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Re: what is RDF for? (was: RDFCore WG 2001-06-01 Teleconference Agenda)

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Tue, 12 Jun 2001 14:03:03 +0100
Message-Id: <5.0.2.1.2.20010612133949.03b1b530@joy.songbird.com>
To: fmanola@mitre.org
Cc: Brian McBride <bwm@hplb.hpl.hp.com>, rdf core <w3c-rdfcore-wg@w3.org>
Frank,

I don't think there's any conflict between what I said and what you are 
saying, apart from lack of clarity on my part.

When I say "attach meaningful semantics", I mean to do this in a formal 
sense, and also to do this in a way that doesn't block the extension of RDF 
in the ways that you reference.  The semantics I would wish to capture are 
roughly what you describe;  one purpose of a formalism would be to be more 
precise about exactly what those may be.

The meaning of some particular attribute+value would, I think, be a matter 
for some particular (model theoretic) interpretation;  I think this mirrors 
your statements like: "RDF was intended to *reference* the semantics of 
predicates like "author"".  But simply saying that a predicate has some 
defined meaning, of itself, doesn't get us very far.  We also need to 
understand what manipulations that can be performed while preserving the 
truth of any statements made using such predicates.

So when I say "useful", I mean that I would hope for some framework that 
provides for some manipulation of expressions that preserves truth.  For 
example, if we are given the truth of two RDF expressions:
     { (s1 p1 o1) }
and
     { (s2 p2 o2) }
we can prove the truth of the expression:
     { (s1 p1 o1) (s2 p2 o2) }

...

I generally agree with your comments about nailing down our assumptions 
about what RDF is *for*.  In talking about "meaningful semantics", it is 
emphatically not my intention to create some kind of extension of the 
expressive capability of (core) RDF -- just to be clear about what that is, 
and to provide a starting point for weaving it into a more expressive 
framework.

...

Why is this topic of possible relevance to this group?  It has been 
asserted and contested that "reification" the the hook by means of which 
additional facilities can be "added to" core RDF.  Some formality may help 
to resolve this unsettling debate.

#g
--

At 01:12 PM 6/6/01 -0400, Frank Manola wrote:
>Graham Klyne wrote:
> >
> > At 09:48 AM 6/7/01 +0100, Brian McBride wrote:
> > >Graham Klyne wrote:
> > >[...]
> > > > More recent discussion on the RDF-logic list has focused on the
> > > > relationship between "reification" and syntax, and the nature of RDF
> > > > "syntax".  A viewpoint that emerges from this discussion is that 
> the RDF
> > > > structure of triples is too simple to attach any meaningful
> > > > semantics;
> > >
> > >Graham, do you really mean 'any meaningful semantics', or do you mean
> > >that it is not syntactically rich enough to support, for example 
> expressions
> > >with quantifiers.  It seem to me rich enough to support the assertion of
> > >'ground facts' with the semantics defined in the definition of the
> > >semantics of the URI's involved.
> >
> > (That was an unfortunate turn of phrase I used ;-)
> >
> > But, yes, for example, the syntax (defined simply in terms of triples) is
> > not rich enough to support expressions with quantifiers.  One can use the
> > triples to encode some richer syntax (e.g. as one uses character sequences
> > to encode programming language constructs) that is capable of supporting
> > this, but that would be a different syntax hence a different language.
> >
> > So, to define a useful level of semantics, there must be a sufficiently
> > rich syntactic structure.
>
>While the above sentence is undoubtedly true, it leaves unaddressed the
>question of what constitutes "a useful level of semantics" in the
>present context.  We also have a general problem of our unstated
>assumptions in a lot of these discussions.  What *I* believe Brian is
>talking about (correct me if I'm wrong) when he says
>
> > >It seem to me rich enough to support the assertion of
> > >'ground facts' with the semantics defined in the definition of the
> > >semantics of the URI's involved.
>
>is roughly this:
>
>1.  Let's assume that the intended purpose of RDF is to provide a
>facility for specifying collections of attribute-value pairs that
>describe Web resources (example:  collections of Dublin Core attributes
>about documents).  Under this assumption, there are some mechanics that
>are really involved in using RDF that aren't currently very explicit,
>like:
>
>a.  there is some mechanism that separates the collection of AV pairs
>that one person created to describe a resource from a collection that
>someone else has created (just as there's an assumption that a
>collection of FOL expressions written by one person to describe one
>circumstance is separable from a collection of FOL expressions written
>by a second person to describe another circumstance).  This is
>presumably some form of context (or at least, scoping) mechanism.
>
>b.  within one of those collections, the AV pairs are implicitly anded;
>as I've mentioned before, I think RDF is intended to be a simple
>existential conjunctive logic;  simple because only binary predicates
>are allowed;  existential because there are also effectively "there
>exists" quantifiers to quantify the resources involved in the
>description (effectively because you can consider the URIs as being
>Skolem constants that remove the need for these quantifiers.
>
>2.  these AV pairs constitute the "ground facts" about certain resources
>using predicates from some domain (like publishing, in the case of
>Dublin Core);  so the fact that Ora Lassila authored some Web page is
>one of these "ground facts"
>
>3.  the "semantics" in question are the semantics of predicates like
>"author" (or the corresponding URI).
>
>4.  RDF was intended to *reference* the semantics of predicates like
>"author" (by using them in AV pairs), but not *define* them;  that
>definition was left for other languages (like DAML+OIL, for example).
>
>5.  Specifying such "ground facts" in RDF, using the semantics of
>predicates in AV pairs that are defined elsewhere and "elsehow", is
>nevertheless "useful" (although perhaps not "a useful level of
>semantics");  just as being able to specify the data in tables in a
>relational database is useful, even though you can't specify the
>semantics of the tables, rows, or columns in that way.
>
>Now, a lot of the debate on rdf-logic has revolved around the fact (I
>repeat, *fact*) that you cannot express negation, universal
>quantification, and other things in RDF triples, and expect them to
>"work" as if these were native parts of the EC logic represented by the
>above interpretation of RDF.  You can certainly encode a lot of
>different things in triples, write triples that (for example) have NOT
>as a predicate (and define engines that process those predicates), and
>use those triples as some kind of exchange notation.  However, it seems
>to me this moves us towards a different assumption about what RDF is for
>than that stated above.  That doesn't disqualify that idea (although if
>exchange of logical expressions were the motivation, I'm not sure I'd
>immediately think of triples), but it is an idea that needs explicit
>consideration as being separate from the idea of RDF as being for the
>simple description of resources using AV pairs.
>
>It seems to me there needs to be some nailing down of our assumptions
>about what RDF is for, and correspondingly of our assumptions about the
>so far implicit mechanisms I've suggested above (contexts/scoping,
>anding of statements, etc.), and how (or whether) they are supported by
>RDF or RDF processors, based on RDF's intended use.  This isn't intended
>to be "philosophical" (to raise a politically-incorrect word here) as
>much as to provide guidance for evaluating many of the issues that have
>been raised regarding reification and other facilities, and for
>clarifying the description of RDF in the specs.
>
>--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
>
>
>
>This footnote confirms that this email message has been swept by
>MIMEsweeper for the presence of computer viruses.

------------------------------------------------------------
Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
                                 <http://www.baltimore.com>
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Received on Tuesday, 12 June 2001 10:25:41 EDT

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