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Re: draft response to #owlref-rdfcore-owl-class-denotation

From: pat hayes <phayes@ihmc.us>
Date: Fri, 13 Jun 2003 18:14:11 -0500
Message-Id: <p05210619bb1007869dc6@[10.0.100.24]>
To: "Jonathan Borden" <jonathan@openhealth.org>
Cc: www-webont-wg@w3.org

>Pat,
>
>May I summarize your post by saying that owl:Class and rdfs:Class need to be
>distinct because the concepts of "Class" in OWL-DL and RDFS are distinct?

Well, the OWL notion defines a subset of the RDFS notion, so 
'distinct' is a bit strong; but yes.

>If so, what about owl:Class in OWL-DL vs. OWL-Full? Ought not OWL-Full be
>defined using rdfs:Class?

That would be a different kind of OWL-Full. It would not satisfy 
Peter's elegant theorem, since in that way of embedding OWL into RDFS 
one could easily tell whether or not one was in OWL_DL or OWL_Full; 
the fact that this is 'invisible' is a feature of the design we have 
come up with. It is however a bug for maintaining a smooth 
interaction between OWL and RDFS.

>If so, when using OWL-Full should owl:Class retain
>its OWL-DL meaning?

That is what I would prefer to have done, yes, and what we still could do.

>  Doesn't this 'break' layering? (perhaps I am being
>naive?)

You are not being naive. It would produce a different kind of 
layering, but one that would be (IMO - and of course nothing here in 
uncontroversial) more 'honest' in allowing the RDFS and OWL-DL views 
of the world to coexist. Call this OWL-in-RDFS; this would actually 
be simpler than OWL Full, but it would have a more complicated 
relationship to OWL-DL. For example, owl:complementOf in OWL-DL would 
have to map to a construction which intersects the complement with 
owl:Thing; the simple complement in OWL-in-RDFS would contain all 
sorts of stuff which wasnt even in the OWL-DL universe.

>It seems to me that the meaning of owl:Class does depend on whether we are
>using OWL-DL vs. OWL-Full in which case the meaning of the URIref depends on
>the framework we are using ... in which case could not the 'meaning' of
>rdfs:Class depend on whether we are using OWL-DL vs. OWL-Full/RDFS?

Well, I suppose it could, but that seems dangerous to me. Some people 
will use the URI in one way, some in another, and there is no way to 
'signal' which sense is intended.

>
>
>however,
>
>>...
>>  And then with these assumptions, we can identify the OWL and RDFS
>>  universes. This hybrid is called OWL Full. It allows the same
>>  syntactic freedom as RDFS - it is in fact an RDF semantic extension
>>  of RDFS - and Peter has proven an ingenious theorem to the effect
>>  that if one restricts oneself to the OWL-DL syntactic case, then
>>  being in OWL-DL and being in OWL Full are indistinguishable states,
>>  as it were, so that OWL-DL is a genuine sublanguage of OWL Full,
>>  which itself is a genuine RDFS semantic extension.
>>
>>  However, notice that there is a genuine tension here. This Full
>>  hybrid asserts that the OWL and RDFS universes are identical; but
>>  they are not, if we understand the OWL terminology according to
>>  OWL-DL. So OWL Full is constructed so that *either* it must be
>>  incommensurate with OWL-DL - ie the OWL terminology must have
>>  different meanings in the two versions of OWL - *or* it is a
>>  restriction of RDFS to the OWL-DL universe, which makes it
>>  incommensurate with much of RDFS - for example, many RDFS logical
>>  truths will be false in OWL Full, under this interpretation. So OWL
>>  Full either breaks with OWL-DL, or amounts to a grand claim that the
>>  entire RDFS universe must be understood as conforming to the limited
>>  OWL-DL 'layered' framework.
>>
>>  It is easy to see that both versions of OWL (DL and Full) could
>>  safely, each in their own terms, merge the concepts of owl:Class and
>>  rdfs:Class without actually breaking 'internally': OWL-DL could
>>  because it is simply unable to express the ways that RDFS classes
>>  differ from OWL classes - what might be called a 'thought police' way
>>  of making the vocabularies similar - and OWL Full could simply
>>  because it is simply asserts that they are identical.  But as I hope
>>  the above makes clear, this assertion would be troublesome for any
>>  kind of consistent interaction between OWL-DL and other RDFS
>  > applications. For example, if OWL-DL were to use the
>>  rdfs:Class/Resource/rdf:Property terminology while in fact referring
>  > only to the OWL-DL subsets of these classes, then OWL-DL theorems
>  > would become sharply false when transcribed into RDFS, and vice
>  > versa; for example,
>  >
>  > _:x rdf:type _:x .
>>  _:x rdf:type rdfs:Class .
>>
>>  is logically true in RDFS (by rules se1 and se2 from the
>>  self-membership of rdfs:Class) but would be a logical contradiction
>>  in OWL-DL if rdfs:Class were identified there with owl:Class. Again,
>>  many other examples could be given.
>>
>
>Understood, but what about 'translating' between OWL-DL and OWL-Full -- same
>problem??? e.g.
>
>_:x rdf:type _:x .
>_:x rdf:type owl:Class .
>
>logically true in OWL-Full, but contradiction in OWL-DL?

Unsayable in OWL-DL: syntactically illegal.

>(I am assuming that
>since owl:Class and rdfs:Class have the same extensions in OWL-Full and
>RDFS,

Well, no, they don't necessarily  have the same extensions in RDFS. 
As far as RDFS is concerned, owl:Class is just another RDFS class 
name.

>that rules se1 and se2 for rdfs:Class apply to owl:Class.)

the se1/2 rules would only get you

_:x rdf:type _:y
from
_:x rdf:type owl:Class .

>
>Jonathan (who doesn't really have a clue about this, but ...)

is doing very well...

Pat


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Received on Friday, 13 June 2003 19:14:13 GMT

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