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Re: Classes and predicates as first class objects

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Mon, 19 Aug 2002 09:44:57 +0100
Message-ID: <15712.45065.988420.709463@merlin.oaklands.net>
To: "R.V.Guha" <guha@guha.com>
Cc: www-rdf-logic@w3.org

On August 16, R.V.Guha writes:
> Ian,
>  I have to beg to differ with you on this point. This feature of RDF is 
> one of its main virtues. Many systems I have helped build (including Cyc 
> and tap) make extensive use of this feature. We have found it quite 
> useful to be able to make statements both about classes and properties. 
> This is why Tim Bray and I included this feature in [1].
>  For example, TAP includes a category of properties corresponding to 
> "abbreviation codes", i.e., airportCode, cityCode, partCode, etc. TAP 
> also states many properties of classes (such as Musician) that help us 
> represent their mappings to dmoz, etc. In Cyc, there is a whole big 
> hierarchy of classes of classes which help make the distinction between 
> substances and individuals (this work was derived from Montague).
>  I submit that the experience to date in the usefulness of this feature 
> suggests that RDF retain it.
>  I do agree that this feature might prove fatal when combined with 
> certain other features (as described by Peter's papers), but this is not 
> always the case. As shown by Hayes & Menzel and Fikes & McGuinness, it 
> is also fairly straightforward to construct first order logics that are 
> quite comfortable with this feature. Further, as explained by Hayes and 
> myself, the introduction of the concept of a reserved vocabulary, which 
> keeps constructs such as first and last, which are essentially syntactic 
> constructs, from showing up in the model, these problems can be avoided.

This last point is the nub of the matter. Of course the problem can be
resolved by treating some RDF triples as a reserved vocabulary - this
much is obvious. It does, however, require a modification of RDF to
which some are strongly opposed.

My point is that, without such a modification, extending RDF with the
expressive power of OWL would result in a language so seriously broken
that the question as to whether classes/predicates can be treated as
arguments to other predicates would become an irrelevance.


> guha
> [1] MCF using XML: http://www.w3.org/TR/NOTE-MCF-XML/
> Ian Horrocks wrote:
> >Guha,
> >
> >You chose to ignore my second point, so I will repeat it:
> >
> >    The ability to treat classes/predicates as arguments to other
> >    predicates is of secondary importance. The crucial thing with RDF
> >    is that it treats the vocabulary of the language itself as
> >    standard classes/predicates that can be arguments to other
> >    predicates. This is beyond the ability of almost all logics. It is
> >    relatively harmless for a language as weak as RDF, but causes
> >    fatal complications when more expressive power is added.
> >
> >Ian
> >
> >  
> >
Received on Monday, 19 August 2002 04:47:24 UTC

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