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

From: Dan Connolly <connolly@w3.org>
Subject: Re: MISC: Internet Media Type registration: proposed TAG finding
Date: 24 May 2002 16:13:32 -0500

> On Fri, 2002-05-24 at 12:56, Peter F. Patel-Schneider wrote:

[...]

> That seems counter to common sense. That seems like saying
> an IOU document
> 
> 	"I owe you one dollar"
> 
> doesn't mean that I could pay you with 4 quarters, since
> the relationship between a dollar and quarters isn't
> specified in the IOU.

Absolutely, unless there was some agreed-upon RDF document to the effect
that 4 quarters was equivalent in monetary value to one dollar.

> The author of the best-friend document, by choosing to use
> ont:UniqueProperty class, licensed inferences
> based on the specification of that class. The conclusion
> that "35" is an :age of :margaret is supported
> by the DAML+OIL spec.

Yes, but not by the RDF spec, and any agent has no business labelling
anything as RDF inferences that are not sanctioned by the RDF model theory.

> In some ways, this example is quite crisp, but there
> seems to be some ambiguity in the discussion... I'm
> not exactly sure what you mean by 'RDF agent', 
> and the term 'entails' is used ambiguously: your
> agent seems to claim that anything beyond
> simple entailment
>   http://www.w3.org/TR/2002/WD-rdf-mt-20020429/#entail
> is dangerous and should not be trusted.

Anything that is labelled as an RDF entailment has to follow from the RDF
model theory, yes.  Any other way of doing  things is sure to lead to
disaster.  Any communication with an agent that is only known to understand
RDF should only use RDF entailment.  

> I can think of two agents (cwm and Euler) that
> do a lot more than simple entailment, when
> asked to. I think of them as RDF agents.

They are not.  If they label their entailments as RDF entailment then they
are being deceitful, moreover.

[...]

> > > 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.
> 
> Hmm... it seems by 'RDF agent' you mean some sort of diagnostic
> testing tool for RDF documents, not any sort of interesting
> agent.

No, I mean an agent that makes RDF conclusions from RDF documents.

> Clearly, by that definition, my tool that concludes "35"
> is an age of :margaret isn't an RDF agent.

Correct, it is not.

> >  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.
> 
> er... 'RDF conclusion'... another new term. I suppose
> you mean the sort of conclusion supported (only)
> by simple entailment.
> 
> But yes, the conclusion that "35" is an age of :margaret
> isn't supported by simple entialment alone, and
> justification of that conclusion needs to be clear
> about including the DAML+OIL spec.

How can this be done in RDF?  It cannot.

> > 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).
> 
> Yes, justifications have to cite their sources.

Yes, and if the reasoning steps are not RDF reasoning steps, then they
should not be so labelled.

> 
> > > > 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>
> 
> I'm not sure how to parse that. If I presume that fol:negation
> is an RDF typednode, and convert to (something close to) KIF, I get:
> 
>   (exists (?n)
>     (rdf:type ?n fol:negation)
>     (fol:statement ?n #John)
>     (rdf:type #John rdf:Person)
>     (loves #John #Mary)
>   )
> 
> so... ?n is a negation, and it's somehow related to John.
> I think you meant to express something like
>   (not (loves John Mary))

That is indeed what I meant. I expressed it in RDF syntax, which according
to you, should be perfectly fine.  I used my extension to RDF reasoning,
which, according to you, should also be perfectly fine.

> but that's really awkward to do in RDF, but I suppose
> it could look like:
> 
>   <fol:Negation>
>     <fol:predicateSymbol>http:...#loves</fol:predicateSymbol>
>     <fol:subjectSymbol>http:...#John</fol:predicateSymbol>
>     <fol:objectSymbol>http:...#loves</fol:objectSymbol>
>   </fol:Negation>
> 
> where fol:Negation is specified ala:
> 
>   (forall (?n ?s ?p ?o)
>     (<=>
>       (and
>         (predicateSymbol ?n ?p)
>         (subjectSymbol ?n ?s)
>         (objectSymbol ?n ?o)
>         (wtr
>            '(not (holds `(str->sym ?p) `(str->sym ?s) `(str->sym ?o))) )
>       )
>       (rdf:type ?n fol:Negation)
>   )
> 
> But even in that case, I *only* eliminate models when I
> learn the meaning of fol:Negation (i.e. when I add that axiom).

Yes, and this is no longer RDF.

> I can only constrain the meaning by learning more.
> 
> > > > 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?
> 
> Hmm... good question. Maybe it's been implicit all along.
> Maybe it belongs in the RDF model theory spec, in
> a discussion of... umm... 'extensions' to RDF,
> as you call them.
> 
> I suppose I'll send a comment to www-rdf-comments.

Go ahead, but where in the RDF specification is this *now* specified?  If
it is not there, then you have no business assuming it.

[...]

> > 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.
> 
> I remain hopeful those problems can be solved.

Well, I don't think that the paradoxes that result are going to go away any
time soon.

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

peter

Received on Friday, 24 May 2002 20:09:31 UTC