[foaf-dev] Group vs Class

Ian Davis me at iandavis.com
Fri Jun 11 18:07:22 CEST 2010

Hi Dan,

I was hoping you would have thoughts on this :)

On Fri, Jun 11, 2010 at 2:03 PM, Dan Brickley <danbri at danbri.org> wrote:
> The original reasoning was that groups are in a lowercase-sense
> 'reified' to be things-in-the-world so we can attach extra info to
> them, such as homepages, icons, founders, etc. So in that context is
> more than just the mathematical set of members. However there are many
> cases in FOAF where we want to talk about a group and it is much more
> like a set, like the set of W3C staff for example. In this situation
> the membershipClass construct is somewhat longwinded, and hasn't as
> far as I can see got much adoption. So I think we're missing out here
> on a lot of the power of OWL and the tooling built around it, and I've
> been wondering how to bridge that; perhaps just by saying that it's ok
> to treat a foaf:Group as a class.

That would imply foaf:member could be regarded as the inverse of
rdf:type wouldn't it?

> ( BTW I think there's a parallel discussion to have about FRBR in RDF
> and documents, but let's resist that for now :)

Yes! Let's not complicate things any further :)

> I mentioned this to Dave Reynolds recently, and he mentioned that the
> main problem is around identity conditions; in OWL (or DL flavours at
> least?) classes are identical by membership.

Identical or equivalent?

I suspect the nuance here is that classes are only distinguished by
their members whereas groups may have other distinguishing
characteristics (like a name)

> [snip]

> Here's what Dave said in an offlist discussions (w/ permission here):
>> > And at the back of my
>> > mind is a trickier topic: can a Group simply be a class, eg. a
>> > subclass of Person? Does OWL2-DL hate that kind of thing?
> Dave Reynolds:
>> Don't think is a problem for DL specifically but it is a problem from an
>> ontology design perspective. The question is around identity criteria.
>> In an Ontology (DL or otherwise) the Class is just the set of things in
>> it, two classes with the same members are automatically equivalent
>> classes. Whereas for foaf:Group you want the identity to be independent
>> from the group members (there's much more about this in the OntoClean
>> stuff from Chris Welty et all [1]).

A question for Dave: what are the real consequences of two classes
being equivalent in this case? Are there things you can't say
distinctly about the two sets of individuals?

> [snipped stuff I'll need to think about more]
> cheers,
> Dan


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