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RE: How do RDF and Formal Logic fit together?

From: Peter Crowther <peter.crowther@networkinference.com>
Date: Mon, 8 Oct 2001 14:15:48 +0100
Message-ID: <B6F03FDBA149CA41B6E9EB8A329EB12D05A816@vault.melandra.net>
To: "'Sandro Hawke'" <sandro@w3.org>, www-rdf-rules@w3.org, www-rdf-logic@w3.org
> From: Sandro Hawke [mailto:sandro@w3.org]
[Similarly cross-posted]

> RDF is a weak knowledge representation language.  When we want to say
> something with a very simple logical structure ("Luke's father is
> Anakin"), we can say it directly with RDF Statements.  If we want to
> say something with a more complex logical structure ("The force is
> strong with *everyone* in Luke's family"), we can't use RDF as the
> only language.  We need all parties to understand a different
> language.

*All* parties?  Whatever happened to the 'pure' RDF parser, or the idea of
layering different languages on top of RDF?

This gets right to the core of the current discussions on rdf-logic.  Is RDF
merely an unnaturally complex way of encoding and transferring graphs, or is
there something more to it?  Are literals somehow 'above' RDF, or part of
it?  Does a minimal RDF engine merely have to read RDF and produce some
representation of that graph, thereby excluding it from 'all parties' above?

> Why use a logic language instead of, say, Java byte code?

Why use a relational database instead of, say, a persistent object store and
a raft of custom code?  Because it's simpler (indeed, possible) to prove
properties of a relational database, due to the presence of relational
theory/algebra/calculus.

Why use a decidable fragment of first order predicate logic such as DAML+OIL
instead of, say, a full logic language?  Because it's simpler (indeed,
possible) to prove properties of a set of assertions involving only the
decidable fragment.

> Why do *you* want something more expressive than RDF1

I want to represent, transfer and use both decidable and undecidable logical
assertions within and between organisations.  Principally decidable.

> and how do you think we should get there?

I'm increasingly unsure that 'there' should involve RDF at all; I can no
longer see how RDF per se helps me to "represent, transfer and use both
decidable and undecidable logical assertions within and between
organisations" any more than does, say, XML.  I do, however, feel that a
useful distinction can be made between an expressive but decidable and
tractable subset of FOPL (such as DAML+OIL) and the very expressive but
potentially undecidable and potentially intractable assertions that lie
outside that area.

		- Peter
--
Peter Crowther, Network Inference Limited
Received on Monday, 8 October 2001 09:16:34 GMT

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