On Jul 19, 2011, at 2:16 AM, sowa@xxxxxxxxxxx wrote:
> It is true that talking about sets of possible worlds has the appearance of
>being extensional, but possible worlds are not observable. (01)
John, you appear to be connecting extensionality with observability. The two
are entirely orthogonal. Distant planets aren't observable either. But the
set of planets is entirely extensional (fuzziness about the concept "planet"
aside). (02)
> They can only be specified by some intensional axioms or descriptions. (03)
That is not true. Lewis provided axioms for worlds in completely extensional
terms, using the (extensional) part-of relation as a primitive. (04)
> In general, any set of possible worlds used for any version of modal logic
>(or any other kind of logic that uses Kripke semantics) can be replaced by
>some set of axioms (or laws) that generate exactly the same results without
>making any assumptions about possible worlds. (05)
True, which means simply that the approaches are equivalent and so the choice
of whether to use worlds or an alternative boils down to pragmatic
considerations. (06)
-chris (07)
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