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

Date: Fri, 24 Jan 2003 18:13:30 -0500
To: "Jonathan Borden" <jonathan@openhealth.org>
From: David Booth <dbooth@w3.org>
Subject: Re: Clarifying what a URL identifies (Four Uses of a URL)

Jonathan,

At 09:49 PM 1/23/2003 -0500, Jonathan Borden wrote:
>David Booth wrote:
> > . . . when
> > you use a URL to identify an abstract concept (such as a particular concept
> > of love), it is common to use that same URL in conjunction with identifying
> > 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 it
> > provides a powerful "view source" effect[1].  It is also "good practice" as
> > recognized by the TAG.[2]
>
>It is not accepted practice to consider a URI to identify 4 things.

You've misunderstood what I'm talking about, which make me wonder if you 
read http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm too 
quickly. The section 
http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm#OneURLAndFourThings 
explains what I mean by these "four kinds of things".  I would be grateful 
for any feedback on what parts of that document you find unclear, because 
I'd like to improve it.

>If we
>are considering the Semantic Web, the RDF model theory
>http://www.w3.org/TR/rdf-mt/#intro 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
>use
> > "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.

Yes, but I'm a little puzzled that you seem to be questioning this 
assertion, since the "different context" approach just means that something 
other than "different names" is being used to prevent ambiguity, so my 
assertion is clearly a tautology.  Were my descriptions of the "different 
names" and "different context" approach unclear in 
http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm ?  Again, I 
would very much appreciate specific feedback on any unclarity.

> > 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
>using
> > 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
>car:
>
>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?

You seem to be implying that I shouldn't try to use "#foo" to refer to both 
the car and the picture of the car.  That's true enough.  But the point is 
that it's very helpful (and "good practice" according to the TAG), that if 
you're using a URI to identify something that does not exist directly on 
the Web (such as a car), you should place a document at the end of that 
URI, which describes the thing that you're using that URI to identify.   In 
TimBL's example, that document is a picture of the car.  The car and the 
picture are two different things.  Yet somehow I should be able to make 
unambiguous statements about either one.  For example, I should be able to 
make two statements of the form:

       X #is #beautiful
       Y #is #beautiful

(where X and Y represent some kind of designation involving the URI) and be 
unambiguously referring to the car in the first statement, and the picture 
of the car in the second statement.
Therefore I need a way to distinguish which one I mean when I make a 
statement.  There are two ways that can be done -- different names or 
different context -- and they have different pros and cons.

Sandro proposes some specific conventions[7] for making this 
distinction.  You may already have conventions that you like (such as the 
use of FragIDs), which may be why you don't think this is an issue.  Any 
convention must use either the "different names" approach or the "different 
context" approach (or even a mixture).

Again, I'd be very grateful if you could point out what parts of 
http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm are unclear 
in explaining this issue.

> > 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
>identify
> > -- 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.
>
>Jonathan

1. 
http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm#ViewSourceEffect
2. http://www.w3.org/TR/webarch/#uri-use (section 2.2.3)
3. http://www.w3.org/TR/webarch/#uri-use (section 2.2.5)
4. http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm#DifferentNames
5. 
http://www.w3.org/2002/11/dbooth-names/dbooth-names_clean.htm#DifferentContext
6. http://lists.w3.org/Archives/Public/www-tag/2003Jan/0287.html
7. http://www.w3.org/2002/12/rdf-identifiers/

-- 
David Booth
W3C Fellow / Hewlett-Packard
Telephone: +1.617.253.1273

-- 
David Booth
W3C Fellow / Hewlett-Packard
Telephone: +1.617.253.1273

Received on Sunday, 26 January 2003 22:31:15 UTC