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RE: semantics status of RDF(S)

From: Danny Ayers <danny@panlanka.net>
Date: Sun, 1 Apr 2001 21:25:48 +0600
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>, <www-rdf-logic@w3.org>
While I can follow enough of your arguments to give me the impression there
may well be some important flaws in the semantic model of RDF, I have
trouble trying to fully pin down the problems identified. I'd like to act as
Devil's Advocate for a moment :

The first point seems to be that some definitions are too loose be useful,
which seems a bit like I can't walk down a street because it isn't on the
map. Ok, solid formalisms should really underlie the model for
interoperability (so two interpretations will be identical), but are they
essential for the model to be useful? Isn't it likely that the assumptions
made about the gaps in the foundations by different agencies are going to be
close enough? (at least for the time being...). I'm also having severe
difficulty trying to figure out what you mean by the 'encoding' that
provides the representational power - isn't a triple a perfectly good way of
modelling a bit of semantic information? Also, why can't quantification be
represented in triples?

Personally I would appreciate one or two example cases where the model comes
unstuck (ideally in informative prose), and a suggestion as to what
formalisms & extensions are required to allow RDF(S) to transfer interesting
semantic content. Also, a clearer idea of what you mean by 'interesting
semantic content' would be most welcome.


<- There has recently been a discussion on the semantic problems with RDF.
<- I strongly agree with Pat Hayes's characterization of RDF (and RDFS) as
<- based on a fundamentally flawed semantic model.  I feel that RDF
<- (and RDFS)
<- have at least two fundamental flaws when considered as vehicles for
<- transfering interesting semantic content.  These two flaws are not just
<- little problems, but instead mean that any attempts to use RDF (or RDFS)
<- for transferring interesting semantic content are doomed to failure.
<- [One could argue what ``interesting semantic content'' means.  To satisfy
<- the assumptions of my argument here it suffices to have to provide a
<- non-RDF(S) meaning for the triples.]
<- This is a very strong statement.  I really do mean to make it.  Note,
<- however, that I am NOT saying that formalisms built on top of
<- RDF (or RDFS)
<- cannot be used to transfer semantic content.
<- The two biggest flaws that I see are:
<- 1/ There are many places in the RDF and RDFS documents where vitally
<- important statements about the semantics of RDF or RDFS are
<- presented with
<- no formal backup, and in a totally inadequate fashion.
<- For example, in Section 2.3.2 of the RDFS Specification 1.0 there are two
<- paragraphs that give conditions on rdfs:subClassOf.  As far as I
<- can tell,
<- this is the only place that attempts to provide a meaning for
<- rdfs:subClassOf.
<- However, these two paragraphs use terms that are not defined in
<- RDF or RDFS
<- (such as set - classes are not sets and thus the subset
<- relationship is not
<- meaningful on classes), are sloppy in their terminology (rfds:subClass
<- vs. sub/superset vs. subclass), and make unenforceable pronouncements
<- (there is nothing in RDF(S) to prevent someone from creating the triple
<- {rdfs:subClassOf, foo, foo}).
<- 2/ The triple model does not provide sufficient representational power to
<- transfer interesting semantic content.   Yes, I know that
<- ``anything'' can be
<- encoded in triples.  However, ``anything'' can also be encoded in bit
<- strings or Goedel numbers.  It is not the underlying transfer mechanism
<- that provides the semantic content---a sequence of bits (or triples, or
<- Goedel numbers)  does not provide any meaning in and of itself.
<- Instead, it is the encoding that provides the representational power.
<- To make this point even more clear, propositional logic does not provide
<- sufficient representational power to transfer interesting
<- semantic content.  It
<- is missing at least one vital component (quantification).  One can
<- encode quantification in propositional formulae and transfer first-order
<- formulae using this encoding, but this trick does not do anything to
<- increase the representational power of propositional logic.
<- The impact of flaw number 1 is that any attempt to use RDF or
<- RDFS without
<- providing a separate semantics for its constructs is built on an unusable
<- foundation.  The impact of flaw number 2 is that any attempt to
<- use RDF or
<- RDFS to represent proofs, etc., will have to provide its own
<- constructs for
<- most of the interesting information.
<- Peter F. Patel-Schneider
<- Bell Labs Research
Received on Sunday, 1 April 2001 11:29:01 UTC

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