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Re: weird example in RDF-MT document...

From: Graham Klyne <GK@ninebynine.org>
Date: Mon, 05 Feb 2007 10:18:51 +0000
Message-ID: <45C7048B.4010405@ninebynine.org>
To: axel@polleres.net
CC: www-rdf-logic@w3.org

Axel Polleres wrote:
> 
> Hi all,
> 
> I was studying the RDF semantics document once again in some detail and
> it looked to me I found a bug in an example in the end of section 3.3.1.
> Since I doubt that, I was asking myself whether somebody here can help
> me to get the knot out of my head....
> 
> 
>    http://www.w3.org/TR/rdf-mt/#Reif
> 
> 
> 
> In the end of that section, it is stated that
> 
> "For example,
> 
> _:xxx rdf:type rdf:Statement .
> _:xxx rdf:subject <ex:subject> .
> _:xxx rdf:predicate <ex:predicate> .
> _:xxx rdf:object <ex:object> .

> _:yyy rdf:type rdf:Statement .
> _:yyy rdf:subject <ex:subject> .
> _:yyy rdf:predicate <ex:predicate> .
> _:yyy rdf:object <ex:object> .

> _:xxx <ex:property> <ex:foo> .
> 
> does not entail
> 
> _:yyy <ex:property> <ex:foo> ."
> 
> 
>  This is at the very least strange for me...  and I think simply wrong.

(From my distant recollection of working group discussion on this point...)

This example was added to the spec to make a specific point about the nature of
the "reification" vocabulary in RDF -- it is possible to define the semantics to
work either way and reasonable people may disagree about whether this is the
right choice.

In this case, the nodes _:xxx and _:yyy are effectively described as referring
to specific instances of statements -- it is possible for two logically
equivalent statements to be made under different circumstances.

The motivation for this particular choice was that the more compelling
reification use cases surveyed were related to provenance applications (who said
what, and when), which required the (weaker) semantics exemplified above.
(consider that <ex:property> and <ex:foo> correspond roughly to "said by" and
"John", and that one might also want to describe a logically equivalent
statement that was "said by" "Jack", maybe made at a different time (i.e. the
example offered by Alex).

There were examples raised requiring a stronger form of semantics (i.e. that the
statement given IS entailed), but they were felt to be more marginal.  But
mainly, for the sake of consistency, the working group had to make a choice, and
that was it.  The wider problem of solving semantics for all these use cases was
out of scope for the working group charter (which was to clarify RDF as
originally designed).  Personally, I think there are more fruitful ways than
"reification" to address these problems, such as named graphs.

#g

-- 
Graham Klyne
For email:
http://www.ninebynine.org/#Contact
Received on Monday, 5 February 2007 15:44:41 UTC

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