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Re: Web Rule Language - WRL vs SWRL

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Thu, 30 Jun 2005 11:59:06 +0100
Message-Id: <8cd46b13ab8948c29b400df796f7f46f@cs.man.ac.uk>
Cc: www-rdf-rules@w3.org
To: drew.mcdermott@yale.edu

On 30 Jun 2005, at 03:29, Drew McDermott wrote:

>> [Bijan Parsia]
>> The old Layering Story has been bankrupted in several different
>> ways.
> I'll take advantage of this crack in the orthodox Wall of Opprobrium
> here to jump to Michael Kifer's defense.  Someone should.
> For one thing, I find the argument against his position to sound a
> little like this one:
> "Don't mix coal dust with the pixie dust --- it burns smoky and
> pollutes the air."

This isn't the argument: the argument is about (lack of) 
interoperability between two formal systems. No one is (yet) claiming 
that either of these systems is pixie dust.

> Yes, pixie dust is much better than coal, except that it doesn't
> exist.  I believe that there are applications in which the logical
> purity of Owl, DLP, etc. can be preserved, but (as Michael has
> implied, I think) there are many domains for which people want rules
> in order to express problem-solving strategies, and I have serious
> doubts that those will be expressible in pure FOL.  The reason why
> nonmonotonicity is (regrettable or not) ubiquitous in real-world
> applications is that most inference is not deductive.  Any attempt to
> reduce an inference task to deduction is either completely misguided,
> or ends up compromising the purity of the deductive notation.  Feel
> free to outlaw this practice; you won't succeed.  If the semantic web
> succeeds, one reason will be that someone implements a "killer app"
> rule system that solves a lot of realistic problems, and it will
> surely have a quasi-procedural --- and nonmonotonic ---
> interpretation.

I don't think that anyone is claiming that *everything* can be done 
with logic - at least I am not claiming that. What I am claiming is 
that a logic based KR system can be a core component in many 
applications. I don't doubt that surrounding this core will need to be 
less formal systems, and even code, that deal with the grubbiness of 
many real world situations, but having a "clean" core with formally 
specified semantics is still tremendously useful. This is more or less 
what we see with database systems today.

> Speaking of semantics, there is something about Ian Horrocks's
> oft-repeated claim (to pick one recent iteration) --
>   "An LP language would find an entailment that is *not* supported by
>    RDF semantics .... Ergo, LP is semantically incompatible with RDF."
> -- that I find dubious, in spite of its apparent obviousness.  Suppose
> one adopts a standard Tarskian account of semantics.  It seems to me
> that a monotonic and nonmonotonic system could have exactly the same
> Tarskian semantics, and still disagree on what followed from a given
> set of facts (call it S).  That's because it's only in classical logic
> that the facts entailed by S are exactly the facts that follow from S.
> In a nonmonotonic system the set of facts that are inferred from S is
> a superset of the set of facts that are entailed.
> This point may be obscured by the way LP is presented nowadays.  In
> the past two decades logic programming and logic have drifted apart,
> to the point where their are routinely characterized in basically
> different ways.  I think this is unfortunate.  I prefer to think of
> nonmonotonic systems as being "basically" ordinary logic, with an
> escape hatch here or there.  I grant that this is hard to formalize,
> but that's because it's precisely the nondeductive parts of inference
> that are hard to find a uniform formal framework for.  But consider
> this hypothetical example: a logical system with procedural hooks to
> connect to a Bayesian inference system.  It might have a nonmonotonic
> rule to the effect that "If the best estimate of the probability of P
> is > 0.9, infer P."  One could develop a pretty formal account of how
> this thing worked, including an account of what inferences were
> licensed under what circumstances.  But it would have nothing to do
> with the _semantics_, which could be specified in advance,
> independently of the details of the inferential mechanisms.

It might be better if we went back to talking about Datalog, which is 
the basis of the "LP" proposal, as there seems to be some confusion 
about exactly what constitutes LP. The standard semantics for Datalog 
and FO (Horn clauses) share the same model structure, the difference 
being that Datalog semantics rules out many of the models that would be 
admissible under FO semamtics. We all agree that, if we limit our 
attention to the entailment of ground Datalog atoms, then we can't 
distinguish between these two semantics. My argument is that, if we 
don't impose this restriction (think more expressive query language), 
then the underlying difference in the models will become apparent: as 
Datalog semantics admits less models, it supports more entailments.


>                                              -- Drew
> -- 
>                                          -- Drew McDermott
>                                             Yale University
>                                             Computer Science Department
Received on Thursday, 30 June 2005 10:59:15 UTC

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