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Re: What do the ontologists want

From: Frank Manola <fmanola@mitre.org>
Date: Mon, 21 May 2001 13:08:56 -0400
Message-ID: <3B094BA8.D853BD48@mitre.org>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
CC: www-rdf-logic@w3.org
Peter--

It seems to me when you say "makes it essentially impossible to use the
semantics for the positive ground triples to represent domain
information" you're going a bit beyond what Pat said.  It seems to me
that Pat was pointing out that all RDF allowed you to do in the example
was define a collection of relationships between some instances
("positive ground triples");  it didn't give those relationships any
special semantics, either logical (to support inferencing) or domain
semantics.  If I say "author(a,b)", RDF doesn't define the semantics of
what an "author" is either (we want DAML+OIL to do things like that). 
However, I presumably can use a collection of "author(X,Y)" triples to
represent useful domain information (given that my processor knows how
to interpret "author" properly, of course).  This is what databases do
all the time, and what the simplest examples of RDF illustrate.  

Now suppose I introduce some new relationships like "premis" and
"conclusion" (and possibly some other encoding tricks to get everything
into triples).  It seems to me it's certainly possible to mix triples
involving "premis" and "conclusion" (and any other relationships needed
for the encoding) with triples involving "author", "subject", and other
domain relationships, and not mess up whatever domain semantics I get
out of the domain relationships, provided I can properly separate the
stuff that is to be interpreted as part of the "more-expressive"
formalism from the stuff that is to be the ground facts.  
Or, to translate this into database terms, suppose I take my relational
database and store my ground triples representing authors in it.  Now
suppose I add some additional tables to the database to support an
encoding of logical rules, to be operated on by some inference engine. 
The same author information (domain semantics) can be extracted from the
database as before those additional tables were added, so I haven't lost
the ability to represent domain information (any ability you already
had, that is).  But obviously you need to make sure that you keep the
use of the domain tables appropriately separated from the tables
representing the "more expressive formalism" if you're going to make any
sense of what you have.  In other words, RDF doesn't restrict the
relationships you define to any one level of abstraction (whether one of
those levels is called "encoding" or not), but it doesn't provide any
magic means of defining (much of) the semantics of those relationships
(no matter what level they are), or sorting those levels out either.  

--Frank


"Peter F. Patel-Schneider" wrote:
> 
> In defense of stripped-down RDF, there is nothing technically wrong with a
> logical formalism that can represent only positive ground triples.  Such a
> formalism can certainly convey some useful semantic information.
> 
> It is just that such a representation formalism cannot be used to
> *represent* anything more than positive ground triples.  Using positive
> ground triples to encode a more-expressive formalism requires encoding, which
> requires a new semantics, defined on top of the semantics for the positive
> ground triples, and makes it essentially impossible to use the semantics
> for the positive ground triples to represent domain information.
> 
> Peter F. Patel-Schneider
> Bell Labs Research
> 
> From: Graham Klyne <GK@ninebynine.org>
> Subject: Re: What do the ontologists want
> Date: Mon, 21 May 2001 13:25:04 +0100
> 
> > I think this example+explanation is one of the clearer expositions of what
> > RDF can and cannot do.  I suspect that many proponents of RDF recognize
> > this without being very clear about how to articulate it.
> >
> > I don't think any of the long-term advocates of RDF expect it to be able to
> > express all meanings (or even any universal truths?) using only semantics
> > defined for core RDF.
> >
> > I think the interesting question is:  can the RDF core framework alone
> > convey *any* useful semantics, or is it no more than an abstract syntax
> > over which semantics must be defined?
> >
> > #g
> >
> >
> > At 08:54 PM 5/18/01 -0500, pat hayes wrote:
> > >>For an example, let me introduce a propositional logic and provide a
> > >>rule R which says that given triple <a,b,c> anyone may infer triple
> > >><d,e,f>.  This logic is not very expressive; it does not even allow
> > >>conjunction in the premise:
> > >>
> > >>    <R, premise, RP>
> > >>    <RP, subject, a>
> > >>    <RP, predicate, b>
> > >>    <RP, object, c>
> > >>    <R, conclusion, RC>
> > >>    <RC, subject, d>
> > >>    <RC, predicate, e>
> > >>    <RC, object, f>
> > >>
> > >>Each of these triples is true itself, while also building a structure
> > >>for us.
> > >
> > >How does this convey the meaning that you indicate, ie that <d,e,f> can be
> > >inferred from <a,b,c> ? It simply says that some things exist called 'R',
> > >'RP' and 'RC', which stand in some undefined relationship to a, b, c, and
> > >so on. The RDF data model provides no further meaning, and the model
> > >theory for RDF provides no further meaning. So no inferences are sanctioned.
> > >
> > >If you want this kind of structure to actually mean somethingmore than
> > >this - in particular, if you want it to have the force of an implication,
> > >as indicated - then you need to state truth-conditions which support that
> > >larger meaning. But those truth-conditions will have to refer not more
> > >than  the RDF syntax; they depend on the particular relation symbols you
> > >have used: in this case, 'premis' and 'conclusion'.  (You will also need
> > >to relate <a,b,c> to the three triples with 'RP' in the subject, but I
> > >presume that this wil be done by reification, so I won't dwell on it.) In
> > >other words, you have now given those symbols a *logical* meaning: they
> > >have become part of the logical syntax. This isn't RDF any more: it is
> > >something else, implemented in RDF.
> >
> >
> > ------------
> > Graham Klyne
> > GK@NineByNine.org
> >

-- 
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 Monday, 21 May 2001 13:09:56 UTC

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