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

David Booth wrote:

> At 10:56 PM 1/22/2003 -0500, you wrote:
> > >
> >
> >Is this attempt at clarifying what a URL identifies intended to shed a
> >glimmer of light on what a URI is intended to identify? Seriously, since
> >are attempting to be more precise, what exactly are you talking about?
> It is an attempt to document reality, to explicitly acknowledge that when
> you use a URL to identify an abstract concept (such as a particular
> of love), it is common to use that same URL in conjunction with
> four kinds of things: the name of that concept (i.e., the URL string
> itself); the concept; a Web location from which a description of that
> concept might be retrieved; and a document instance that is retrieved from
> that Web location.  This is not only common, it is very helpful, because
> provides a powerful "view source" effect[1].  It is also "good practice"
> recognized by the TAG.[2]

It is not accepted practice to consider a URI to identify 4 things. If we
are considering the Semantic Web, the RDF model theory is reasonably precise in stating how URIs
are used in RDF -- at least in the formal definition of RDF as captured by
the model theoretic semantics. Read this paragraph: "There are several
aspects of meaning in RDF which are ignored by this semantics; in
particular, it treats URI references as simple names..."

What this says is that a URI *is* a simple name, *not* that a URI denotes a
simple name. A URI is a simple name. The name is treated as a logical
constant in the same fashion that when you might say

x = 1
xx = 2

> On the other hand, ambiguity about what a URI denotes is a Bad Thing, as
> the TAG has stated[3].  To prevent ambiguity, it is necessary to either
> "different names"[4] or "different context"[5].

This is your assertion. By 'different names' if you mean that different
constants may have different values, then this is certainly true.

> These observations have helped me (at least) mentally reconcile the
> positions that I think I've heard on the httpRange-14 issue[6], so I'm
> hoping they will help others.  Tim Berners-Lee describes[6] the issue
> an example in which a URL is used to identify an actual car, but the
> document instance that can be retrieved from that URL is a picture of the

TimBL's issue is well known, yet I've not seen a concrete example where this
is a real issue for something like an OWL reasoner (as a concrete example of
'Semantic Web' software).

> >The issue only arises when, in the semantic web, [. . .] 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? [. . .] I want to use the URI to
> >identify the picture. Roy has always felt it identifies the car.

Shrug, shrug, shrug. I can say:

#foo a #Car .
#foo a #Picture

#Car owl:disjointWith #Picture

and I have a contradiction, so what?

> In particular, if you believe that it's adequate to use "different
> context"[5] to distinguish the different uses, then there is no need for
> the TAG to definitively say whether the URI identifies the car or the
> picture of the car.  On the other hand, if you believe that it's important
> to use "different names"[4] to distinguish the different uses, then there
> is a need for the TAG to decide which thing the URI is supposed to
> -- the car or the picture of the car.

The TAG might decide something, for sure. On the other hand there is the RDF
model theory/semantics and the upcoming OWL model theory/semantics which
define a reasonably precise way for processing documents (ontologies and
knowledge bases) that contain URIs. There is software that implements OWL
reasoners. It works. I see ***no problem here***

I really wish folks would stop pointing to the "Semantic Web" as evidence
that there exists some deep problem with URI ambiguity, because the
"Semantic Web" as incarnated by working software that implements the
upcoming sets of working drafts, actually works.


Received on Thursday, 23 January 2003 22:11:43 UTC