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Re: Can RDF say anything about anything?

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Fri, 31 Jan 2003 09:45:59 -0500 (EST)
Message-Id: <20030131.094559.07321967.pfps@research.bell-labs.com>
To: bwm@hplb.hpl.hp.com
Cc: www-rdf-comments@w3.org

From: Brian McBride <bwm@hplb.hpl.hp.com>
Subject: Re: Can RDF say anything about anything?
Date: Fri, 31 Jan 2003 06:51:29 +0000

> At 09:48 30/01/2003 -0500, Peter F. Patel-Schneider wrote:
> 
> >Can RDF say anything about anything?
> >
> >The RDF documents are contradictory on this point.  The Primer indicates
> >that RDF can be used to let anyone ``say anything they want about existing
> >resources'' with no exception for the resources used by RDF.  [Section
> > 3.2] Concepts says
> >that ``RDF is an open-world framework that allows anyone to make simple
> >assertions about anything''.  [Section 2.2.6, and elsewhere]
> > However, Concepts also says that ``Certain
> >URIs are reserved for use by RDF, and may not be used for any purpose not
> >sanctioned the RDF specifications.'' [Section 3.7]
> >
> >What is the situation here?
> 
> Peter,
> 
> As this comment affects several documents, I'll respond.
> 
> As a general point, it is helpful if you can provide links to the sections 
> of documents where you have a problem with the text, or at the least 
> section numbers.

Isn't that what the Search/Find capabilities of browsers are for?  I would
expect that an interested reader would want to know where else Concepts
talks about being able to say anything about anything.  I've added section
numbers to my comment above.

> Specifically about this point, I don't yet see the contradiction to which 
> you refer.  Can you offer a test case to illustrate this contradiction.

Well, if I believed the ``say anything about anything'' wording, I should
really be able to say anything about, for example, rdf:ID, perhaps even
saying that it is a class with an instance, as in 

	...
	<rdf:ID rdf:about="#IDinstance">
		<rdfs:comment>An instance of rdf:ID.</rdfs:comment>
	</rdf:ID>
	...

A better example, perhaps, would be using RDF to critique RDF, as in

	...
	<rdfs:Class rdf:about="#broken">
	  <rdfs:comment>The class of broken features in RDF.</rdfs:comment>
	</rdfs:Class>
	<broken rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID">
	  <rdfs:comment>rdf:ID is broken because it can only occur once in
			       an rdf document.</rdfs:comment>
	</broken>
	<broken rdf:about="http://www.w3.org/2000/01/rdf-schema#comment">
	  <rdfs:comment>rdfs:comment is broken because it brings in social
				     meaning.</rdfs:comment>
	</broken>
	...


A third example, would be to use one of the RDF URIs as a property, as in:

	...
	<rdf:Property rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID">
	  <http://www.w3.org/1999/02/22-rdf-syntax-ns#ID
	    rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID"/>
	</rdf:Property>
	...

Are these three examples legal in RDF?  The first two quotes I presented
above would indicate so.  

The first example, however, is illegal according to RDF Syntax.  

The second example is, I think, *legal* but this goes counter to the third
quote I presented above.

The third example is illegal according to RDF Syntax, and I don't think
that there is any way of generating this triple in RDF/XML.

> Brian

It would be interesting to determine just which sets of triples whose first
element is a URI reference or a blank node; whose second element is a URI
reference; and whose third element is a URI reference, a literal, or a
blank node cannot be generated from RDF/XML.  (I have deliberately not
called this set the set of RDF graphs.)  I have already mentioned that I
believe that there are some that cannot be so generated.

peter

PS:  On a related note, is

	rdfs:Class rdfs:Class rdfs:Class .

     allowed? 

     Section 3.7 of Concepts would indicate not, as it uses rdfs:Class for
     a ``purpose not sanctioned [by] the RDF specification''.  This
     normative caution needs *much* more care in its formulation.
Received on Friday, 31 January 2003 09:47:35 GMT

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