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RE: RDF semantics: applications, formalism and education

From: Danny Ayers <danny@panlanka.net>
Date: Mon, 9 Apr 2001 23:22:43 +0600
To: "Drew McDermott" <drew.mcdermott@yale.edu>, <www-rdf-logic@w3.org>
Message-ID: <EBEPLGMHCDOJJJPCFHEFCEDODBAA.danny@panlanka.net>
Thanks for this - your description is very helpful.

IANAL, and have been arguing from a point of view of considerable ignorance
of the actual problem (can you have a large quantity of something missing?),
but I worry when a lawyer/logician produces a bill - I prefer to see a
pretty clear cut case. Having said that, I really do hope it won't be long
before the community reaches for its chequebook, one way or another.

I think I'd better do some more reading before I dig myself too big a
hole...

---
Danny Ayers
http://www.isacat.net

<- -----Original Message-----
<- From: www-rdf-logic-request@w3.org
<- [mailto:www-rdf-logic-request@w3.org]On Behalf Of Drew McDermott
<- Sent: 09 April 2001 21:14
<- To: www-rdf-logic@w3.org
<- Subject: Re: RDF semantics: applications, formalism and education
<-
<-
<-
<-    [Danny Ayers]
<-    > The alternatives
<-    > then are 1. to kill RDF and/or 2. create an altogether new
<-    > framework and/or 3. extend RDF to make it do what we want.
<-
<-    [Peter Crowther]
<-    I'd add (4) AMend RDFS to make it do what we want.
<-
<-    > 3. would seem to be the least bad/most likely option - add
<-    > functionality in
<-    > the form of external schemas, if need be gaffer (duct) tape in
<-    > characteristics from different domains (calculi/algebras
<-    > whatever - any volunteers for the Chinese Room?)
<-
<-    These can only be added if the core is sufficiently
<- expressive and flexible
<-    to allow them to be added.  Pat, Peter and others seem to be
<- pointing out
<-    that:  ....
<-
<- Let me put it this way.  RDF in its original form is a very simple
<- language, which does not contain negation, disjunction,
<- quantification, modality, etc.  It is good that there is a simple core
<- language, because all those features come at a fairly high price.  The
<- more complex the language the more difficult it is to manipulate it.
<- (The difficulty increases very rapidly.)  The question is, How do we
<- get the more complex features when we need them?  There are two basic
<- answers:
<-
<- 1 View the original RDF, call it RDF_0, as the minimal subset of a
<- more complex language.  Set up the subsets in some rational way, so
<- that it's fairly easy for users to know which subset they need.  Adopt
<- a system of labels so that a given RDF description can be clearly
<- flagged as requiring subset I.
<-
<- 2 Introduce quotation into the language, and use RDF_0 purely as a
<- vehicle to describe more complex languages.
<-
<- Perhaps I'm biased, but option 1 seems so obviously to be preferable
<- that no argument is required.  In fact, I know of no argument in favor
<- of option 2, only an argument against option 1, which goes as follows:
<-
<- i. Every RDF expression is equivalent to a conjunction of statements,
<- that is, "triples," of the form p(a,b), where p is a relation and a
<- and b are atomic names (URIs, e.g.); furthermore, if e2 occurs as a
<- subexpression of e1, the triples of e2 are a subset of the triples of
<- e1.
<-
<- ii. Therefore, if you assert an RDF expression you must assert every
<- element of this conjunction, and hence assert all of its
<- subexpressions.
<-
<- iii. Therefore, it is impossible to add negation or disjunction to the
<- language, because p is a subexpression of (not p), so asserting (not
<- p) requires asserting p.
<-
<- iv. Therefore, there are no nontrivial supersets of RDF_0
<-
<- In my opinion, this should be viewed as a reductio ad absurdum of
<- premise (i).  However, instead it is mostly viewed by the RDF
<- community as a knockdown argument against extending RDF, and
<- therefore, by default I suppose, in favor of using quotation to
<- describe some super-RDF in terms of RDF_0.
<-
<- But Ayers and Crowther, and others, are beginning to grant that
<- "extending" and "amending" RDF might be a good idea.  In that case, we
<- need to amend the way triples figure into the language.
<-
<- I think the original motivation for the triples model was to allow
<- one to follow pointers all over the web without worrying what they
<- were pointing into.  That is, if a document points to a triple in
<- another document, I could go there and figure out what that one triple
<- asserts without having to figure out what its context is.  (Its
<- context might not even be well defined if different people made use of
<- it in different ways from different places.)
<-
<- Perhaps others could shed some light on whether this is indeed the
<- underlying motivation for the triples model.  However, I think one can
<- have pointers all over the place without the triples model.  What we
<- should have instead is a well defined notion of "expression."
<-
<- I'm visualizing something like this:
<-
<- I set up a web page where I stoutly deny that Francis Bacon wrote
<- "Hamlet."
<-
<- (not <a id="foo">(wrote Bacon Hamlet)</a>)
<-
<- [usual disclaimer about how ridiculously oversimplified this is]
<-
<- I've thoughtfully provided an anchor point "foo" that others can use
<- to point to this assertion.
<-
<- [another disclaimer about how improbable this is as actual XML/RDF]
<-
<- Now someone else disagrees with me, perhaps by listing all the plays
<- Francis Bacon wrote:
<-
<- (and (wrote Bacon Coriolanus)
<-      <a ref="foo"/>
<-      (wrote Bacon HeddaGabler))
<-
<- where he has pointed to the claim about Hamlet rather than copying it.
<-
<- I don't see any problem with this, provided we have a clear definition
<- of what counts as a subexpression of formula foo.  In the present
<- simple-minded example, it has no nontrivial subexpressions, but in
<- more realistic examples it would be some large entity with pointers
<- off to other places.  That's okay, provided we can in principle
<- follow all those pointers, retrieve the subexpressions they refer to,
<- and build a coherent expression that is what the person must be
<- claiming if he includes a pointer to foo in one of his formulas.
<-
<- Of course we can run into problems, where someone writes something like
<-
<-   <a id="ying">(not <a ref="yang">)</a>
<-   <a id="yang">(not <a ref="ying">)</a>
<-
<- If we try to resolve this into a coherent expression, we get
<-
<-    (not (not (not (not .....))))
<-
<- But the fact that some apparent pointers can't be resolved
<- doesn't kill the idea of pointing to subexpressions.  We just
<- stipulate that there are no cycles in the pointer graph.
<-
<- Note that we can still have triples if we want; we just do away with
<- the idea that an expression is *equivalent* to the triples from all
<- its subexpressions.
<-
<-                                              -- Drew McDermott
<-
Received on Monday, 9 April 2001 13:26:56 GMT

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