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Re: log:forSome/#rdfms-identity-anon-resources

From: Dan Connolly <connolly@w3.org>
Date: Thu, 28 Jun 2001 16:38:28 -0500
Message-ID: <3B3BA3D4.D7D70899@w3.org>
To: fmanola@mitre.org
CC: pat hayes <phayes@ai.uwf.edu>, w3c-rdfcore-wg@w3.org
Frank Manola wrote:
> 
> Dan Connolly wrote:
> >
> snip
> > A prolog query
> >         author(X, "b"), title(X, "f e")
> > is a request to prove the theorem
> >
> >         (exists (?x) (author ?x "b") (title ?x "f e))
> > right? and the query results are proofs; i.e. substitutions
> > of the variables that show that it's satisfyable.
> >
> > That's how queries work, right? They're existential formulas,
> > and the job of a query engine is to give proofs, by
> > example, of the theorem. This is completely traditional, no?
> >
> > [[[
> > Free variables in a query
> > are assumed to be existentially quantified.
> > ]]]
> >
> > --        Knowledge Interchange Format
> > http://logic.stanford.edu/kif/dpans.html
> > Thu, 25 Jun 1998 22:31:37 GMT
> 
> Sorry if this appears to be barging in out of the blue,

On the contrary; the pointer is there just so that
folks like yourself can follow it and share the
background I have and join the discussion.

This is the second time somebody has apologized
for responding to a message that I sent to this
group. Hmm... I hope everybody
feels welcome to give their two cents on any issue...


> but the KIF spec
> you cited says:
> 
> * Note that the significance of free variables in quantifier-free
> sentences depends on context.
> * Free variables in an assertion are assumed to be universally
> quantified. Free variables in a
> * query are assumed to be existentially quantified. In other words, the
> meaning of free variables
> * is determined by the way in which KIF is used. It cannot be
> unambiguously defined within KIF itself.
> * To be certain of the usage in all contexts, use explicit quantifiers
> 
> Based on this, it seems to me that we'd need to say more than that we're
> interpreting anonymous resources as free variables.

I have said more: I've said I'm interpreting them as
existentially quantified. I my research, I have
found that this is fairly traditional; from 1996:

[[[
2.2 Existential Quantifiers

[...] the meaning of 

 
<citation>
<title>On the pulse of morning</title>
</citation>

is not, in general, merely "The title is On the pulse of morning" but
something more
like "(There is an object, described by this record, and) the title (of
the object
described by this record) is On the pulse of morning." That is, there is
an implied
existential quantifier inherent in the existence of a metadata record,
and there is an
implied argument for each metadata element, viewed as a logical
function.
]]]

--        On Information Factoring in Dublin Metadata Records
http://tigger.uic.edu/~cmsmcq/tech/metadata.factoring.html
Thu, 18 Apr 1996 00:56:35 GMT

This is also the way it works in "conceptual graphs",
these days advocated by John Sowa by also written
about by Pierce as far back as 1909.

see

[[[
4. A person is between a rock and a hard place.
[...]

Following is the KIF representation: 

(exists ((?x person) (?y rock) (?z place) (?w hard))
        (and (betw ?y ?z ?x) (attr ?z ?w)))
]]]

--        Conceptual Graph Examples
http://www.bestweb.net/~sowa/cg/cgexampw.htm#Ex_4
Thu, 22 Mar 2001 01:45:12 GMT
linked from http://www.bestweb.net/~sowa/cg/
linked from http://www.w3.org/DesignIssues/CG#[2]

>  We'd also need to
> say whether we're expressing an assertion or a query.  So far, I thought
> we'd been assuming that RDF was making assertions.

Yes... the issue (#rdfms-identity-anon-resources) is
whether these assertions are all ground (i.e. use
only logical constants) or whether, in RDF, one
can assert the existence of something without naming it.

>  On the other hand,
> it might be that RDF is in the same position as KIF is here, and we'd
> need to take context into consideration.

No, RDF isn't as expressive as KIF. RDF doesn't have
free variables nor universal quantification.
And as folks have pointed out, there are some formulas
like
	(exists (?x) (title ?x ?x))
that can't be expressed in RDF 1.0 syntax.

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Thursday, 28 June 2001 17:38:32 EDT

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