W3C home > Mailing lists > Public > www-rdf-logic@w3.org > January 2002

Re: reification

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Thu, 24 Jan 2002 22:08:27 -0600
Message-Id: <p0510101fb876886b424d@[]>
To: "Seth Russell" <seth@robustai.net>
Cc: www-rdf-logic@w3.org
>At http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2002Jan/0257.html
>Pat Hayes said:
>The trouble with this kind of approach (not to reification in
>particular, but more generally) is that if there is no way in the
>language to state a distinction, and if the same constructs are used
>to say different kinds of things, then confusion will be inevitable.
>Maybe some people will manage to get by, but someone else will be
>confused (or, worse, some other piece of software will get confused.)
>Surely it is better to provide a way to state or somehow indicate the
>distinction [*1], and allow both kinds of things to be said more clearly.
>If you want strict backward compatibility and don't like new
>syntactic conventions, then make the one you like best be the
>unmarked case and mark the other one.
>... ]]
>Would you agree that whatever a reification node is, it is certainly is a
>blank node?
>_:s1 rdf:type rdf:Statement
>_:s1 rdf:predicate :pred
>_:s1 rdf:subject    :sub
>_:s1 rdf:object     :obj
>Doesn't the MT say that a blank node simply indicates "the existence of a
>thing".  So the Ntriples above just say:
>(exists (?x)
>      (and
>           (rdf:type ?x  rdf:Statement)
>           (rdf:predicate ?x :pred)
>           (rdf:subject ?x :subject)
>           (rdf:object ?x :obj)
>So our interpretation is any of the things which qualify according to the
>above formula that exist in our universe.

OK, that's fine. That just treats a reification as a piece of 
ordinary RDF with the standard MT reading. But that means that the 
reification "of" a piece of RDF now has absolutely no connection 
with, or particular relationship to, the piece of RDF that it is a 
reification "of". That thing that exists is in the semantic universe: 
the piece of RDF that got reified was in the syntax. Nothing 
establishes any kind of connection between them. To get such a 
connection we need to extend the MT in some way.

>  If we want to further qualify
>those things to statings in a particular document, then we could add further
>restraints to the formula like:
>(exists (?x)
>      (and
>           (rdf:type ?x  rdf:Statement)
>           (rdf:predicate ?x :pred)
>           (rdf:subject ?x :subject)
>           (rdf:object ?x :obj)
>           (dc:auther ?x :Seth)
>           (dc:document ?x "http://foo/bar.rdf" )
>My question is:  Does considering reified statements to be the blank nodes
>("existential assertions") means that we can draw whatever distinctions your
>refer to above [*1] with additional Ntriples on the same blank node
>identifier ... and that it is not necessary for the MT to further refine the
>definition ... but rather allow the user to draw those distinctions with
>additional triples?

with additional triples, no; not without assuming some special kinds 
of semantic constraint to capture the intended special meanings of 
properties like rdf:predicate. Now, we can do that, of course: but 
then we have some choices to make, was my point. There are several 
ways we can do it, not all the same and probably not mutually 
compatible. So we have to choose. Not a deep point, and the choices 
can each be made precise reasonably easily, I think; but we can't do 
them all. We can do none of them and leave it all in confusion, of 
course :-)


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Received on Thursday, 24 January 2002 23:08:20 UTC

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