X-Message-Number: 11577
From: Thomas Donaldson <>
Subject: To Mike Perry: why computability is not the same as unreachable worlds
Date: Mon, 19 Apr 1999 23:37:03 +1000 (EST)

To Mike Perry:

While I agree with the arguments about "computable functions" it's not
clear at all that they work the same in the case of possible worlds. Be
warned, however, that when we speak of real worlds rather than
mathematical entities I become very much a constructivist. It is easy
enough to postulate (say) an arbitrary endless decimal number, but unless
you have some means to produce the integers contained in it, you are
postulating far too much for the real world. 

Yes, in one sense we might have a world produced by a number of infinite
length: if we can compute each integer in that number then in a sense we
can characterize that world. The computation rule, of course, would have
to be one we can do, even if it takes increasing time for each integer.

I am happy to put up with nonconstructivist arguments when we do
theoretical math. They bring with them lots of other problems, but also
have some convenience, too. You takes your choice. But when we talk about
the REAL world we are not just doing theory. We want to live and act
in that world.

			Best and long long life,

				Thomas Donaldson

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