- From: Tim Berners-Lee <timbl@w3.org>
- Date: Tue, 24 Sep 2002 12:24:51 -0400
- To: Tim Bray <tbray@textuality.com>
- Cc: WWW-Tag <www-tag@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 UTC