Re: [SMW-devel] Classes vs. Categories

[Jeff T. <je...@th...>]

Jeff Thompson wrote:
> Another example
> of a "class of classes" is Heads of State.  For example, President and Emperor are
> instances of the class Heads of State.  But President itself is *already" a class, which
> can have instances like George Washington.  

Sorry, I screwed up this example.  (I'm even getting confused.)  Head of State is *not*
a second-order class containing President.  It is the superclass of President.  Here's the test:
George Washington is a President, and George Washington is a Head of State.

However, Official Title is a second order class containing instances like President and Emperor.
Here's the test: President is an Official Title, but George Washington is *not* an Official Title.

So SMW should support separate properties for "is a" and "subclass of" (apart from Category) because
we have to handle:
* a page like George Washington is a President, but the President category does not explain
what a president is.  That is on the President *page*, so you want to like there with "is a".
* The President page can link to the Heads of State page through a "subclass of" relation.
* The President page can also link to the Titles page through an "is a" relation.
* Hence, a page can be an individual, a first-order class or a second-order class (like Title), etc.
   and the category system is not able to deal with this.
* The only good argument for using Category for classes is that SMW already supports transitive
   lookup from a category to a super category.  But that is laziness.  We should be able to support
   that "subclass of" is also transitive.

- Jeff