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RE: New Intro to RDF

From: Martin Hepp \(DERI extern\) <martin.hepp@deri.org>
Date: Wed, 5 Oct 2005 01:31:51 +0200
To: <tim.glover@bt.com>, <tauberer@for.net>, <semantic-web@w3.org>
Message-ID: <E1EMwGO-0004Vs-J2@maggie.w3.org>

>2. Using URLS does not guarantee uniqueness. Many people may choose to use
the same URL to mean different things. 
(I assume you mean URIs, not URLs)

I disagree, see Tim-Berners Lee: Cool URIs Don't Change. ("Not URIs change,
but humans change them").
It is just a question of proper definition, and TBL assigns the duty of
defining the concept identified by a given URI to the "inventor" of this
URI. In other words: Who introduces a URI has the right to define its
intension.

Often URIs are mixed with the file that can be retrieved if this URI is used
for retrieval, but they are not. Even if a Web resource changes very
frequently (e.g. a news page), it is still possible to define and keep
unchanged the meaning of the respective URI, e.g. read
"htpp://www.news.com" as "The current version of this news service" instead
of  "The news of October 5, 2005".

So defining the notion of identity is just part of defining the intension of
the concept.

Also, I do not share your concerns about lack of unambiguity. In fact, URIs
are a pretty good naming scheme that has scaled up to the size of the Web we
have today. Much more, since URIs can be of unlimited length, one can import
any other naming scheme space into the URI space.
 
Martin
---------------------------
martin.hepp@deri.org, phone: +43 512 507 6465
http://www.heppnetz.de / http://www.deri.org

-----Original Message-----
From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org] On
Behalf Of tim.glover@bt.com
Sent: Dienstag, 04. Oktober 2005 12:02
To: tauberer@for.net; semantic-web@w3.org
Subject: RE: New Intro to RDF




Hi,

Thanks for this. I think it is particularly useful to distinguish RDF from
its XML serialization. 

I have a couple of comments, but I am not a regular contributor to this
list, so this is a personal view - please excuse me if I am out of place.

Under the section "What is RDF" you say "RDF is nothing more than a general
method to decompose information into pieces". I think that this cuts
straight to the heart of two views of RDF.

On the one hand, RDF is commonly used and treated as a database of
information held in "triples". RDF has stimulated a lot of research into
databases built on triple stores, and for some applications these offer
significant advantages over conventional relational databases. I think the
research is very valid and interesting. I also think that this view is the
most likely to bring immediate benefits to a "Semantic Web". 

On the other hand, actually RDF IS more than this. It has a formal logic
which allows you to deduce, for example, that 

	<a foo b> implies <foo rdf:type rdf:Property>

This is pretty feeble on its own, but as you point out towards the end of
the article, RDFS introduces a wider vocabulary with a richer semantics. The
point is that the RDF semantics allow you to add extra triples that were
**not in the original data**. This is part of the distinction between a
database and an ontology, and I think this distinction is worth some extra
clarification. 



Secondly, I would like to make a comment about the use of URLs as unique
identifiers. This idea is central to RDF and is always included in a
discussion of its benefits, but this has been an area of some controversy,
and I think counter arguments deserve a mention. Some counter arguments are

1. Names only have to be unique IN CONTEXT. For example, I can write a
program using variable x without any danger of interfering with your
program, also containing variable x. And in natural language, the phrase "I
am going to try to catch the plane" has a different meaning in the context
of an airport and the context of a woodwork shop, but there is no difficulty
in using the same word for two different things because the meaning is clear
from the context. 

2. Using URLS does not guarantee uniqueness. Many people may choose to use
the same URL to mean different things. 

3. By avoiding the problem of using the same word for different things, you
multiply the problem of using different words for the same thing, and this
problem is probably more difficult to resolve. 

4. Using URLs makes many people believe that the URL is an address of some
useful information, and this is not the case. They are just names.
The URLS can be completely fictitious. More dangerously, the content of a
URL can change over time. 

5. URLS make RDF difficult for humans to read and understand. The problem is
compounded by that fact that prefixes can be used in some places but not in
others. 

In other words, URLS are not a silver bullet, and I think the advantages
should not be overstated. 

Tim Glover




-----Original Message-----
From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org]
On Behalf Of Joshua Tauberer
Sent: 03 October 2005 23:22
To: 'SWIG'
Subject: New Intro to RDF


Hi,

As probably everyone on the list knows, there's a lot of negative opinions
of RDF out there, and it seems like some of this stems from a confusion of
RDF the XML format and RDF the general method for expressing knowledge.
But, I haven't come across a deep explanation of what RDF-the-method is that
we can point people to so they know there's more to RDF than the
serialization format.

I know such a document may very well exist, but I figured I would take a

stab at writing one myself.  (If it has no value for anyone else, at least I
gained a deeper understand of RDF by writing it :-).  What I wrote is posted
at:

http://taubz.for.net/code/semweb/whatisrdf/

The goal was to introduce RDF from the beginning, show why it's useful for
modeling knowledge in a distributed way, and to give a basic presentation of
RDFS and OWL.

It's long for an introduction as I tried to be as explicit as possible about
what defines RDF (at least in my understanding of RDF).  A shorter

to-the-point version could be synthesized from this.

Comments welcome, especially if you think it was worth the time writing.
:)

--
- Joshua Tauberer

http://taubz.for.net

** Nothing Unreal Exists **
Received on Tuesday, 4 October 2005 23:32:07 UTC

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