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Re: I have a trouble with The RDF Model

From: Stefan Decker <stefan@db.stanford.edu>
Date: Sun, 26 Nov 2000 14:07:14 -0800
Message-Id: <4.3.2.7.2.20001126123207.042c1688@db.stanford.edu>
To: pat hayes <phayes@ai.uwf.edu>
Cc: RDF-Logic <www-rdf-logic@w3.org>
Hi,

At 02:15 PM 11/26/2000 -0600, pat hayes wrote:
>Stefan Decker, replying to Seth Russel, wrote:
>
>>a triple is unique - nobody can distinguish
>>between the triples [Bush, wonThe, Election]
>>and [Bush, wonThe, Election] per se.
>
>That is correct if 'triple' means an abstract mathematical object. However 
>if these 'triples' are syntactic entities (kinds of expression) then one 
>most certainly can distinguish two distinct such entities which have 
>identical structure. This is called the "type-token distinction" in 
>linguistics: its the commonplace observation that one can say or write the 
>same 'thing' twice (two tokens with the same type).

how are they distinguished?


>>However, as you have already observed, the source of
>>the triple might be relevant for believing a fact or not believing a fact.
>>A model theory (assigning true or false)
>
>That isnt quite what is meant by a model theory. (It would be a very 
>trivial model theory.)

Nevertheless - a mapping from a kind of expression (a mathematical object) 
to some interpretation.

>>therefore has to include the source of the triple.
>
>No, the model theory simply assigns interpretations to the syntactic 
>constructions; it does not control the syntax. If the 'at' construction is 
>part of the syntax then the model theory should assign it a meaning, and 
>if it is not part of the syntax then it should ignore it. So, is 'at' part 
>of RDF syntax or not?

I never claimed that the model theory should control the syntax. I just 
said that the
domain of the interpretation mapping has to include the source of the 
triples into account.

>BTW, a model theory should assign a meaning (referent) to every 
>well-formed expression of the language, so even if the language contains 
>expressions of the form
>([Bush, wonThe, Election] at RobustAI)
>the question still arises of what exactly the subexpression
>[Bush, wonThe, Election]
>denotes, since it is not a truthvalue (it would probably be something like 
>a function from the things denoted by the expressions after the "at" to 
>truthvalues, ie a predicate on those things, whatever they are.)
>>That means it should act on syntactic constructs like
>>([Bush, wonThe, Election] at RobustAI)
>>and
>>([Bush, wonThe, Election] at Electoral College).
>>These constructs are different even if the triples are identical.
>>
>>This violates neither the uniqueness of triples nor
>>the Law of the Excluded Middle.
>
>Yes, but you seem to now have changed the language into something else, 
>and the original question (and this mailing list) is supposed to be about 
>RDF. If someone asks whether there is an English word for "Schadenfreude", 
>it isn't much use to tell him that there is one in German.
It was always clear, that RDF-interpretations and applications have to look 
at the source
of RDF triples for judging the quality and truth value of statements - thus 
this belongs to RDF.

The open question is how to represent the source in RDF.
The result of an earlier discussion was to represent it by reification.
(See: http://www-db.stanford.edu/~stefan/updates.html for links to the 
earlier discussion.).
The language has not changed  -  I used  ([Bush, wonThe, Election] at 
Electoral College) as an abbreviation for
{[Bush, wonThe, Election])
  [id1, type, statement]
  [id1, subject, Bush]
  [id1, predicate, wonThe]
  [id1, predicate, Election]
[id1, at, Electoral College]
}
(although I still would vote to extend the RDF model)

>>One could vote to include the source of the triple into the RDF datamodel
>>itself.
>>Former discussion of this question in the rdf-interest group resulted in the
>>expressed opinion to use reification for this purpose
>>(see ).
>
>I have a slightly more basic trouble with the "RDF Model". I really cannot 
>understand what it is supposed to be a description OF.  Are these 
>'triples' to be considered syntax or interpretation? Since RDF has an 
>XML-ish syntax which is quite different (it involves many angle brackets 
>and quotation marks, for example), the 'triples' are apparently not the 
>syntax. So I presume that they are intended to be part of the 
>interpretation of the syntax, ie the semantics of RDF is defined in terms 
>of abstract entities called 'triples'.
>But if that is so, then Stefan's reply to Seth, above, doesn't make sense, 
>since obviously semantic interpretations aren't the kind of thing that can 
>be located on websites, or which have 'sources' in this sense, or which 
>are assigned truthvalues; and Stefan seems to refer to the model theory 
>*of* the triples, rather than to the triples as constituting the model theory.
You seem to assume, that on the web we will have one (or at least a small 
number)
of sophisticated language(s) that will be sufficient for everybody
to express their data/information/knowledge it.

I don't believe that this will happen. We have a multitude of formats 
already -
and we will see even more different languages for various purposes.
Naturally, this will lead to a multitude of interoperation problems.

The best one can do is to provide a common ground for all the different 
languages, such that
establishing interoperation if as cheap as possible.
(even if the others do not base their languages on top of it - it can be 
used as a more
  abstract base for technology development).

 From database research, it is well known, that semi-structured data
(a graph form) is useful for mapping between heterogeneous datasources.
(see eg.
Papakonstantinou, Y.; Garcia-Molina, H.; Widom, J.
Object Exchange Across Heterogeneous Information Sources
1994,ICDE '95
http://dbpubs.stanford.edu/pub/1994-8

Correspondence and Translation for Heterogeneous Data,
Serge Abiteboul, Sophie Cluet, Tova Milo, ICDT, 1997.
ftp://ftp.inria.fr/INRIA/Projects/verso/VersoReport-102.ps.gz

or a newer paper:
Representing and Transforming Model-Based Information
by Shawn Bowers, Lois Delcambre
ECDL 2000 Workshop on the Semantic Web
21 September 2000, Lisbon Portugal
http://www.ics.forth.gr/proj/isst/SemWeb/program.html

RDF is exactly this kind of semi-structured data - even if you think the
syntax is ugly (it's just one particular syntax)

So the triples at first can be understood as assertions.
But the triples are also the syntactic material we are dealing with.

>  So I am left completely confused about the meaning of RDF, as indeed I 
> have been ever since first meeting it. I have been assuming in the DAML 
> discussions that it is basically simply a syntactic specification 
> (possibly the ugliest ever devised by any human being, but let that pass) 
> without any actual semantics. However, some people seem to think that it 
> has a semantics. Can anyone point me to a specification of a model theory 
> for RDF?

I would say, given a specific set of RDF triples, most of this apply:

Raymond Reiter. Towards a logical reconstruction of relational database 
theory. In M. L. Brodie et al. (Hg.), On Conceptual Modeling, S. 191-238. 
Springer, Berlin, 1983.

Best,

         Stefan


>(Just to save time, there isn't one in: 
>http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/#model )
>
>Pat Hayes
>
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Received on Sunday, 26 November 2000 17:14:51 GMT

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