the various URIs, with those 
from an alternative
Received on Tuesday, 24 September 2002 12:24:56 GMT

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