On Sun, July 8, 2012 22:11, Pat Hayes wrote:
> On Jul 8, 2012, at 3:41 PM, Jack Park wrote: (01)
>> It seems to me that if "MorningStar" is the name given to Venus when
>> it is visible in the morning
>> (http://en.wikipedia.org/wiki/Morning_Star) and if "EveningStar" is
>> the name given to Venus when visible in the evening (linked from same
>> Wikipedia entry), (02)
> OK so far. (03)
Agreed. Note further that the name "Morning Star" can be given to
Mercury when it is visible in the morning (and -- unstated -- Venus is not). (04)
>> then, at least when representing Venus as a topic
>> (class) in a topic map (05)
> AFAIK, topic maps have no coherent semantics, but it seems very odd
> to say that a planet is a class. While not strictly illegal in OWL-Full,
this
> would be very bad ontology design (and it is illegal in OWL-DL). (06)
Agreed. A class is something that can have instances. A planet can not.
However, MorningStar can be a class. At certain times Venus is the sole
instance of that class. At other times Mercury is the sole instance of that
class. Perhaps when neither of them are visible in a clear morning sky,
some other object would fall within the class definition, e.g. "the last
natural visible object that is visually similar to an extra-solar system star
before the sky becomes two bright in the morning." (07)
>> , both are used as *scoped* names for that object. (08)
> I have no idea what ypou mean by a scoped name. (09)
It seems to me that he means that in certain contexts that name
refers to that object, but in others it does not. (010)
> Names in the RDF language family are IRIs, so have no local scope. (011)
The scope (context) is not for the name, but for the name being a
referent for a specific object. You certainly aren't suggesting that
RDF can not have IRIs such as .../PresidentOfIreland or .../KingOfNorway. (012)
-- doug f (013)
>> It's confusing, I suppose, that topic maps use the term
>> "name" for what is known as "label" in OWL, and not for the URI
>> (identifier) of the object. I don't recall reading about scoped
>> labels in OWL. (014)
> AFAIK, there are no scoped labels or names in any OWL dialect.
>
>> Not sure what to make of all this...
>
> I dont think scoping has anything at all to do with the
> extensional/intensional distinction.
>
> Pat
>
>>
>> Jack
>>
>> On Sun, Jul 8, 2012 at 11:46 AM, Obrst, Leo J. <lobrst@xxxxxxxxx> wrote:
>>> Pat,
>>>
>>> So if I understand your statements correctly, two OWL classes
>>> EveningStar and MorningStar will be equal if their extensions are the
>>> same, i.e., {venus}, or:
>>> EveningStar = MorningStar. And that this holds of OWL-DL, but not of
>>> OWL-Full, correct? In OWL-Full, EveningStar \= MorningStar, even if
>>> they have the same extension (apparently because OWL-Full allows
>>> classes to be instances, and that therefore, one does not know if the
>>> extension of a given class includes the instance or the class).
>>>
>>> Thanks,
>>> Leo
>>>
>>> -----Original Message-----
>>> From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx
>>> [mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx] On Behalf Of Pat Hayes
>>> Sent: Sunday, July 08, 2012 11:12 AM
>>> To: [ontolog-forum] ; David Price
>>> Subject: Re: [ontolog-forum] Truth
>>>
>>>
>>> On Jul 8, 2012, at 8:34 AM, David Price wrote:
>>>
>>>>
>>>> On 8 Jul 2012, at 03:52, Pat Hayes wrote:
>>>>
>>>>>
>>>>> On Jul 7, 2012, at 12:54 PM, Chris Mungall wrote:
>>>>>
>>>>>>
>>>>>> On Jul 6, 2012, at 7:08 PM, Pat Hayes wrote:
>>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> To be pedantic - in OWL-DL there are object properties (individual
>>>>>> to individual), data properties (individuals to literals) and
>>>>>> annotation properties (these are invisible in the direct semantics,
>>>>>> but in practical terms these can link classes, provided you don't
>>>>>> need inferences from them)
>>>>>>
>>>>>> Regarding classes being the same as their extents in OWL: I don't
>>>>>> think this view is universally shared.
>>>>>
>>>>> Well, I havnt checked the OWL2 specs in detail, I confess, but it is
>>>>> certainly true in the original OWL-DL, stated quite explicitly in the
>>>>> semantics. Mathematical statements in a normative specification are,
>>>>> fortunately, not "views" to be shared or not, at will.
>>>>
>>>>
>>>> The OWL 1 Language Reference says:
>>>
>>> Yes, this is for all the OWLs, so to speak, as a general statement.
>>> OWL Full does indeed treat classes intensionally. OWL-DL, however,
>>> treats them extensionally. See the 'direct semantics' (which is
>>> normative) for OWL-DL in
>>> http://www.w3.org/TR/owl-semantics/direct.html, where the
>>> interpretation of a class name is simply a subset of the universe. That
>>> is an extensional meaning for classes. RDFS and the RDF-based semantics
>>> for OWL both distinguish between the class itself I(<name>) and the
>>> class extension CEXT(I(<name>)), which distinction allows for an
>>> intensional interpretation.
>>>
>>> Pat
>>>
>>>
>>>> 3. Classes
>>>>
>>>> Classes provide an abstraction mechanism for grouping resources with
>>>> similar characteristics. Like RDF classes, every OWL class is
>>>> associated with a set of individuals, called the class extension. The
>>>> individuals in the class extension are called the instances of the
>>>> class. A class has an intensional meaning (the underlying concept)
>>>> which is related but not equal to its class extension. Thus, two
>>>> classes may have the same class extension, but still be different
>>>> classes.
>>>>
>>>> So, if "Classes are extensional" means two OWL 1 classes with the same
>>>> extent are the same class, then clearly OWL 1 classes, while having
>>>> extents, are not extensional - or else this paragraph in the OWL 1 LR
>>>> is wrong. FWIW I checked the errata and this paragraph is not
>>>> mentioned so it seems to stand as-is.
>>>>
>>>> The OWL 2 new features document claims "More importantly, backwards
>>>> compatibility with OWL 1 is complete, both syntactically and
>>>> semantically." even though I can't find any mention of the intensional
>>>> meaning vs. class extension relationship in any of the OWL 2
>>>> documents. So what does Pat's "assumption of extensionality" mean wrt
>>>> OWL 1 and OWL 2 and the question of whether two classes with the same
>>>> extent are the same class?
>>>>
>>>> Cheers,
>>>> David
>>>>
>>>>>
>>>>> The "argument" given in the blog cited below is completely spurious:
>>>>> it is based on a common misunderstanding about model theory, that the
>>>>> individuals in models are "mathematical" entities rather than real
>>>>> things in the world, which is complete nonsense. It (the cited blog)
>>>>> also confuses extensionality with the idea of knowing or explicitly
>>>>> listing the elements of a set.
>>>>>
>>>>> Pat
>>>>>
>>>>>
>>>>>> In fact, one of the authors of the OWL2 direct semantics
>>>>>> specification states otherwise here:
>>>>>> http://ontogenesis.knowledgeblog.org/1004
>>>>>>
>>>>>>> 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..
>>>>>>
>>>>>> Indeed!
>>>>>>
>>>>>>> (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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