W3C home > Mailing lists > Public > www-rdf-logic@w3.org > May 2001

Re: metadata vs. meta-(meta)-data

From: Dan Connolly <connolly@w3.org>
Date: Tue, 22 May 2001 11:46:27 -0500
Message-ID: <3B0A97E3.D2695EE2@w3.org>
To: Larry Masinter <LMM@acm.org>
CC: www-rdf-logic@w3.org
Larry Masinter wrote:
> I'm hoping that RDF can be modified in a way that
> distinguishes between the use of RDF to make assertions
> about actual web resources (data content) and its use
> to make assertions about things that are described or
> referenced by things on the net.
> The use of "http" URIs as unique identifiers for things
> that are not resources backed by HTTP web servers is
> a serious confusion of levels.

Er... it's a straightfoward contradiction, no?
http://mycollege.edu/students/Amy is definitely
an HTTP thingy; if something tries to say that
it's not, that's a contradiction between that
something and the (axioms taken from the) HTTP/URI specs.

> The model for the sentence
>  The students in course 6.001 are Amy, Tim, John, Mary, and Sue.
>  is written in RDF/XML as
> <rdf:RDF>
>   <rdf:Description about="http://mycollege.edu/courses/6.001">
>     <s:students>
>       <rdf:Bag>
>         <rdf:li resource="http://mycollege.edu/students/Amy"/>
>         <rdf:li resource="http://mycollege.edu/students/Tim"/>
>         <rdf:li resource="http://mycollege.edu/students/John"/>
>         <rdf:li resource="http://mycollege.edu/students/Mary"/>
>         <rdf:li resource="http://mycollege.edu/students/Sue"/>
>       </rdf:Bag>
>     </s:students>
>   </rdf:Description>
> </rdf:RDF>
> This example shows the confusion -- the description is not
> about the web page at http://mycollege.edu/courses/6.001, but
> it's about the course which is described by the web page at
> http://mycollege.edu/courses/6.001.

It's less than ideal, yes, but I can see several ways to
make sense of it:

(a) in the schema for s:students, it could say

	This property relates a web page about a class
	with a collection of web pages about students in the class.

(b) if the schema syas

	this property relates a class with a collection
	of students in the class

Then the above RDF blurb says that the class and the
web page are the same thing. In the absense of
axioms/constraints to the contrary, that makes
logical sense, no?

i.e. there's no logical problem with saying that
the integer 2 and the pencil I'm holding in
my hand are identical, are there?

I'd recommend against it as a modelling practice,
but I don't see a problem with RDF per se.

> However, we may also want
> to make assertions about the web page: when it was written,
> who wrote it, when it was last updated, where it is archived,
> etc.

What's to prevent them from doing so?

The only issue is that if they take approach (b) where
they equate the HTTP resource with the class, they can't
say that X wrote the class and Y wrote the page
without also saying that X wrote the page and
Y wrote the class.

> The web page and the thing the web page is about are at
> different levels of quoting, but these levels are confounded
> in RDF today.

Hmm... to say that a whole document denotes something is
interesting; I wouldn't have looked at that as quoting
so much as a normal relationship, sorta like dublin
core Subject.

I dont' see anythingabout RDF that forces folks
to confuse levels. (There's nothing that
prevents them either, as demonstrated above.)

> Adding a "#" at the end of the Description's about attribute's
> URI doesn't really help, since it resolves into the structural
> granuarity of the web page rather than into the next level of
> reference.

I don't think that http://www.w3.org/People/Connolly/#dan
necessarily denotes a structural part of a document.
What's wrong with saying that it denotes me?

i.e. http://example/#FRAG denotes whatever
#FRAG denotes in messages received in replies
to GET requests to http://example/.

But maybe this is a different sort of reference
than the one used in <a href="...">. Hmm...

> I can think of several ways of fixing this, but they're all somewhat
> unpleasant:
> - leave RDF as is, add another URI scheme that means
>   'the thing described by this URI'
>   <rdf:description about="ttdb:http://mycollege.edu/courses/6.001">
>   (ttdb - the thing described by )

That doesn't appeal to me at all. I'd rather use a

	<rdf:Description rdf:about="http://mycollege.edu/courses/6.001">

(I've wondered if dc:subject should be used this way;
i.e. does the Dublin Core community expect dc:subject
to be used to relate a photo to a person that
appears in a photo? or does it always relate
the photo to subject keywords?)

> - leave RDF as is, assume ttdb, and add a level of quoting if
>   you want to use RDF for metadata, e.g.,
>   <rdf:description about="data:text/uri,http://mycollege.edu/courses/6.001">

Again, why make an expensive new URI scheme when you
can express it with a cheap new RDF property?

> - Define that each relationship should be specific about its level
>   of indirection, e.g., "dublin core RDF relations are about the
>   URI as a web resource, but other kinds of RDF assertions might
>   really about the thing described by the web resource rather than
>   the web resource itself".

I wouln't call it "level of indirection," but yes,
the specification of each RDF property needs
to be clear about what its domain and range is,
ala (a) above.

Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Tuesday, 22 May 2001 12:46:32 UTC

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