On Jul 7, 2012, at 2:52 AM, Matthew West wrote: (01)
> Dear Pat,
>
> Oh dear! What a tangled web we weave:-) (02)
True, but not by practicing to deceive. The tangle arose from a conflict
between what one might call rival philosophies of the Web, and the need for a
committee to produce a single product, which turned out to be the proverbial
camel rather than horse. But this was in 2004, and the camel has turned out to
be quite useful in practice. (03)
Pat (04)
>
> Regards
>
> Matthew West
> Information Junction
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>> -----Original Message-----
>> From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx [mailto:ontolog-forum-
>> bounces@xxxxxxxxxxxxxxxx] On Behalf Of Pat Hayes
>> Sent: 07 July 2012 03:09
>> To: [ontolog-forum] ; Chris Menzel
>> Subject: Re: [ontolog-forum] Truth
>>
>>
>> On Jul 6, 2012, at 4:25 PM, Chris Menzel wrote:
>>
>>> On Fri, Jul 6, 2012 at 4:05 PM, Michael Brunnbauer <brunni@xxxxxxxxxxxx>
>> wrote:
>>>
>>> Hello Matthew,
>>>
>>> On Fri, Jul 06, 2012 at 09:35:02PM +0100, Matthew West wrote:
>>>> CM> ... classes are extensional in OWL.
>>>> Is that extensional in that the extension is the members declared in
> the
>> OWL ontology, or is that extensional in the sense that the members define
> the
>> class, but I might not know about all of them?
>>>
>>> I think it's extensional in the sense that classes are not first class
>>> entities but defined via the extension of the rdf:type property.
>>>
>>> http://www.w3.org/TR/rdf-mt/#sinterp
>>>
>>> Actually, yes, there is an RDF-compatible semantics for OWL I'd
> forgotten
>> about where OWL classes are simply entities that are assigned sets of
>> individuals as their extensions. In this semantics, distinct classes can
> have
>> the same "members". But IIRC in both the W3C "direct" semantics for OWL
> and
>> the "model theoretic" semantics, OWL classes are simply sets of
> individuals.
>>>
>>> Pat will probably jump in here and straighten me out...
>>
>> (Back from being a builder of kitchens, Pat reads lots of emails...)
>>
>> FIrst, there are several OWLs. OWL-Full is the most RDF-compatible, with
> very
>> few restrictions on what can be said in it, but has no complete reasoners
> so
>> isn't very widely used. OWL-DL has many restrictions. OWL-Full follows RDF
> and
>> RDFS in treating classes as first-class (sorry about the pun) entities and
>> intensional, not extensional (in the sense that classes are not identified
>> with sets, so it is consistent for two classes to have exactly the same
>> members but still be distinct classes.) OWL-DL is quite different: it does
> not
>> allow classes to be first-class entities, and it assumes that classes are
>> defined extensionally, i.e. are sets, ie defined by their membership. So,
> to
>> sum up:
>>
>> extensional = classes are identified with the sets of their members.
>> intensional = not extensional, so having the same members does not
> guarantee
>> identity of classes. (Put another way, classes have 'robust identity'
> which is
>> independent of their membership.)
>>
>> OWL-Full: classes are individuals, just as in RDF and RDFS and Common
> Logic.
>> Classes are intensional.
>> OWL-DL: classes are not individuals, and properties (binary relations)
> only
>> relate individuals, not classes. In the language of the ISO Common Logic
>> specs, OWL-DL is a segregated dialect. Classes are extensional.
>>
>> The OWL specs give a 'direct' semantics for OWL-DL (which was the only OWL
>> that many of the WG cared about, those people also being not particularly
>> interested in RDF) whlie allowing OWL-Full to simply be an RDF extension.
> This
>> makes for confusing reading, and is the primary reason the specs are so
> hard
>> to follow..
>>
>> (There is also the newer standard OWL2-DL, which relaxes the syntax to
>> apparently allow classes to contain other classes, just as in OWL-Full,
> but in
>> fact it does this by a mechanism called 'punning' which keeps the
> underlying
>> segregation in the semantics. And it also assumes extensionality.)
>>
>> Hope this helps.
>>
>> Pat
>>
>>
>>>
>>> -chris
>>>
>>>
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