Re: rdf inclusion

Pat Hayes wrote:
> 
> >Jeff Heflin wrote:
> >>  ... Since I was the initial proponent of daml:imports on the Joint
> >>  Committee, let me address this issue. You are absolutely correct that
> >>  the imports statement must be used. Simply refering to a namespace does
> >>  not include any ontology definitions. You must make the imports
> >>  statement explicit. Period. ...
> >
> >So, what does it mean if one refers to a term in an ontology
> >without importing it? Should this be considered an error?
> 
> I would say not.
> 
> >  If
> >so, is there a reasonable recovery technique, like ignoring
> >triples using externally defined terms not imported?
> 
> In RDF, the official line (as I understand it) is that any published
> RDF which uses a term is (asserted by the publisher of the RDF
> document to be) a true assertion, and anyone who draws any RDF
> conclusion from any combination of such RDF is entitled to conclude
> that the conclusion is true also. That is, publication is assertion,
> and all assertion is unqualified. There is no way to explicitly agree
> or disagree with another piece of RDF.
>
> >  > ... The problem with using RDF namespaces to decide which schemas are
> >>  relevant is that multiple files may contain different definitions about
> >>  the same URI.
> 
> Indeed, but RDF is designed to work in a more perfect world where
> people do not disagree about definitions. Or at any rate, it
> delegates responsibility for dealing with such conflicts to some
> other mechanism.

This is exactly why RDF as-is is not appropriate for the Semantic Web.
It really wasn't designed for the problems of a distributed,
heterogeneous information environment. There is no way we can get the
entire world to agree on the definitions, so our architecture must
accomodate differences of opinions.

The RDF specs never discuss how one combines information from multiple
sources. It is implied that you simply merge all of the graphs. This
isn't too bad when you can't express a contradiction, but once you
include additional semantic primitives (as DAML+OIL does) and scale for
use in distributed environments you are bound to get contradictions if
you simply merge the information sources. Some have suggested that in
such situations that you just select one of the contradictory axioms and
throw it out. However, if different systems choose different things to
throw out, then they no longer agree on the semantics of terms used. The
only way around this problem would be to have a consistent, world-wide,
determinisitc method for resolving these contradictions. Even if it was
possible to design such a thing, it would be highly impractical because
people often cannot agree on things (some of the worst cases of this
lead to wars, etc.) We can never expect the Semantic Web to be a single
consistent knowledge base!

Jeff

Received on Wednesday, 8 May 2002 14:34:25 UTC