> 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.
> 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
>> 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
>> 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
>> 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:
>> 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:
>> 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
>> 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.
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