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Re: concerning lite, fast, large versions of OWL

From: Jim Hendler <hendler@cs.umd.edu>
Date: Thu, 17 Oct 2002 20:12:57 -0400
Message-Id: <p0511171eb9d501f93b79@[]>
To: Enrico Motta <e.motta@open.ac.uk>, Frank van Harmelen <Frank.van.Harmelen@cs.vu.nl>, Christopher Welty <welty@us.ibm.com>, www-webont-wg@w3.org

Enrico - all those arguments address why it would not be so bad to 
include classes as instances in OWL Lite, but I'm not sure they 
address the specific need to include these things in Lite.  That is, 
it would be perfectly fine for a tool developer to say "I include all 
of OWL Lite and also classes as instances"  or such.  This is not 
meant to be a refutation, I mean it as a real question - why do you 
feel we need these metaclasses in the simplest subset of our language 
rather than in the langauge as a whole?

At 1:00 AM +0100 10/18/02, Enrico Motta wrote:
>It is an incredibly rare event for me to say that I do not  agree 
>with something you say, but I guess this is one of those rarities!
>You say that classes as instances should not be in OWL-lite 
>basically because of three reasons:
>1) they break the beautiful inclusion hierarchy (OWL Light < 
>OWL/FOL-style < OWL/RDF-style)
>2) they make reasoning more complicated
>3) they make tool implementation more complicated.
>I am not convinced by either of these three arguments
>1) If we add RDF to the picture, what we get is 2 distinct 
>hierarchies in your scenario
>RDF <  OWL/RDF-style
>OWL Light < OWL/FOL-style <  OWL/RDF-style
>This is not brilliant either, given that OWL is supposed to be based 
>on RDF.  If we make OWL-light RDF-compliant, then we get the 
>following picture
>RDF <  OWL Light <  OWL/RDF-style
>OWL/FOL-style <  OWL/RDF-style
>This isn't great either, but it is no worse than the alternative. 
>It basically shows an inclusion hierarchy baed on top of RDF, with 
>an alternative, FAST-OWL, for those applications/scenarios/groups 
>who need both powerful and complete reasoners and do not care about 
>the (very) basic metalevel support provided by RDF.
>2) I am not sure why reasoning should be more complicated. It may be 
>undecidable, but this is different from being complicated, in the 
>sense of ease of implementation of reasoners, rather than formal 
>complexity. Do you really think FAST-OWL is going to be that easy to 
>implement? Both FAST-OWL and LARGE-OWL have very powerful constructs 
>and inevitably they will BOTH be tricky to implement correctly. But 
>I don't really understand why having metaclasses should seriously 
>make implementing OWL-lite more difficult.
>3) In our group we have have been implementing KB tools for almost 
>20 years and I really can't see why having classes as metaclasses 
>makes tool implementation more difficult.  Instance links from a 
>class to a metaclass do not affect the display or the browsing of 
>subclassOf hierarchies. And in any case such problem is a lot 
>simpler than supporting browsing and visualization of ontology 
>entities in a language that allows DL-style flexibility in the 
>association between properties and classes.
>My view is that the need to link classes to their metaclasses is so 
>ubiquitous in knowledge representation that it will be a problem to 
>leave it out of OWL-lite.  I guess this is why the RDF folks added 
>it to something as 'bare' as RDF.  Indeed, the wine ontology 
>subgroup in Bristol spent quite a bit of time trying to come up with 
>examples where such need (of representing metaclasses) would not 
>arise, given that we did not know whether OWL would support classes 
>as metaclasses.  It is a truly ubiquitous problem, which is often 
>solved by modelling classes as instances, thus making it difficult 
>to teach students how to build proper ontologies and limiting 
>interoperability. For instance, it is very difficult to talk both 
>about individual grapes and about types of grapes that are excellent 
>for wine, without the ability to treat classes as individuals. In 
>Bristol we solved it simply by ignoring instances of class GRAPE and 
>talking only about GRAPE-TYPE, but this was simply a way to avoid 
>the problem, rather than solving it.
>PS Btw, at the cost of sounding real pedantic I should also object 
>to the implied statement that having classes as instances means that 
>we will go beyond OWL/FOL-style.  Maybe it is only a matter of 
>labels, but, as  Pat Hayes has stated many times in this list, 
>treating classes as instances is not really a problem for FOL 
>treatments. I stopped being a formal logician back in 1986, but I do 
>remember that even back then this was not a problem for 1st order 

Professor James Hendler				  hendler@cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
Univ of Maryland, College Park, MD 20742	  240-731-3822 (Cell)
Received on Thursday, 17 October 2002 20:13:17 UTC

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