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Re: ISSUE 131 (OWL R Unification): Different semantics on syntactic fragment

From: Jim Hendler <hendler@cs.rpi.edu>
Date: Tue, 12 Aug 2008 08:19:01 -0400
Cc: Bijan Parsia <bparsia@cs.man.ac.uk>, "Michael Schneider" <schneid@fzi.de>, "Ian Horrocks" <Ian.Horrocks@comlab.ox.ac.uk>, <public-owl-wg@w3.org>
Message-Id: <EE5CC0B2-4A53-45E0-8340-2CC0E45609E3@cs.rpi.edu>
To: Jim Hendler <hendler@cs.rpi.edu>

Oops - apologies to the WG, didn't mean to send this to the whole  
group - accidently hit the reply-all, JH

On Aug 12, 2008, at 8:13 AM, Jim Hendler wrote:

>
> Bijan- before you accuse Michael of making a point for "debate" I  
> suggest you check the logs -- the idea of a rule-based fragment per  
> se was raised by me in the context of what we were then calling RDFS  
> 3.0, and it was distinct from the DLP-oriented solutions (very  
> distinct).  The WG over time pushed these two together, causing the  
> problems Michael is trying to solve.  The log is pretty clear on  
> this -- and I have the scars to prove it...
>
>
> On Aug 12, 2008, at 4:00 AM, Bijan Parsia wrote:
>
>>
>> On Aug 12, 2008, at 1:08 AM, Michael Schneider wrote:
>>
>>> Hi Ian!
>>>
>>> Ian Horrocks wrote:
>>>
>>>> Michael,
>>>>
>>>> It has never been claimed that OWL R DL and OWL R Full are  
>>>> completely
>>>> equivalent on the syntactic fragment.
>> [snip]
>>> In the original issue, it was stated that
>>>
>>> "The main benefit would be that we would not need  
>>> owl:intendedProfile"
>>>
>>> For me, this is at best a very minor nice-to-have benefit.
>>
>> That's not the main benefit for *me*. The main benefits are:
>> 	1) Profiles all (mostly) work the same (i.e., the core bit is a  
>> syntactic fragmetn)
>> 	2) Extensibility on the semantics makes sense (i.e., tools aren't  
>> *forced* to be semantically restricted outside ethe core fragment)
>> 	3) We don't have axioms which have semantically meaningful  
>> constructs only on one side.
>>
>> For me 1 and 3 are *killers*.
>>
>>> And now, we are near to close this "intended profile" issue,  
>>> anyway, by not having such a signaling URI at all. So, the  
>>> "unification" issue can even be regarded to be kind of moot.
>>
>> No.
>>
>>> But we are still talking about the unification, for which the  
>>> price to pay would be pretty high for the RDF side, which  
>>> originally was the only side that asked for such a rule-based  
>>> language.
>>
>> First, this isn't true. DLP was included in the original fragments  
>> document, IIRC. Please don't rewrite history esp. to make a  
>> debater's point.
>>
>> Second, we have to consider the ecosystem, not just a single  
>> species. We also have to consider the range of users and the range  
>> of possible confusions.
>>
>> We're adding a *lot* of real constructs to an RDF fragment...we  
>> should expect that people will be less indifferent to the  
>> entailments they get.
>> [snip]
>>>> Your example is a good illustration of why it would be *a very bad
>>>> idea* to define a 3rd semantics for OWL based on the OWL RL rules.
>>>> According to this semantics, it would NOT be the case that
>>>> owl:intersectionOf (C D) is a subClassOf D. Any reasoner finding  
>>>> this
>>>> entailment would be unsound and non-conformant w.r.t. this  
>>>> semantics.
>>>> This would, IMHO, be highly counter-intuitive.
>>>
>>> Now this was actually my counter example, so one can easily take  
>>> it as an example for an "unintuitively" missing entailment. But  
>>> equally well, from a rules perspective, one could also claim that  
>>> producing this entailment is counter intuitive.
>>
>> What? Are you seriously coming from a user perspective *at all*?  
>> They aren't going to look at the rules. They are going to *write  
>> something down*.
>>
>>> Actually, there are enough examples for derivations by applying  
>>> the rules, where there is no respective entailment by OWL R DL,  
>>> simply because it would fall outside OWL R DL's syntax (at least,  
>>> an OWL R DL reasoner wouldn't be required to produce it there).
>>>
>>> Consider that very asymmetric syntax of OWL R DL (e.g., unions,  
>>> and existential and universal restrictions may only occur on one  
>>> side of subclass axioms, respectively). Isn't this alone already  
>>> "counter-intuitive", at least to people who do not understand the  
>>> theoretic background behind this language? In comparison to this,  
>>> the OWL R Full ruleset looks pretty coherent to me.
>>
>> Only by deception. You allow the syntactic freedom without the  
>> semantics. That's not coherent, that's *bad*.
>>
>> At least by highlighting the asymmetries, you clearly indicate  
>> where what you say *has no effect*.
>>
>>> Well, so we have claims about "counter-intuitive" reasoning  
>>> results on both sides. I would call that a draw! :)
>>
>> I wouldn't. We don't have mere claims.
>>
>> [snip]
>>> The point is that it would really not be a great marketing  
>>> statement to say: "We have a sound OWL RL reasoner!".
>>
>> I don't see how you can simultaneously claim that users won't care  
>> about silent no-semantics axioms and yet care about calling it  
>> sound. Calling the rules complete for all graphs is *just*  
>> marketing and *bad* marketing because it relies on some fairly  
>> unpretty sophistry.
>>
>>> Building sound-only reasoners is a trivial task (just take a "zero- 
>>> reasoner", which does not produce any inferences).
>>
>> Oh man, you're clearly not in marketing land now.
>>
>>> And a reasoner, which just implements the "official OWL R  
>>> ruleset", wouldn't be more than just an OWL R sound-only reasoner.  
>>> While there would be other reasoners around, which would really be  
>>> sound and complete w.r.t. the OWL R semantics. Would anyone buy  
>>> the former reasoner under these circumstances?
>>
>> Sure, for scalability, for brand name, because they don't care  
>> about completeness, or for any of a  number of reasons.
>>
>> (BTW, in bioinformatics circles, soundness is *much* less  
>> interesting than completeness (assuming it's not overnoised).  
>> Talking with folks like Robert Stevens,  I find that they would be  
>> *much* more perturbed by constructs that *didn't* have effects  
>> though they look like they should. I *imagine* HCLS is a field that  
>> would be highly interested in OWL-R (as a compilation target, and  
>> directly)).
>>
>> I think it's a *very* bad idea to get too creative in profile land,  
>> in the standard. I'm not saying it's exactly analogous, but OWL  
>> Lite should be a cautionary tale.
>>
>> Cheers,
>> Bijan.
>>
>
> "If we knew what we were doing, it wouldn't be called research,  
> would it?." - Albert Einstein
>
> Prof James Hendler				http://www.cs.rpi.edu/~hendler
> Tetherless World Constellation Chair
> Computer Science Dept
> Rensselaer Polytechnic Institute, Troy NY 12180
>
>
>
>
>

"If we knew what we were doing, it wouldn't be called research, would  
it?." - Albert Einstein

Prof James Hendler				http://www.cs.rpi.edu/~hendler
Tetherless World Constellation Chair
Computer Science Dept
Rensselaer Polytechnic Institute, Troy NY 12180
Received on Tuesday, 12 August 2008 12:19:45 GMT

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