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Re: Trust, Context, Justification and Quintuples

From: Jeremy Carroll <jjc@hpl.hp.com>
Date: Fri, 19 Dec 2003 22:06:36 +0100
To: www-rdf-interest@w3.org
Message-Id: <200312192206.36607.jjc@hpl.hp.com>

On Fri, Dec 19, 2003 at 01:31:14PM +0100, Jeremy Carroll wrote:
> > {reification] does not work - see RDF Semantics and its 
   discussion of reification
> > - it does not quote the triples, which is what is needed.

Dmitry
> Can you elaborate or provide a reference on what you mean by "quote" and
> why/how is it needed here? I feel like I'm missing some background: I've
> read RDF Semantics (that's my favorite RDF spec, in fact), but I still
> don't see your point.

RDF Concepts is much better :)

OK we look at RDF Semantics 3.3.1
http://www.w3.org/TR/rdf-mt/#Reif

The key text is:
[[
Notice that the value of the rdf:subject property is not the subject URI 
reference itself but its interpretation, and so this condition involves a 
two-stage interpretation process: one has to interpret the reified node - the 
subject of the triples in the reification - to refer to another triple, then 
treat that triple as RDF syntax and apply the interpretation mapping again to 
get to the referent of its subject. This requires triple tokens to exist as 
first-class entities in the universe IR of an interpretation. In sum: the 
meaning of the reification is that a document exists containing a triple 
token which means whatever the first graph means.Note that this way of 
understanding the reification vocabulary does not interpret reification as a 
form of quotation. Rather, the reification describes the relationship between 
a token of a triple and the resources that triple refers to. The reification 
can be read intuitively as saying "'this piece of RDF talks about these 
things" rather than "this piece of RDF has this form".
]]

and

[[
Since an assertion of a reification of a triple does not implicitly assert the 
triple itself, this means that there are no entailment relationships which 
hold between a triple and a reification of it. Thus the reification 
vocabulary has no effective semantic constraints on it, other than those that 
apply to an rdf-interpretation. 

A reification of a triple does not entail the triple, and is not entailed by 
it. (The reification only says that the triple token exists and what it is 
about, not that it is true. The second non-entailment is a consequence of the 
fact that asserting a triple does not automatically assert that any triple 
tokens exist in the universe being described by the triple. For example, the 
triple might be part of an ontology describing animals, which could be 
satisfied by an interpretation in which the universe contained only animals, 
and in which a reification of it was therefore false.)

Since the relation between triples and reifications of triples in any RDF 
graph or graphs need not be one-to-one, asserting a property about some 
entity described by a reification need not entail that the same property 
holds of another such entity, even if it has the same components. 
]]

basically Pat is listing all the things you think might be true and pointing 
out that they do not necessarily hold.

The clearest examples are ones in which we have owl:sameAs to say that two 
urirefs have the same denotation.

Here we go ..

eg:a owl:sameAs eg:b .
eg:r rdf:type rdf:Statement
eg:r rdf:subject eg:a .
eg:r rdf:predicate owl:sameAs .
eg:r rdf:object eg:b .

SinceI(ag:a) = I(eg:b) we have that this graph means exactly the same as the 
following graph

eg:a owl:sameAs eg:b .
eg:r rdf:type rdf:Statement
eg:r rdf:subject eg:a .
eg:r rdf:predicate owl:sameAs .
eg:r rdf:object eg:a .

i.e. if eg:a owl:sameAs eg:b is true then its reification is also the 
reification of eg:a owl:sameAs eg:a (and eg:b owl:sameAs eg:a, and eg:b 
owl:sameAs eg: b). 

For trust mechanism, provenance etc. this really is not what is wanted - a 
true quoting mechanism leaves the urirefs unchanged and does not interpret 
them. (Although I think it is OK to rename bnodes - not thought that one 
through properly yet) 

The classic example is "I saw Superman fly" and "I saw Clark Kent fly" are two 
different sentences even if we know that Superman = Clark Kent. RDF Semantics 
does not allow you to capture this using reification.

Thus any quad mechanism, or the named graph stuff, associates the quad ID or 
the graph name, not with the interpretation of the s,p,o but with the actual 
nodes s, p, o - quoting them, rather than interpreting them.

Jeremy
Received on Friday, 19 December 2003 16:08:43 GMT

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