Re: MISC: Internet Media Type registration: proposed TAG finding

WARNING:  My conclusions below depend on the assumption that RDF agents do
not need to understand the implications of natural language prose as the
objects of rdfs:comment statements.  If assumption this is not true, and,
given the recent statements on W3C mailing lists, it may indeed not be
true, then, of course, many of the conclusions about RDF agents below are
completely wrong.  


From: Dan Connolly <connolly@w3.org>
Subject: Re: MISC: Internet Media Type registration: proposed TAG finding
Date: 23 May 2002 22:08:16 -0500

> On Thu, 2002-05-23 at 19:14, Peter F. Patel-Schneider wrote:
> 
> > From: Dan Connolly <connolly@w3.org>
> > Date: 23 May 2002 17:31:20 -0500

[...]

[ example document, modified
	:best-friend rdf:type ont:UniqueProperty.
	:bob :best-friend :margaret .
 	:bob :best-friend :peggy .
	:peggy :age "35" .
]

> > Sure, but what you have done is *not* RDF, it is RDF plus DAML+OIL.
> 
> I disagree that it's not RDF.

Well, then suppose that our RDF agents have a disagreement on the meaning
of this RDF document.  My RDF agent says that the above document is a
perfectly fine RDF document, and that, according to that document, 
:bob has a :best-friend that is :margaret
     and a :best-friend that is :peggy 
and that the :age of :peggy is the two-character english string "35".
Your RDF agent says that the above is a fine RDF document but that it also
entails that the :age of :margaret is the two-character english string "35". 
At this point my RDF agent publishes on the web that your RDF agent makes
unsupported conclusions and should not be trusted.

I claim that my RDF agent is completely justified in publishing a claim
that your RDF agent is broken in a dangerous fashion and that no one should
trust anything it says.


[...]


> In what way does it not fit into the framework? I don't see
> the distinction you're trying to make. Are you talking
> about daml:collection syntax? Yes, that was (an attempt
> at) an extension to the framework, I suppose. But otherwise,
> DAML+OIL ontologies seem to use the framework without
> changing it; they seem to fit right in.

I'm not talking about daml:collection.  I'm talking about meaning, not
syntax.  The meaning that comes from the DAML+OIL model theory, or the
meaning that comes from the commonly-understood meaning of natural language
utterances that are objects of RDF triples, is not part of the RDF
meaning of these triples.

> Let's put it this way: does dublin core fit into the framework?
> Or is RDF+dc an extension? How about a document that
> uses RDFS, DAML+OIL, and dublin core together? Is that
> another sort of extension?

Any part of Dublin Core that has is part of an RDF(S) document can be
considered to be part of RDF(S).  However, if Dublin Core has formal
portions that are not captured by an RDF(S) document, such as functionality
conditions on some of its properties, then Dublin Core is a strict
extension of RDF.  As DAML+OIL is an extension of RDF(S), so DAML+OIL +
Dublin Core is, of necessity, an extension of RDF(S).


[...]


> Er... so you expect every agent to completely understand every document?
> Surely that's a non-starter, no?

No.  I expect an RDF agent to completely understand the RDF meaning of
every RDF document.  At the very least, I expect an RDF agent to not
understand anything else.  Any agent that makes conclusions that are not
sanctioned by the RDF model theory is doing more than RDF, and had better
be very sure not to label its conclusions as RDF conclusions.

Think of the example where two agents are trying to determine whether one
owes the other some money.  If one agent presents the other with a
derivation labelled as a valid RDF (or first-order logic) entailment, then
it had better be a valid RDF (or first-order logic) entailment, not
something that only comes from some extra-RDF semantic conditions (or
conditions outside of first-order logic).  


> > > [...]
> > > > >At the instance data level, all this shouldn't matter. (Thankfully, for
> > > > >the poor end users...)
> > > > 
> > > > It has to matter. If someone marks up their webpage using WebOnt, 
> > > > then an RDF engine isn't going to be able to understand it, right? 
> > > 
> > > It will understand it partially.
> > 
> > Sure, but the non-understood part may completely change the meaning of the
> > other part,
> 
> No, I don't think so...

Why not?  What about 

	<fol:negation>
	  <fol:statement>
	    <rdf:Person rdf:about="#John">
	      <loves rdf:resource="#Mary">
	    </rdf:Person>
	  </fol:statement>
	</fol:negation>

> > so the partial understanding may not be related to the ``real''
> > meaning in any worthwhile fashion.
> 
> it's monotonic, in that the more you understand, the fewer
> interpretations are models. You can't rule out models by
> failing to understand something.

Sure, if you require a monotonic relationship, then the relationship is
monotonic.  However, where does the requirement for a monotonic
relationship come in?

> Can you think of an example where the non-understood part
> changes the meaning of the other part?

Sure.  See above.

> Maybe this monotonicity is a constraint on vocabulary
> specifications that I didn't mention earlier.
> You can't, for example, specify that the semantics
> of my:magicThing are such that
> 	my:magicThing dc:title "abc".
> means that "abc" is *not* a title.

Well, of course, if you require the relationship to be a monotonic
extension, then you get monotonic relationships.  However, as discussed
extensively, requiring the relationship to be monotonic leads to severe
problems.

[...]

> >  If that is what you want then go right
> > ahead, but don't expect me to send any business to a system that acts in
> > this fashion.
> > 
> > > > This isn't rocket science: all of computation is like this.
> > > 
> > > No, some computation degrades gracefully. Try looking
> > > a the W3C home page, written in XHTML 1.0 circa 1999, with
> > > a web browser written in 1994. I think you'll find
> > > a remarkable degree of fidelity.
> > 
> > Sure, sometimes you win.  However, you can also lose.
> 
> Well, partial understanding in the semantic web isn't
> just a matter of syntactic sloppiness... it's constrained
> to be monotonic. So I don't see how you can lose.

Again, how is the relationship constrained to be monotonic?  Who is
mandating this?  Why can't I just do the fol:negation thing above?


> -- 
> Dan Connolly, W3C http://www.w3.org/People/Connolly/
> 


Peter F. Patel-Schneider

Received on Friday, 24 May 2002 13:57:01 UTC