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Re: concerning lite, fast, large versions of OWL

From: Christopher Welty <welty@us.ibm.com>
Date: Thu, 17 Oct 2002 21:21:32 -0400
To: www-webont-wg@w3.org
Message-ID: <OFC1363BC6.116315A8-ON85256C56.0006A914@pok.ibm.com>

Classes as instances are allowed in OWL.   This is not an argument about 
whether or not it is possible to implement, nor is it about how important 
a feature it is in a KR language.

The simple issue is that adding this as a feature to OWL Lite creates a 
complicated relationship between three languages, whereas keeping OWL Lite 
as a subset of OWL/FOL keeps a simple layering.  If OWL Lite is not a 
subset of Fast OWL, and in fact if classes as instances are allowed, then 
there would be things you could say in each that you couldn't say in the 
other, and this would create interoperability problems.  There is simply 
no way around this argument.

There is a strong aesthetic argument for this, and a strong political one 
as well.  I have already heard outside mumblings that we are creating a 
complicated "network" of languages (I think this may have been a result of 
the original OWL Lite debates), and I believe a simple layering addresses 
that criticism.


Dr. Christopher A. Welty, Knowledge Structures Group
IBM Watson Research Center, 19 Skyline Dr.
Hawthorne, NY  10532     USA 
Voice: +1 914.784.7055,  IBM T/L: 863.7055
Fax: +1 914.784.6078, Email: welty@us.ibm.com

Dieter Fensel <dieter@cs.vu.nl>
10/17/2002 08:45 PM

        To:     Enrico Motta <e.motta@open.ac.uk>, Frank van Harmelen 
<Frank.van.Harmelen@cs.vu.nl>, Christopher Welty/Watson/IBM@IBMUS
        Subject:        Re: concerning lite, fast, large versions of OWL


I agree with Enrico. It is just wrong that meta classes are beyond FOL.
Just take Flogic as an example. Clearly one needs some syntactical
tricks but that is all.

At 01:00 AM 10/18/2002 +0100, 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/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 
>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 
>ease of implementation of reasoners, rather than formal complexity. Do 
>really think FAST-OWL is going to be that easy to implement? Both 
>and LARGE-OWL have very powerful constructs and inevitably they will BOTH 

>be tricky to implement correctly. But I don't really understand why 
>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 
>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 
>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 
>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 
>not really a problem for FOL treatments. I stopped being a formal 
>back in 1986, but I do remember that even back then this was not a 
>for 1st order logic.

Dieter Fensel
Tel. (mobil): +31-(0)6-51850619
Fax: +43-512-940686
http://www.google.com/search?q=dieter or http://www.fensel.com
Received on Thursday, 17 October 2002 21:22:08 UTC

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