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Re: what is the meaning of the RDF model theory?

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 04 Oct 2001 14:43:28 -0400
To: seth@robustai.net
Cc: phayes@ai.uwf.edu, www-rdf-interest@w3.org, www-rdf-logic@w3.org, peters@earlham.edu
Message-Id: <20011004144328Q.pfps@research.bell-labs.com>
I'm going to step into this fray a bit. ...

From: "Seth Russell" <seth@robustai.net>
Subject: what is the meaning of the RDF model theory? 
Date: Thu, 4 Oct 2001 08:19:55 -0700

> From: "Pat Hayes" <phayes@ai.uwf.edu>
>
> > You are living in a paranoid fantasy world. There are no Priests of
> > logic. There is however a working group charged with defining a
> > formal language called RDF, and the model theory (soon to be
> > improved, BTW) is what that body says RDF means. Of course you are
> > free to treat RDF files in any way you like; but just as a practical
> > matter, if you plan to use RDF to communicate with others, it might
> > be wise to adopt the same conventions that everyone else is adopting,
> > if you intend to be understood by them. Or, you can use some other
> > language; nobody is calling you to prayer here. The MT just sets the
> > RDF standards into public view to help people avoid mutual confusion.
>
> Yeah sure ... I'm well aware of the nature of standards and the consequences
> of deviating from them.    Sorry if my questions have been asked with such
> bumbling confusion.   I'm just trying to understand, not only the technical
> issues of the RDF Model Theory [1], but also how it will be used, what it
> means to me as a developer, and how that  meaning will flow to users
> cruising the Semantic Web [7].     So let me go out and come in again ....
> 
> It seems to me, and you seem to have confirmed this, that this document will
> tell us, in no uncertain terms,  what circumstances must obtain for a RDF
> graph to be true or false.  Is that a fair summary ?    ... and may I
> proceed as if it were ...

No it is not and no you may not.  

Model theory does not tell one whether a syntactic structure (such as an
RDF graph) is true or false.  It does however, tell one which states of
affairs (interpretations) are consistent with a syntactic structure.  From
this you get a notion of entailment, which can more-or-less be read as

	Given that this (RDF graph) is true, is that (RDF graph) also true?

From this you can determine some things related to validity and
satisfiability, which can more-or-less be read as

	Can this (RDF graph) ever, or never, be true?

But there are only a very few RDF graphs that are valid, and all RDF graphs
are satisfiable, so this is certainly not your notion of truth or falsity.

[...]

> Now let me ask a more practical question which, hopefully, will help me
> focus my understanding of your document.   Before the Model Theory [1], we
> had RDF graphs and detailed specifications of how to form and communicate
> them [2 - 6].  Can you provide an actual example of a RDF graph that adheres
> to those specifications, yet is invalid according to the Model Theory ?
> 
> [1 RDF Model Theory] http://www.w3.org/TR/2001/WD-rdf-mt-20010925/
> [2 RDF M&S] http://www.w3.org/TR/REC-rdf-syntax/
> [3 RDF Schema Specification ]
> http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
> [4 RDF revisions draft] http://www.w3.org/TR/rdf-syntax-grammar/
> [5 RDF W3C homepage ]http://www.w3.org/RDF/
> [6 DAML ] http://www.daml.org/2001/03/daml+oil-index.html
> [7 Semantic Web] http://semanticweb.org/

I would put it to you that [2-3] do NOT form a detailed specification of
how to form and communicate with RDF graphs.  The whole thrust of model
theory, on the other hand, is precisely to provide detailed specifications
of how to communicate with syntactic structures.  

> Seth Russell
> Logic is great!  Survival is better :)

Peter F. Patel-Schneider
Received on Thursday, 4 October 2001 14:43:58 GMT

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