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Re: [ontolog-forum] owl2 and cycL/cycML

To: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
Cc: Bernardo Cuenca Grau <bcuencagrau@xxxxxxxxxxxxxx>
From: Pat Hayes <phayes@xxxxxxx>
Date: Sun, 1 Aug 2010 12:54:59 -0500
Message-id: <D055A796-A044-48CF-9725-D4C470E06E81@xxxxxxx>
Just to correct a point of terminology:    (01)

On Jul 29, 2010, at 4:48 PM, Ian Horrocks wrote:    (02)

> OWL and CycL are not really comparable, because OWL is based on a  
> fragment of First Order Logic that is known to be decidable, for  
> which provably correct reasoning algorithms are known and for which  
> effective implementations based on said algorithms are available.    (03)

True, though the CycL reasoning engine also takes advantage of many of  
these decideable cases. (The real difference here is one of academic  
style: the CycL developers are ruthlessly pragmatic and do not care a  
whit for theoretical analyses of completeness or for proving  
correctness.) It should be stated also that proving *correctness* of a  
reasoner is usually fairly easy.    (04)

> OWL's expressive power could, of course, be easily (indeed  
> arbitrarily) extended if one were prepared to compromise on some or  
> all of these design constraints. For example, SWRL extends OWL with  
> Horn clauses (AKA rules), but the resulting language is undecidable,    (05)

True. Strictly, it is semi-decideable.    (06)

> and so only incomplete reasoners are available.    (07)

False. "Complete" means that if a sentence is a theorem, then the  
prover will (eventually) tell you that it is. Complete reasoners (in  
this, textbook, sense) are available for FOL and indeed have been for  
decades.    (08)

Ian is using "complete" here to mean "complete and decideable", which  
can be characterized as: if a sentence is a theorem, then the prover  
will tell you that - completeness - AND if it isn't a theorem, the  
prover will also tell you that it is not. Full FOL is not decideable  
in this sense. But even when the logic is decideable it can still be  
the case that the complexity of the decision process is arbitrarily  
high, and if you have to terminate it early, you are left in a don't- 
know situation, whether the logic is decideable or not.    (09)

> In fact such reasoners are typically used in a way that is actually  
> incorrect, in that failure to find an entailment is treated as a non- 
> entailment, whereas it should be treated as "don't know".    (010)

I dont think it is fair to say that they are *typically* used in this  
incorrect way. (?)    (011)

Pat Hayes    (012)

>
> Regards,
> Ian
>
>
> On 29 Jul 2010, at 20:15, Zhuk, Yefim wrote:
>
>> Ian,
>>
>> Thanks for another great event related to OWL2.
>>
>> What is your opinion on CycL and CycML (see http://cyc.com)?
>>
>> From my point of view, this language is more expressive than OWL  
>> and has naturally embedded rules features.
>> I used this language to describe complex objects and rule-based  
>> business scenarios.
>> I think, this comes much closer to mimicking real world than OWL  
>> can provide today.
>>
>> Do you see this one as gaining bigger acceptance and getting to the  
>> level of a standard?
>>
>> Or maybe there are some limitations that I donít see?
>>
>> Thank you,
>>
>> Jeff
>>
>
>
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