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

From: Frank Manola <fmanola@mitre.org>
Date: Fri, 29 Jun 2001 11:37:05 -0400
Message-ID: <3B3CA0A1.EFBC2E46@mitre.org>
To: Dan Connolly <connolly@w3.org>
CC: pat hayes <phayes@ai.uwf.edu>, w3c-rdfcore-wg@w3.org
Dan--

Comments embedded below.

Dan Connolly wrote:
> 
> 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]

Yes, that was the interpretation I'd been assuming as well (see my
message
http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0326.html)

> 
> >  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.

I was assuming the latter.

> 
> >  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.
> 

I agree with everything you've said above, except I have a tiny quibble
with the initial "no". Part of the problem, it seems to me, is sometimes
we seem to be talking about what literally is or is not in RDF, and
sometimes we seem to be talking about what is in a presumed translation
of RDF into logic, based on an interpretation of RDF as making
assertions (I discuss this in my earlier message too;  I think it's the
same interpretation you described earlier).  The "quibble" has to do
with the fact the extract from the KIF spec says that the interpretation
of the KIF syntax (free variables in this case) depends on whether it's
being used in a context that uses the syntax as an assertion, or in a
context that uses the syntax as a query.  What was confusing me about
some of the interaction you were having (and the reason I cited this bit
of the KIF spec) was the introduction of queries and querying into the
discussion.  As I noted in my earlier message, the initial M&S example
of anonymous resources:

"The individual whose name is Ora Lassila, email <lassila@w3.org>, is
the creator of http://www.w3.org/Home/Lassila." 

hasn't the semantics of a query.  It's an assertion that a given
individual exists (that I don't happen to have a name for).  It's not an
invitation for a processor to go off looking for a matching individual. 
If you look at the equivalent graph in the M&S, there isn't any
"variable" to represent that "individual";  there's just a resource
without an identifier.   So I don't understand how queries come into
this.

--Frank

-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752
Received on Friday, 29 June 2001 11:37:41 EDT

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