Re: Where DAML+OIL deviates from the RDF-Schema spec.

>On Sun, 4 Mar 2001, Tim Berners-Lee wrote:
>
> > DAML is in my opinion right.  If a is a subclass of b and b of a mean
> > that a and b are equivalent classes - this is not a bug.  You can't go
> > peperring the underlying logical framework with exceptions just because
> > in some cases a loop is formed by mistake.
>
>Tim, I agree that this isn't a bug. It is a feature, and you may or may
>not like that feature. If you're an implementor features and bugs might be
>difficult to distinguish.
>
>This has very little to do with underlying logical frameworks, but rather
>with the semantics of "subPropertyOf" and "subClassOf" on the one hand,
>and "samePropertyAs" and "sameClassAs", on the other. And in particular if
>samePropertyAs is a subPropertyOf subPropertyOf ;)
>
>I belong to those that feel any set of sub-properties to a certain
>property X, should not contain any property which is equivalent with X.
>This is like the distinction between less than or equal to '<=', or just
>less than '<'. You want subPropertyOf and subClassOf to be like '<='. But
>I want them to be like '<', for reasons similar to those put forward by
>Ian and DanBri, among others.
>
>Your view is perfectly logical, as is that x1 <= x2 && x2 <= x1 implies
>that x1 is equal to x2. However, I want subPropertyOf (X,Y) imply that Y
>has a STRICTLY NARROWER semantics than X, and the same should go for
>sameClassAs.

Well, from a strictly abstract point of view this can be done either 
way, as you point out. But it is usually easier to reason with 
conjunctions than with disjunctions, so I would just on those grounds 
tend to prefer the combination
(<= primitive; < means  (<= AND not =) ) to the other one
(< primitive; <= means (<  OR =)), which would suggest having 
subPropertyOf not be strictly narrower. Also, often one might not 
actually know whether a subclass is strict or not, and it doesnt 
matter to the things in the subclass. A while ago California changed 
its employment laws and many local companies fired or reclassified 
all their part-time employees; with a strictly-narrower 
interpretation you would have to say that
FullTimeEmployee subClassOf Employee suddenly went from true to false 
at that point, but the full-time employees didn't feel the heat.

Pat Hayes

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Received on Monday, 5 March 2001 14:06:16 UTC