W3C home > Mailing lists > Public > www-webont-wg@w3.org > January 2003

Re: Reference, beginnings of HP review?

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Thu, 23 Jan 2003 14:31:54 +0000
Message-Id: <>
To: Guus Schreiber <schreiber@swi.psy.uva.nl>, Jeremy Carroll <jjc@hplb.hpl.hp.com>
Cc: www-webont-wg@w3.org

At 10:26 23/01/2003 +0100, Guus Schreiber wrote:


>I guess this is my fault. I agree the wording should be more careful, 
>maybe something like "One way of looking at a class ..", etc., while also 
>explaining the intensional view. The problem is that if you want to 
>explain to people how OWL classes work, the notion of a class as a set of 
>individuals is an absolute must for making soime sense. How can we 
>otherwise explain, for example, the notion of a owl:Restriction class?


First a suggestion, then an explanation of why I'm making a fuss about this.

Does adopting the following terminology help?

   o classes have instances, sets have members
   o an instance of a class is a member of the class extension of the class

With this we can write things like:

[[An owl restriction expresses a constraint on the instances of a 
class.]]  which is simpler than

[[An owl restriction expresses a constraint on the members of the class 
extension of a class.]]

i.e. it allows us to write in a similar style to treating classes as sets, 
whilst retaining accuracy.  It doesn't work all the time.  Sometimes you 
just have to write explicitly about the class extension of the 
class.  That's the price of accuracy.

I'm making a fuss about this because I think accuracy in the specs is 

As such I think its a bad thing if the reference document says something 
different to the semantics document.  I have observed the vast amount of 
energy that can be wasted interpreting documents that are not clear - viz 
the recent TAG discussions over what a URI denotes and many past 
discussions on RDF interest with regards to reification, contexts, etc.  If 
OWL publishes documents that are inconsistent in their description of what 
a class is, either within themselves or with RDF Schema, then we are 
creating an opportunity for the pointless waste of similar amounts of energy.

As I understand it, an owl:Class is not a set, and as such, the reference 
document should not say that it is, that it can be thought of as if it 
were, that "One way of looking at a class is as a set" or any other such 
weasely phrases.  The reason for this is that such distortions may lead 
people to the wrong conclusions.  For example, if I am encouraged to "look 
at" classes in the way you suggest, then if I have two classes that have 
the same members, the I am also encouraged to "look at" any statement about 
one as also being a statement about the other and that just ain't so.

Why is it so hard to just to tell it as it is?

Received on Thursday, 23 January 2003 09:31:00 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 21:56:50 UTC