Forgive me if I'm misusing some terms, I'm just becoming familiar with RDF and reification in particular.
What I'm trying to understand is if/how you can make a statement about a statement that you don't control and which isn't actually set up as an rdf:Statement (or any other resource, i.e., reified).
For instance, if some semantic website makes the claim:
ex:elvis-presley
ex:is-alive "true"^^xsd:boolean .
There is an implicit rdf:Statement resource here:
_:x
a rdf:Statement ;
rdf:subject ex:elvis-presley ;
rdf:predicate ex:is-alive ;
rdf:object ex:true "true"^^xsd:boolean .
Now suppose I have my own semantic website and I would like to refute this statement, or affirm it, or make any other kind of meta-statement about this statement. The statement resource doesn't have a global identifier, so I can't reference it.
Is there any way to handle this, or can you only make statements about statements that are explicitly formed as identified resources in their own right?
I think that reification is a topic that initially seems more useful than it actually tends to be in practice. You can have a triple in a graph:
s p o .
and you can have four triples in a graph:
x a rdf:Statement .
x rdf:subject s .
x rdf:preficate p .
x rdf:object o .
but that's about it. If someone happens to have four triples of the second form, and x happens to be a URI, then then you write triples about x. If it's a blank node, then you don't have a way of referencing it. In either case, x is said to be a reification of the triple s p o. That means that the question
Can you only reify statements that are explicitly formed as identified resources in their own right?
doesn't make a whole lot of sense. The reificiation of a statement s p o is a resource x that has the associated properties. "To reify s p o" doesn't really mean anything except "pick an x, and assert the corresponding triples about it."
It's very unlikely that anyone trying to assert s p o would write the second form. The second form tends to arise if you're trying to represent some statements about triples, e.g., "john says x . x a rdf:Statement . …".
If you want to decry someone's claim that Elvis lives, you'd probably just do
:elvisLives a rdf:Statement ;
rdf:subject ex:elvis-presley ;
rdf:predicate ex:is-alive ;
rdf:object true ;
:claimedBy <http://example.org/whoeverSaidIt> ;
:hasValue false .
Alternatively, if you're in the realm of OWL, you can use a negative property assertion:
NegativeDataPropertyAssertion( ex:lives ex:elvis-presley "true"^^xsd:boolean )
The RDF representation would look like
_:x rdf:type owl:NegativePropertyAssertion .
_:x owl:sourceIndividual ex:elvis-presley .
_:x owl:assertionProperty ex:lives .
_:x owl:targetValue true .
You can see a similarity between the two approaches. OWL includes a sort of reification vocabulary with its owl:sourceIndividual, owl:assertionProperty and owl:targetValue.