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Re: A Rough Guide to Notation3

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Sat, 24 Aug 2002 14:30:05 -0400 (EDT)
Message-Id: <20020824.143005.125120844.pfps@research.bell-labs.com>
To: sandro@w3.org
Cc: seth@robustai.net, sean@mysterylights.com, www-rdf-interest@w3.org

From: Sandro Hawke <sandro@w3.org>
Subject: Re: A Rough Guide to Notation3 
Date: Sat, 24 Aug 2002 07:16:10 -0400

[...]

> > Following the RDF philosophy that any RDF graph,
> > particularly non-tree graphs, should be allowable, non-tree versions
> > of the logic's sentences should also be allowed.
> 
> I don't completely accept that last sentence.   DAML+OIL says the RDF
> graph 
>     _:x daml:differentIndividualFrom _:x.
> is not allowed.  

DAML+OIL does indeed not say that at all.

_:x daml:differentIndividualFrom _:x.

is a perfectly good DAML+OIL knowledge base.  It just happens to be
inconsistent, and thus have no intepretations, but this does *not* prevent
it from being a DAML+OIL knowledge base.

By the way, _:x pl:negation _:x is only a very simple example of the kind
of sentences that cause problems.  More complex ones can easily be
constructed, such as
	_:x pl:negation _:y .
	_:y pl:conjunct _:x .
	_:y pl:conjunct _:x .

> Each new vocabulary can do that.  The RDF philosophy
> of "anyone can say anything about anything" does not extend to the
> point of a receiver having to make sense of a contradiction.  

Why not?  DAML+OIL makes sense of these contradictions.  (DAML+OIL has its
own lossage, by the way, but a different sort of lossage.)

[...]

> Can't we just just disallow RDF graphs which describe self-referencial
> sentences (or at least self-negating) ones?

How can you do this, in RDF?  They are still RDF graphs, and need to be
handled by the model theory.  In fact, there is no real problem with
self-negating sentences, by themselves, in RDF (as opposed to the situation
is some other styles of providing semantics).

[...]

> Is there some reason we need any other kind of sentences to exist?

The problem is not their (potential) existence.  It is their existence
everywhere.  The problem is that if you allow self-referential
sentences and also need to have sentences exists everywhere, removing just
the problematic ones is problematic. 

>    -- sandro

peter
Received on Saturday, 24 August 2002 14:30:16 GMT

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