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

On Thu, 2002-05-23 at 19:14, Peter F. Patel-Schneider wrote:
> [Moved from WebOnt to RDF Logic per the direction of the WebOnt chair.]
[oops; quite...]

> From: Dan Connolly <connolly@w3.org>
> Subject: Re: MISC: Internet Media Type registration: proposed TAG finding
> Date: 23 May 2002 17:31:20 -0500
[...]
> > But that's not all there is to it. dc:title is a term
> > with widely deployed semantics/meaning/definition/specification.
> > The dublin core folks have some reasonably clear notion
> > of which interpretations are consistent with its
> > intended use and which are not.
> 
> I fail to see how the intended meaning of dc:title can be considered to be
> part of RDF, except for that portion of the intended meaning that is
> captured by RDF triples that are available on the web and are readily
> retrieved from a well-known place. To make it otherwise would turn RDF
> into an undefined thing.  

Hmm... incompletely defined, yes, of course.
i.e. tomorrow I expect to find out more about how
folks are using the framework; i.e. to see how
they've further elaborated it.


> For example, I could claim that pfps:Truth is a part of RDF, whose intended
> meaning is the type of all true propositions in first-order logic.  Does
> mean that RDF captures first-order truth?  Not at all!

I'm not sure what you mean by 'captures', but yes, your pfps schema/spec
would provide a term that folks can use to claim, in RDF syntax,
that things are first-order truths.

It wouldn't be unprecedented, by the way:

  http://www.w3.org/2000/10/swap/log#Truth

> > These
> > informal prose documents, e.g. the dublin core spec,
> > still have semantics: they still divide interpretations
> > into true and false.
> 
> Sure, but this informal part is *not* part of RDF.

I accept that as your opinion. I disagree.
Perhaps if you'd share the argument that leads
you to that conclusion, we could discuss it further.

> > I don't think that notion of "meaning of a document"
> > is specified very well, but I think
> > it's what most RDF authors/implementors have
> > in mind.
> 
> If so, then they are seriously deluding themselves that they are doing
> RDF.  They are instead doing RDF + what I mean, or RWIM.

Again, I'm not sure what you mean by 'doing RDF', but it's
plain to me, from the usage of RDF that I can see (RSS,
PRISM, dublin core, XMP, ...) that
the framework includes this sort of thing.

> > Now likewise, the DAML+OIL spec divides interpretations
> > between those that are consistent with it and those
> > that are not.
> 
> Well, aside from the fact that DAML+OIL interpretations are not RDF
> interpretations, sure.

I consider that a bug in the DAML+OIL spec; I hope we fix
it in OWL.


> > If I say
> > 	:age rdf:type ont:UniqueProperty.
> > 	:bob :age "10".
> > 	:bob :age "20".
> > 
> > and I investigate its meaning in the Resource Description
> > Framework, I start with the conjunction of the three
> > facts there, and then I'll look up rdf:type; its
> > spec tells me the extension of rdf:type is computed
> > from the class extension of its object; so I go
> > and look up ont:UniqueProperty, and I discover
> > that its class extension is properties whose
> > subjects determine their objects uniquely.
> 
> Who discovers this, and how?  

As explained above, I (i.e. anybody using
the framework) use the deployed URI infrastructure
to dereference http://www.daml.org/2001/03/daml+oil#UniqueProperty
and I see:

<rdfs:Class rdf:ID="UniqueProperty">
  <rdfs:label>UniqueProperty</rdfs:label>
  <rdfs:comment>
    compare with maxCardinality=1; e.g. integer successor:
    if P is a UniqueProperty, then if P(x, y) and P(x, z) then y=z.
    cf OIL FunctionalProperty.
  </rdfs:comment>
  <rdfs:subClassOf
rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/>
</rdfs:Class>

The comment there is reasonably clear as a constraint
on interpretations, no? I didn't know much about
model theory when I wrote it, but it seems to work OK.

> > But the RDF MT tells me that "10" and "20"
> > denote distinct things, so there isn't any
> > way to satisfy the combination of this
> > document, the rdf:type spec, and the
> > ont:UnambiguousProperty spec, no matter
> > what specification for :age I might find.
> > i.e. this document, combined with the
> > specifications for the terms it uses,
> > is false.
> 
> Sure, but what you have done is *not* RDF, it is RDF plus DAML+OIL.

I disagree that it's not RDF.

That seems like saying that XHTML documents are not XML documents...
or that TCP packets aren't IP packets. They clearly are.

> > That's how DAML+OIL fits into the Resource Description Framework,
> > and how I hope/expect OIL will too.

[oops... s/OIL/OWL/]

> That is, DAML+OIL does not fit into the Resource Description Framework,
> instead it extends the Resource Description Framework (or, more accurately,
> DAML+OIL could extend the Resource Description Framework if the natural
> extension didn't give rise to paradoxes).

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.

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?


> > An RDFS tool can handle a document that uses WebOnt terms
> > much more gracefully than a version 2 word processor
> > usually handles version 3 documemtns: halt and catch
> > fire totally.
> 
> Sure, an RDFS tool can handle a DAML+OIL document - by treating it as an
> RDFS document and completely ignoring the DAML+OIL part.

No, not completely ignoring the DAML+OIL part: it can
still tell that daml:Class is a subclass of rdfs:Class
and come to lots of relevant/licensed conclusions.

> > e.g. given the age 10/20 example above, an RDFS-capapble
> > tool might not detect the inconsistency by reducing
> > the possible interpretations to none, but it can
> > tell that in all satisfying interpretations,
> > :age has rdf:type rdf:Property; it can derive
> > conclusions by erasure and existential introduction,
> > by subPropertyOf and subClassOf rules, etc.
> > 
> > That's partial understanding.
> 
> Well, maybe, but such partial understanding is prone to misunderstandings.
> (Think of propositional negation.)

What about it?

>  I don't see how it could be more gain
> than loss, in general.

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

> 
> > [...]
> > > >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...

> 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.

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

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.

Er... well, you could, but that would say that documents
that use your vocabulary are always unsatisfyable/false.
So that case is still monotonic.

In general, vocabulary specs start with property extensions,
and rdf:type delegates to class extensions.

>  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.


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

Received on Thursday, 23 May 2002 23:08:19 UTC