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Re: Difference between dc:subject and foaf:topic

From: Dan Brickley <danbri@w3.org>
Date: Fri, 12 Mar 2004 11:47:45 -0500
To: Charles McCathieNevile <charles@w3.org>
Cc: Libby Miller <Libby.Miller@bristol.ac.uk>, "Miles, AJ (Alistair) " <A.J.Miles@rl.ac.uk>, "'public-esw-thes@w3.org'" <public-esw-thes@w3.org>
Message-ID: <20040312164745.GC12366@homer.w3.org>

* Charles McCathieNevile <charles@w3.org> [2004-03-12 11:34-0500]
> It isn't clear from that explanation what the difference is, since unless you
> talk about the domain and range of foaf:topic it's hard to understand how
> there is any.

dc:subject relates a document to a code that stands for some thing the
doc is about.

foaf:topic relates a document to some thing that the doc is about.

...the difference is w.r.t. layers of indirection: where dc:subject uses 
an external taxonomy, set out in advance, foaf:topic relies on any chunk
of RDF that is handy for describing the thing. It's an interesting 
stylistic and representational difference that seems really quite 
unfortunately hard to explain clearly...
> The fact that foaf:topic has a defined domain of foaf:Document and a range of
> rdf:Resource means that it's a bit more restricted than dc:subject which can
> happily live with a literal value, but there doesn't seem to be much else
> that can be used to pick between them (on my reading of the two sets of
> specifications).

They differ importantly, just as you differ from your homepage, and XML 
differs from library subject codes for XML. With dc:subject there is a
whole other world squeezed into the representation: the world of
subject/topic codes. With foaf:topic, the RDF itself does that work,
removing subject/topic codes from the list of things the RDF needs to
describe. See above re this being hard to describe :(

> If I was trying to define the way I see the web I would write somewhere that
> foaf:topic seems to me like a subProperty of dc:subject. 

that would imply that any pair of things related by foaf:topic are also 
related by dc:subject. This isn't so, since the values taken by
dc:subject are various forms of subject code, whereas the values taken
by foaf:topic are the things that those subject codes denote. So
declaring a subPropertyOf relation would confuse those two levels.

>	Being a believer in
> living language, and in language as an attempt to provide identifiers for
> concepts in such a way thhat we are convinced we mean the same thing, this
> doesn't strike me as a bad thing to do. But since we are inventing RDF
> vocabulary creation, it is something I would treat with care - especially
> while we don't yet have good methods for dealing with conflicting statements,
> including conflicting statements about how vocaublaries are defined.

We defintely need to to allow for vocabulary evolution, it might be 
interesting to compare the two approaches...

Received on Friday, 12 March 2004 11:47:45 UTC

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