> Thanks John, that makes a lot of sense.
>
>
The way I try to explain it to business domain folks is that if
>
something walks like a duck, quacks like a duck and swims like a
duck
> then it is a member of the set of all things that are a duck.
Assuming
> of course that there is a class of things in the ontology
with the
> properties "walks like a duck" etc.
>
>
Mike
>
> John F. Sowa wrote:
>> Jonathan, Mike,
Pavithra, and Ed,
>>
>> As I said before, my primary
concern was to clarify some confusion
>> about the use of the word
'class'. It is sometimes used as a
>> synonym for 'set',
sometimes for 'type', and sometimes in a way
>> that is not clearly
one or the other.
>>
>> But I admit that the word 'class'
has long been used in various ways
>> in various systems and that
trying to get people to stop using their
>> favorite terminology is
not easy. Therefore, I suggest that the
>> following
convention be used to define the notion of class in
>> whatever
system happens to use the word 'class':
>>
>> 1. If
in system X, the identity conditions for a class are
>>
determined by extension, then a definition of class in
X
>> should begin with a phrase similar to the
following:
>>
>> "Every class in
system X is a set such that...."
>>
>> 2. If in
system X, the identity conditions for a class are
>>
determined by intension, then a definition of class in X
>>
should begin with a phrase similar to the
following:
>>
>> "Every class in
system X is a type such that...."
>>
>> This convention
would allow people to continue to use the word
>> 'class' whenever
they feel the urge to do so, but it would clearly
>> specify
whether a class is considered as a set or as a type.
>>
>>
Some detailed comments on previous comments:
>>
>> JR>
Regarding OWL's choice of 'type' vs. 'class', what one needs to
>>
> know is that RDF already had a notion of "type" when OWL
started
>> > making overtures, so when OWL DL came to be
embedded in RDF, a
>> > different term was needed, because
there were RDF "types" that
>> > were not OWL
"classes"...
>>
>> That indicates that both RDF types and
OWL classes are defined by
>> intension (some rule or description
rather than a set of instances).
>> That would imply that every RDF
type is a type, and every OWL class
>> is a
type.
>>
>> Given the convention above, you could say
something along the
>> following lines:
>>
>>
Every OWL class is a type of entity specified by a
document
>> identified by a particular
URI.
>>
>> MB> I seem to recall that in OWL1, a Class
could be understood both
>> > as extensional (a set of
individuals) and intensional (a class has
>> > a
collection of properties which would define the members of the
>>
> set, i.e. all individuals which have those properties are seen
as
>> > members of that set - so still effectively a set
of individuals,
>> > but arrived at
differently).
>>
>> In linguistics, there is a general
principle that the intension
>> of a word (informally, its
"meaning") determines its extension.
>>
>> For example,
the intensional definition of 'integer' or 'cow'
>> determines the
set of all integers or the set of all cows.
>> If an OWL class is
defined as a type, then the set of all entities
>> of that type
would be the set of instances of that class.
>>
>> PK>
... if you remove that word, it would create a gap from modeling
>>
> to implementation in software world!
>>
>> My
modified recommendation above provides an option for continuing
>>
to use the word 'class' whenever people prefer to use that term.
>>
But it provides a way of stating explicitly whether a class is
>>
considered as a set or as a type.
>>
>> EB> The
percentage of computer science graduate students who are
>>
> incapable of searching the literature that is not available
online
>> > in PDF form must now be well over 75%, judging
from the papers
>> > I have read.
>>
>>
Not only students, but professors as well. The citation
statistics
>> now indicate that for papers published in the same
year, the
>> average number of citations for papers available
online is 10 times
>> the number for papers available only on
paper.
>>
>> EB> ... the concept of abstract types in
programming languages goes
>> > back to 1967 and Simula,
and I have not been able to identify any
>> > earlier
published programming language that has a formal concept
>>
> of abstract type (including a search of Jean Sammet's
survey,
>> > published in 1968-9).
>>
>>
Jean Sammet was not inclined toward formal definitions. Steve
Zilles
>> has a good bibliography of the work in the 1960s and
early '70s:
>>
>>
http://csg.csail.mit.edu/CSGArchives/memos/Memo-75-1.pdf>>
>>
Before he went back to MIT, Steve and I had been designing an
>>
interesting system, but it was declared to be "too difficult"
for
>> the IBM Endicott engineers to understand. That was
probably true.
>> I started scanning in our specification manual
from March 1971:
>>
>>
http://www.jfsowa.com/computer/afs/sl2.htm>>
>>
EB> And therefore, unlike John, I can't fault software
engineering
>> > for having chosen "class" as the term for
"abstract type",
>> > regardless of the usage in other
disciplines.
>>
>> As I said above, I modified my
recommendation to let people continue
>> to use their favorite
terminology, but still clarify whether they
>> mean the word
'class' as a set or as a type.
>>
>>
John
>>
>>
>>
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>
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