On Tue, July 10, 2012 14:47, Chris Menzel wrote:
> 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). (01)
If that class were intensionally defined, one could probably generate a
hypothetical world in which that class had multiple instances, a different
instance, or no instances. (02)
> But whatever one thinks about that, I think it's pretty clear
> one must make room for classes that can have no instances. (03)
Certainly classes that logically can have no instances should be accepted.
This is not what i meant. I meant by "a class can have instances" that a
class is a valid argument to an "instance of" predicate. One could,
using the appropriate logical formalism, hypothesize an instance of the
class, maybe assign it some additional properties, and then apply logic
to determine whether the situation was logically consistent. (04)
This could not be done with an individual such as Venus. (05)
-- doug (06)
> 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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> (07)
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