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Re: comments on http://www.w3.org/TR/2002/WD-rdf-schema-20021112/

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Wed, 27 Nov 2002 16:25:45 +0000
Message-Id: <5.1.0.14.0.20021127154747.0ab62bf8@0-mail-1.hpl.hp.com>
To: "Richard H. McCullough" <rhm@cdepot.net>, <www-rdf-comments@w3.org>

At 06:02 27/11/2002 -0800, Richard H. McCullough wrote:
>Yes, I am assuming that two classes with the same members are the same class.
>
>If that is not true of rdfs:Class,

As I wrote before, it is not.

then either
>1. you are talking about "currently known members" of a class

I don't believe that to be the case, as RDF makes no closed world assumptions.

>or
>2. you are talking about two "different contexts",

I don't believe that to be the case either, as we don't define a concept 
called 'context'.

>i.e., two different ways of viewing the same individuals,

Maybe.

Consider the rdfs:Class A, hypothetically defined by the post office to be 
the rdfs:Class of people whose address has the same zipcode as mine, and 
the rdfs:Class B hypothetically defined by the tax office to be the 
rdfs:Class of people living at the same address as me.  As it happens, 
classes A and B have the same members.

A and B have different properties; they are different things.  rdfs:Class A 
has the property that it is described in a document 
http://example.org/schema/postoffice, a property that is not true of B. 
rdfs:Class B has the property that it is described in a document 
http://example.org/schema/taxoffice, a property that is not true of A.  A 
and B are different things.  They just happen to have the same members.

Let me try and bridge the gap here.  There are two different concepts 
floating around.  I think you call them context and class where a class 
*is* the set of its members.  Two classes with the exactly the same 
membership are the identical.  RDFS has a similar but different model.

In RDFS we have two concepts.  There are rdfs:Class's each of which has an 
associated set that is the set of its members.  But the rdfs:Class is not 
the same thing as the set of its members; to use a term I first heard from 
a colleague that I particularly liked, the set of its members is *nearby*.
Thats just how it works in RDFS.

We are not going to get very far if we get your concept of class mixed up 
with what an rdfs:Class is.

Now I figure we might take this in steps:

   1.  You need to understand what the RDFS model is, and in doing so, we 
need to understand from you what bits of the specs are not clear to you.  I 
think we got the message we should explain the notion of class better :)

   2.  Having figured out what RDFS really is, then you should check it for 
flaws.

You should know that we are (I hope) pretty far down the process of writing 
the spec.  We have decided all our issues and are now just writing them 
up.  What that means is that for comments of the form:

   o Hey, I've got a better idea; its too late for this round - a future 
working group will need to look at those.  You have to understand, if we 
didn't do that we would never get finished.

   o Hey, thats broken because ... we will deal with, though we have to 
hope there will not be too many of those.

>with two different sets of relations to other classes.
>
>If 1. or 2. is not true, then you're not talking about reality.

I am writing about RDFS.  Draw your own conclusions.

Has this helped at all?

Brian
Received on Wednesday, 27 November 2002 11:24:17 GMT

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