Thanks John, that makes a lot of sense. (01)
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. (02)
Mike (03)
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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> (04)
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