> 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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