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Re: Web Rule Language - WRL vs SWRL

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Fri, 1 Jul 2005 21:04:20 +0100
Message-Id: <0ded184c0062de5e948eff447e5cb025@cs.man.ac.uk>
Cc: www-rdf-rules@w3.org
To: drew.mcdermott@yale.edu

On 1 Jul 2005, at 05:03, Drew McDermott wrote:

>
>
>>>> [Bijan Parsia]
>>>> The old Layering Story has been bankrupted in several different
>>>> ways.
>> [me]
>>> I'll take advantage of this crack in the orthodox Wall of Opprobrium
>>
>> I don't know how you determine your factions, but this really makes no
>> sense.
>
> Sorry; I admit that my factionometer is not as accurate as it should 
> be.
>
>>
>> (The Old Layering Story is that every semantic web language should be 
>> a
>> same syntax semantic extension of RDF. In this current thread, I think
>> only Jim ever strongly endorsed that approach, and most of the major
>> proponents, afaict, have weakened or abandoned it. Peter has a paper 
>> in
>> ICJAI showing that it is paradoxical if extended to FOL. I find it a
>> PITA for OWL and have argued vehemently, with major grade vehemence 
>> for
>> the truth of my PITA pain.
>
> Well, Ian said this a couple of days (and many messages) ago ---

Where? I never said any such thing! I have been a long standing 
opponents of the "same syntax extension" idea and have consistently 
argued against it.


>
>> Regarding the existing layered architecture of OWL, and the proposed
>> extension to SWRL, nobody (well not me anyway) claims that it is
>> perfect, and there will (as Holger has pointed out) no doubt be some
>> compatibility issues between different tools, but the situation is
>> hardly comparable to the one that is being proposed in the "updated
>> layer cake": as we have seen, even the very limited degree of
>> interoperability suggested by this diagram turns out to be a chimera.
>> In contrast, RDF, OWL and SWRL share a common semantic framework, and
>> allow for a relatively clean layering: OWL-Lite, OWL-DL and SWRL are
>> layered on a subset of RDF, but share the same semantics (and it can 
>> be
>> syntactically determined when RDF ontologies are within this subset).
>
> --- so the Old Story can still be wheeled out when needed.  Again, if
> I have missed a nuance, I apologize.  As far as I can tell, "layering"
> is completely meaningless, but perhaps there are degrees of
> meaninglessness.

I don't understand your problem. OWL-Lite, OWL-DL and SWRL are 
increasingly expressive subsets of FOL. For the first, many important 
inference problems are decidable in ExpTime, for the second in NExpTime 
and for the third they are undecidable. This seems to be a pretty 
reasonable kind of layering. The relationship with RDF is, admittedly, 
rather more complex, but not "completely meaningless".


>
>> [...]
>> Well, this is familiar from you, Drew, and good to have it on the
>> table. However, I don't think either Ian nor I are arguing against
>> non-mon *per se*. I might argue against quasi-proceduralism, but that
>> doesn't put me on a different side than Michael, as far as I know.
>
> I guess I should have kept my mouth shut, because obviously I don't
> understand the argument.
>
>> [snip]
>>
>>> 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 believe model theory is quite standard for each. I believe the
>> proposal on the table from Michael et al is for the Well Founded
>> semantics. I'd be interested in what you thought of all that.
>>
>>>   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.
>>
>> I wonder what you think of all the work in formal semantics for LP.
>> Minimal model, perfect model, Well founded, answer set....
>
> Here's the nub: We are in the presence of an unfortunate pun.  All of
> these different "semantics" for LP concern the meaning of logic
> programs _as programs_.  But for interoperability what we care about
> is the semantics of logical expressions as _statements_.  So when Ian
> says
>
>> 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.
>
> I am somewhat baffled.  If two systems use the same syntax, and employ
> the same vocabulary with the same (Tarskian) semantics

They use the same model structure, but LP semantics admits many fewer 
models than FO semantics, and fewer models means more entailments.

> , then they can
> interoperate by exchanging messages.  The fact that they may have used
> different methods in order to arrive at their conclusions is
> important, but has nothing to do with interoperability per se.

I'm not interested in methods used to arrive at conclusions, I am 
interested in which conclusions are supported by the semantics. LP 
semantics support conclusions that are not supported by FO semantics, 
and this does have something to do with interoperability.

>   (I'll
> anticipate the objection that there has to be a standard proof or
> justification language so that a module needn't believe a statement
> unaccompanied by a proof; a Methodist system can refuse to believe the
> testimony of a Baptist system.  Having anticipated it, I will simply
> say that I am deeply skeptical about the possibility of such a
> justification system.)

Me too. But I don't see how this is relevant to the current discussion.

Ian


>
>> [...]
>> I'll be interested to know if Michael thinks this is a defense of him 
>> :)
>
> Yes, he's probably preparing a statement disavowing everything I've
> said.  Well, I tried.
>>
>> I guess I just find this too vague to get a grip on. What's your
>> proposal? Is ISO Prolog a reasonable starting place? Some more modern
>> logic/functional programming language? Should we just add a few things
>> to XQuery?
>>
>> I just don't know how to take your POV and generate useful
>> standardization activities. Do you think there are none? (That's not
>> unreasonable.)
>
> My proposal is type theory, or, failing that, Common Logic with a
> strongly typed syntax in front of it.  But this is a proposal for a
> notation for exchanging information, not for a system of inference.
>
>                                              -- Drew
>
> -- 
>
>                                          -- Drew McDermott
>                                             Yale University
>                                             Computer Science Department
>
>
Received on Friday, 1 July 2005 20:04:27 GMT

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