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RE: [OPEN] and/or [PORT] : a practical question

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 30 Mar 2004 16:27:25 -0800
Message-Id: <p06001f27bc8fb733e7cf@[]>
To: "Bernard Vatant" <bernard.vatant@mondeca.com>
Cc: "SWBPD" <public-swbp-wg@w3.org>

>>  >- Is it worth the trade-off to switch one's ontology (otherwise DL)
>>  >to OWL-Full, just to
>>  >allow its classes to be used as objects in 'dc:subject' predicates?
>>  That's a weird way to ask the question.  You mean, is it worth doing
>>  the extra work to break your naturally occuring model just so that
>>  you can be in DL?
>The way I put it might seem weird indeed, but it's the way it was 
>set in the real project
>context (real world is weird). We had an OWL-DL ontology, and wanted 
>to keep it so, and
>suddenly after six months or so some user wants to be able to use a 
>class as a subject of
>a document

This seems to be a general issue, which has already come up in 
several cases I know of and I am sure will continue to come up again 
and again. Stated as neutrally as I can manage,  it can be expressed 
as follows.

The "DL case" embodied in OWL-DL (as well as many other standards and 
reasoners) requires that modelers trade off some expressivity to earn 
the computational advantages of DL reasoners. It will sometimes 
happen that a "natural" way to extend an existing model will take one 
outside the DL case. (By "natural" here I mean only that the 
extension is motivated by considerations which arise in some problem 
or application domain.) However, the pragmatic or managerial or 
political pressures to stay within the DL case may be overwhelming, 
in which case the modeler may need to find a way to work around, 
re-think or otherwise modify the model so as to stay within the DL 
case limitations.

Since this situation is likely to arise quite frequently, and since 
the pressures to stay within the DL case are likely to be too strong 
to resist for many innocent people, I think that we could 
legitimately spend some time describing known cases of this kind of 
phenomenon, with known workarounds, and to generally give aas much 
practical advice as we can.

HOWEVER, I think also that to cast this situation in terms of some 
techniques (ie, in this case, anything outside the DL case) being 
"wrong" or "bad practice" or "mistaken" or in fact in any negative 
way at all other than incompatible with DLs, is intellectually 
arrogant, misleading and completely unnecessary. It also leads us 
into unresolvable areas of intellectual debate where we know we as a 
group will not agree, so might be best avoided. All we need to say is 
that there are work-arounds, or if you prefer, DL-consistent 
practices and guidelines for modelers, which can be used to avoid 
this situation if you are desirous of staying within the DL case.  We 
might also try to give a reasoned statement of the pros and cons of 
staying within that case, if we can possibly manage to do so without 
sliding into rhetoric and mud-slinging.

>  ... which is one case out of one thousand, the 999 others using 'regular'
>subjects. So using a class as subject of a document is not exactly 
>'naturally occuring'.
>It's a borderline case - not to say a weird one :))
>>  I would argue this is indeed a BP issue, but probably for WORLD not
>>  for OPEN... we need to explain why and when you would do the extra
>>  work (and in every case we have explored it is extra work) to make
>>  sure your ontology is in the DL profile of OWL.
>I suggested it might be in PORT scope, because it deals with the 
>terminology vs ontology
>general issue. For me the heart of the question is to know what it 
>means to 'use a
>concept' defined in a terminology (glossary, thesaurus, subject 
>headings, index...) as a
>class (or a property) in an ontology.

Surely we can all agree that there is no general answer to that question.

>Is 'PhD Thesis' class the same 'subject' (using TM language here, 
>sorry) or 'resource'
>than the original concept?

'Original' in what sense?

>The more I think about it, the more I have to deal with it, and
>the more I tend to say that they are distinct animals. Jim's PhD 
>Thesis is an instance of
>the class, but not of the concept.

Do concepts have instances? If so, why isn't Jim's thesis one of them?

>One subject of 'Social Functions of PhD Thesis in
>Occidental University during 20th century', is the concept of PhD 
>Thesis, not the class.

It sounds like you really want the intensional concept of PhD 
thesis-hood, or some such animal. There isn't anything in OWL 
corresponding exactly to that, but the nearest thing to it is 
probably the class of all PhD theses. The RDFS class would be a 
better approximation to the intensional concept, but you probably 
don't want to hear that. BTW, in the common logic framework (I 
promise not to keep on advertising SCL, but its relevant to his 
particular issue) classes (= unary predicates) are both intensional 
objects and first-class entities, and so are quite a lot closer to 
what you need here.

>So it's not just an issue of OWL-DL vs OWL-Full, it's also an issue 
>of making distinct or
>not those two 'things'. This is a core issue in porting thesaurus to 
>the SW, related to
>others of the same kind, like if concepts A and B are interpreted as 
>classes, and there is
>a Broader-Narrower relationship between A and B in the Thesaurus, 
>has it to be interpreted
>as a class-subclass relationship in the ontology etc.

Yes, that is a pity. The RDFS class model allows subClassOf to be a 
stronger relationship than merely a subset relation betwen class 
extensions. This is in many ways closer to NL 'concept' talk, but 
note that in many ways it results in a weaker logic.

>So I think in that case a BP definition would be two-fold
>1. Is it generally a BP to make terminology concepts distinct from 
>ontology classes (and

It is impossible to given an answer to that question. It depends on 
what you want the model to do. There is no "best" answer in general.

Some potentially relevant observations:

subClass is *not* a taxonomy (A can be a subClass of B and C where B 
and C are incomparable), so if your terminology is a taxonomic 
heirarchy then you need at least something stronger than (a 
subPropertyOf) owl:subClassOf.

If you want to talk about terminology and instances of terminology, 
you are likely outside the DL case if you model using subClassOf. In 
this case you should consider modelling terminology relationships 
(subcase, etc) directly as properties on terminological ("concept") 
individuals, and using restrictions to refer to the 'classes' 
indirectly. This enables you to stay inside the DL case longer and to 
express complex relationships in ways that support quite elaborate 
reasoning. The cost is that your descriptions will become less 
direct, less directly intuitive (though you can get used to that) and 
more rebarbative. The other cost is that you have to deal with a lot 
more KINDS of individual, and if you keep choosing this option your 
models tend to get so complicated that they get hard to keep track 
of.  Modelling starts to feel almost like programming.

>2. If agnostic about 1, what is the trade-off when choosing to make 
>them distinct or to
>merge them ?

See above.

>FWIW, having tried both terms of the alternative in the course of 
>time, my personal view,
>for above quoted reasons, is that they shoud be kept separate, and 
>it's worth the extra
>work (even before being aware of the DL vs Full issue)

Right. Unless you have only very simple applications in mind, and if 
you want to stay inside the DL case, this is often going to be true.

Pat Hayes

>Are there other concrete experience on that, not only theoretical 
>considerations? Seems
>like there are not so many people exploring the terminology-ontology 
>interoperability. Or
>are they?
>Bernard Vatant
>Senior Consultant
>Knowledge Engineering
>Mondeca - www.mondeca.com

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Received on Tuesday, 30 March 2004 19:27:07 EST

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