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Re: Logic and Using The Semantic Web Toolbox

From: Dan Connolly <connolly@w3.org>
Date: Wed, 13 Dec 2000 09:11:48 -0600
To: jos.deroo.jd@belgium.agfa.com
Cc: " - *Ora.Lassila@nokia.com" <Ora.Lassila@nokia.com>, " - *www-rdf-logic@w3.org" <www-rdf-logic@w3.org>
Message-id: <3A3791B4.AC9BB6AD@w3.org>
jos.deroo.jd@belgium.agfa.com wrote:
> 
> Ora wrote in a reply to Pat:
> > > Ah, I discern a ray of hope. Am I correct, then, in thinking
> > > that by asserting a reified description of a triple (that is,
> > > the four triples which describe it as being a triple plus
> > > giving its three components), one is NOT thereby also
> > > asserting the triple itself? That would greatly simplify the
> > > semantics of RDF, certainly.
> > Exactly! Asserting the reification is independent from asserting
> > the triple itself. And only those statements we have asserted
> > we consider as "facts".
> 
> Ok, maybe we are completely wrong (please tell us), but we see a
> distinction between asserting
>   <rdf:Statement>
>     <rdf:subject>#cernDoc</rdf:subject>
>     <rdf:predicate>#includes</rdf:predicate>
>     <rdf:object>thisDocument</rdf:object>
>   </rdf:Statement>
> 
> and asserting
>   <rdf:Description>
>     <rdf:type resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
>     <rdf:subject resource="#cernDoc"/>
>     <rdf:predicate resource="#includes"/>
>     <rdf:object>thisDocument</rdf:object>
>   </rdf:Description>
>
> In the first ("assembly" RDF) representation the single statement
>   [#cernDoc] #include [thisDocument]
> is asserted and becomes a fact

Huh? No, it represents the 4 statements:
	[#genid] rdf:type [rdf:Statement]
	[#genid] rdf:subject "#cernDoc"
	[#genid] rdf:predicate "#includes"
	[#genid] rdf:object "thisDocument"

The only difference between the two is that
"#cernDoc" and "#includes" are strings in the
first case, and symbols (URI references) in
the second case.

> 
> In the second (reified RDF) representation the 4 statements
>   [#genid] rdf:type [rdf:Statement]
>   [#genid] rdf:subject [#cernDoc]
>   [#genid] rdf:predicate [#includes]
>   [#genid] rdf:object [thisDocument]
> are asserted as 4 facts and indeed nothing is asserted about
> the triple itself!

Nope, they both give 4 statements-about-a-statement.

> 
> I know that the first representation is very confusing and
> lengthy and not human readable and so on, but it is (I think)
> valid RDF and it is a way to have "compound" subjects and objects
> such as in http://www.agfa.com/w3c/euler/lists.axiom.rdf
> without the need for reification.
> Only the "outer" statements are asserted (as facts, Horn clauses,
> etc) so a "nested" statement is not asserted at all.
> 
> --
> Jos De Roo, AGFA

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Wednesday, 13 December 2000 10:26:32 GMT

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