Dear Matthew, (01)
As we've discussed before, I agree that sets are involved --
as they are in the model theory for any logic. (02)
> MW: Also 4D ontologies that use possible worlds can define
> classes as sets, as has been discussed on this list in the past. (03)
Yes. But when you talk about sets in possible worlds or sets
in 4D regions that are inaccessible because they are outside
the time cone of observability, you can't get any information
from them -- except for whatever assumptions you put into
the definitions of those worlds. (04)
> MW: A problem with defining classes as sets for ontologies with other
> foundations is that you cannot talk about imaginary things like unicorns,
> because they all equate to the null set, and other definitions might
> accidentally give identical sets, even though they are not necessarily
> identical. (05)
I agree. (06)
But I would add that you can have possible worlds with non-null sets
of unicorns. However, any data you get about unicorns in those worlds
are just implications of your starting assumptions about them. (07)
John (08)
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