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Re: Question re: HasKey entailments

From: Jim Hendler <hendler@cs.rpi.edu>
Date: Tue, 30 Dec 2008 09:20:06 -0500
Cc: "OWL Working Group WG" <public-owl-wg@w3.org>
Message-Id: <47F8F56D-22B4-4EB3-A6CA-4DB02DEA7A05@cs.rpi.edu>
To: Michael Schneider <schneid@fzi.de>

Michael-
  I understand that that is the way it works now, and I hadn't really  
thought about the idea of "local" keys -- I wonder about that design  
decision, but won't argue.  However, given the somwwhat non-intuitive  
nature of this (at least to some people, including me) perhaps the  
document should include either a better explanation of why the class  
assertion about Peter is needed and/or an explicit mention of the fact  
that the HasKey doesn't make the :PropertyDomain assertion and/or an  
example that makes some sense of this (the best I've come up with is a  
contrived example where some sort of permission is unique to managers  
but non-key to regular employees, or something like that)
  So I hereby raise this as a LC Document issue (I believe the right  
terminology) -- i.e. the design is okay, but the documentation should  
better reflect it
    -JH
p.s. I would suggest this change in the syntax/structure document  
itself, again, since this will be nonintuitive to at least some users  
and is likely to cause to the creation of buggy ontologies in OWL 2.0  
if not explained

On Dec 30, 2008, at 8:12 AM, Michael Schneider wrote:

> Hi Jim!
>
> The idea behind the class a:Person (the first argument) in the  
> HasKey axiom
> is /not/ to entail that subjects of the key property a:hasSSN are  
> instances
> of the class a:Person. This could easily be achieved by putting an
> additional domain axiom on that property a:hasSSN :  
> PropertyDomain(a:hasSSN
> a:Person).
>
> Instead, the idea is that the given property a:hasSSN is only "key- 
> ish" for
> instances of a:Person. For instances outside of a:Person (or for  
> instances
> that are at least not known to be from a:Person), the property  
> a:hasSSN does
> not need to be a key. This allows to define a sort of "local key",  
> i.e.
> local to some given class, which you cannot define with inverse  
> functional
> properties.
>
> In the given example, both a:Peter and a:Peter_Griffin are known to  
> be in
> class a:Person, and hence the property a:hasSSN really acts as a key  
> for
> them: They are entailed to be the same individual, based on the fact  
> that
> the objects of the two property assertions are equal  
> ("123-45-6789"). But if
> you drop the last axiom, then nothing specific is known about the  
> individual
> a:Peter_Griffin, and so the key axiom will not "fire".
>
> To summarize: In order to make a HasKey(C p) axiom fire for two  
> individuals
> a and b, two premises must be met:
>
>  (1) The objects x and y in two property assertions "a p x" and "b p  
> y"
> must be identical: x = y.
>
>  (2) The individuals a and b must both be instances of the class C.
>
> You can see these two conditions (somewhat buried by other  
> conditions) in
> the definitions of the semantics of Keys, both in the Direct  
> Semantics [1]
> and the RDF-Based Semantics [2].
>
> Cheers,
> Michael
>
> [1] <http://www.w3.org/2007/OWL/wiki/Semantics#Keys>
> [2]
> <http://www.w3.org/2007/OWL/wiki/RDF-Based_Semantics#Semantic_Conditions_for
> _Keys>
>
>> -----Original Message-----
>> From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org 
>> ]
>> On Behalf Of Jim Hendler
>> Sent: Monday, December 29, 2008 9:58 PM
>> To: OWL Working Group WG
>> Subject: Question re: HasKey entailments
>>
>>
>> (Since I'm not a member of the WG this can be considered a LC comment
>> or since my group is in the group, you can consider it an internally
>> discussible comment)
>>
>> I was talking to someone about keys and we were looking at the
>> document section on this (http://www.w3.org/2007/OWL/wiki/ 
>> Syntax#Keys)
>> where the example is given of
>>
>> HasKey( a:Person a:hasSSN )
>> PropertyAssertion( a:hasSSN a:Peter "123-45-6789" )
>> ClassAssertion( a:Person a:Peter )
>> PropertyAssertion( a:hasSSN a:Peter_Griffin "123-45-6789" )
>> ClassAssertion( a:Person a:Peter_Griffin )
>>
>> and my colleague asked why the last axiom wasn't entailed by the
>> HasKey.  We went and looked in the model theory, and it says under  
>> the
>> circumstances of the first four expressions HasKey won't apply
>> (because there's no evidence of a CE for Peter_Griffin) -- so it
>> appears this is not entailed in the current model theory.
>> Guess my question is why does one need the additional condition (and
>> thus the additional axiom) -- wouldn't it follow that if HasKey
>> relates person's via hasSSN (i.e. the HasKey assertion) that anything
>> that has that key (the SSN) would have to be a person?
>> If HasKey would entail that the domain of the property asserted
>> would be the first argument to the HasKey (which is what seems to be
>> intended) then wouldn't the fact that X is an element of
>> (CE)^^C [1] and in fact that it is a Person follow?
>>   Is there a reason we don't do this ?-- would seem to simplify the
>> use of HasKey without causing an obvious semantic harm that I can see
>> (and would make its use more intuitive in many cases).
>>   thanks
>>   JH
>>
>>
>>
>>
>> [1] sorry, my mailer doesn't seem to like the fonts from the document
>> - see http://www.w3.org/2007/OWL/wiki/Syntax#Keys fpr the
>
>
> --
> Dipl.-Inform. Michael Schneider
> FZI Forschungszentrum Informatik Karlsruhe
> Abtl. Information Process Engineering (IPE)
> Tel  : +49-721-9654-726
> Fax  : +49-721-9654-727
> Email: Michael.Schneider@fzi.de
> Web  : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555
>
> FZI Forschungszentrum Informatik an der Universität Karlsruhe
> Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe
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> Studer
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>

"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, 30 December 2008 14:20:46 GMT

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