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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 08:13:42 -0400
Cc: "Michael Schneider" <schneid@fzi.de>, "Ian Horrocks" <Ian.Horrocks@comlab.ox.ac.uk>, <public-owl-wg@w3.org>
Message-Id: <A670405B-3158-44F8-8D77-FB25D37D514C@cs.rpi.edu>
To: Bijan Parsia <bparsia@cs.man.ac.uk>

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
Received on Tuesday, 12 August 2008 12:14:34 GMT

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