Chimezie Ogbuji wrote:
In SPARQL, the combined use of FILTER/!/BOUND effectively gives you a
mechanism for matching records with non-monotonic mechanisms without an
entailment regime. This is how we are able to *explicitly* ask for the
absence of an assertion based only on what the RDF dataset has in
persistence.
A SPARQL endpoint is different from a simple RDF assertion because a
SPARQL endpoint answers within the scope of its knowledge base. The
context is closed by itself. But the semantics of an RDF statement is
always global and open. The issue at hand is not if a CWA can or cannot
be applied to a given set of RDF statements, it is about if two agents
will give consistent answer given the same set of assertions.
Given a KB "_:someone pha:medicine _:aspirin", you agent can sure
interpret that it implies that "_:someone pha:medicin _:viagra" is
false. But you should not assume that other agent will have the same
interpretation.
Think in open world will make the ontology design differently from
thinking in closed world.
I think a proper definition of scoped negation as failure would help
show how SPARQL can be used to match the absence of an assertion against
an RDF dataset that can also be subject to open world assumption s at
the same time:
[[
Related to the notion of scoped inference is an extension of the concept
of default negation, called scoped default negation. The idea is that
the default negation inference rule must also be performed within the
scope of an explicitly specified knowledge base. That is, not q is said
to be true with respect to a knowledge base K if q is not derivable from
K.
]] -- "A Realistic Architecture for the Semantic Web" [1]
But please also see how dangerous such practice will be: "Ian Horrocks1,
Bijan Parsia, Peter Patel-Schneider, and James Hendler, Semantic Web
Architecture: Stack or Two Towers"
at "http://www.cs.man.ac.uk/~horrocks/Publications/download/2005/HPPH05.pdf";
Xiaoshu
