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I completely agree that the planet Venus is not a class and hence that it has no instances (although there is of course a class whose sole instance is Venus). But whatever one thinks about that, I think it's pretty clear one must make room for classes that can have no instances.
On Jul 10, 2012 1:35 PM, "doug foxvog" < doug@xxxxxxxxxx> wrote:
On Tue, July 10, 2012 14:08, Chris Menzel wrote:
> On Tue, Jul 10, 2012 at 12:56 PM, doug foxvog <doug@xxxxxxxxxx> wrote:
>> On Sun, July 8, 2012 22:11, Pat Hayes wrote:
>>
>>> ...AFAIK, topic maps have no coherent semantics, but it seems very odd
>>> to say that a planet is a class. While not strictly illegal in
>>> OWL-Full,
>>> this would be very bad ontology design (and it is illegal in OWL-DL).
>> Agreed. A class is something that can have instances.
> So there's no (necessarily) empty class, e.g., the class of even primes >
> 100?
Touché!
This gets to the definition of "can have". You understand what an instance
of your proposed class would be. Logically the class is necessarily empty.
However, one would stretch to come up with what "an instance of the
planet Venus" would be.
-- doug f
> -chris
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