S Page said: 
Categories aren't conflated in SMW, they're conflated in MediaWiki, because people use them loosely and inexactly as they make up category hierarchies and assign them.

By 'conflated' I meant that the same term is used in SMW for two different meanings: subclass and instance.  I should have used the more accurate term: overloaded. 
I also now see how the context of a use of Category disambiguates the meaning.  Category in a Category is subclass, Article in a Category is instance. However, as others have said, this is problematic, as it requires people to do things very differently in SMW compared to MW, making the transition from one to another error-prone.
An approach that would reduce this difficulty is to keep the usage of Category the same in SMW as it is in MW, and to create new primitives in SMW that mean exactly what you want them to mean.   E.g. subclass or instance (i.e. rdf:type).
The benefits are:
* old usage patterns don't have to change
* the discipline for distinguishing instance from subclass is enforced with the primitive, rather than relying on people having to follow a cconvention (which is asking for trouble)
There is a cost of course to this, so it is a tradeoff.
One cost is the inability to use the inference that you get with Category.
Others have suggested that other relations shold also be supported for things like transitivity etc.
Perhaps what is needed is some generic relationship-handling code that handles what Category already does, as well a new things. Then the new code could be used to re-implement the same behavior that MW has for Category, but it would also be used for other kinds of relations.

Michael Uschold
M&CT, Phantom Works
425 373-2845

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-----Original Message-----
From: S Page [mailto:skierpage@earthlink.net]
Sent: Saturday, August 04, 2007 6:54 PM
To: Uschold, Michael F
Cc: Clark, Peter E; semediawiki-user@lists.sourceforge.net; Kitzmiller, Ted; Jones, David H; Folger, Deborah H; Murray, William R
Subject: Re: [Semediawiki-user] Semantics of MW Categories and OWL

(Following up only to semediawiki-user)

> So the basic question is: *how can I get the Semantic Media Wiki to
> distinguish different meanings of 'Catagory' that OWL supports and
> that I need* given they seem to be all conflated in SMW.

Categories aren't conflated in SMW, they're conflated in MediaWiki, because people use them loosely and inexactly as they make up category hierarchies and assign them.

As has been discussed on semediawiki-user a few times, SMW's RDF Export assigns very specific meanings to categories
*  an article with a category has <rdf:type> of that category
*  a category with a category has <rdfs:subClassOf> of that category (with a distinction between the pages and the "things" they describe).

These are correct semantics for most usage but it's easy to find other usage.

If you care, I think you have a few choices:

1. Change pages that don't match these semantics to use different relations, such as "is a kind of" or "narrows" instead of Category.  You can use http://ontoworld.org/wiki/Help:Import_vocabulary to get RDF Export to export these relations with names from external ontologies.

The downside is you lose the browsing and navigation of category hierarchies that MediaWiki provides, and the nested searching of subcategories that SMW semantic search provides.

(It would be nice if SMW semantic search could generalize its nested searching of subcategories so that searching on any transitive property would do a similar recursive search.)

2. Keep using Category, but add SMW annotations to unusual articles and categories to express the different semantics.  You'd have to modify SMW_SpecialExportRDF.php to pay attention to the additional annotation and export differently.

3. Modify MediaWiki and SMW to have additional namespaces that work similar to Category but with your desired semantics.

In any approach, good luck coming up with names that make sense to your
wiki's users ;-) .