Hi, Jason, and let me join the 'welcoming' members.

(I have no intention to join the professional discussion (Wei Dai et al.) -
what can a former polymer chemist say in it, who's 'scientific'
career.concentrated in  times before the expansion of the computer-usage,
anyway? )
Mark brought my name into your thread so I want to explain what I THINK Mark
might have been referring to (I really do not try to outguess him<G>)
Mark wrote in his post:
"... >Furthermore I am beginning to wonder if the apparently
'straight' and clear cut boundaries to concepts and so forth are not
merely figments of my imagination. I don't think I go anywhere as far as
John M. in this but then maybe that is just because I fear to let go of
my sceptical reductionist walking stick. :-)..."
How far do I think I do go?

In my 5 decades of polymer and technology R&D I arrived at an agnostic
point: to consider common sense instead of the (so far) discovered epistemic
mass in human (scientific??) cognitive inventory. The ever increasing
knowledge-base means a reduced basis for any consciously performed
computation  with the already given (known) conditions. (Cf: Robert Rosen's
unlimited interconnection- based complexity with its Turing  non-emulable
impredicativity).We just don't know (yet?) everything that may be relevant.
Our concept-identifications are formed by the past knowledge-base (from
millennia ago to the preceding second) which is in congruence (in my mind)
with Mark's reductionist walking stick.(maybe 'not quite' the figments of
the  imagination).
I believe in further epistemic enrichment and so whatever we can include NOW
as parameters into our computations will be "improved" later on. I don't
know how Wei Dai's "unbounded memory spread over unbounded space" may work.
but by common sense I hold on to our present ignorance (vs.'later' learning
more) as a support for Mark's uncertainty against our existing 'clear cut'
"I just don't know" (=we are by far not omniscient) is that "all the way" I
go for.
Final (unrestricted) conclusions in ongoing reductionist sciences sound
I cannot help to compare them with earlier 'conclusions' that had to be
changed - and THEN I apply strong induction for the unforeseeable changes
our present wisdom will undergo over future epochs.

People involved in practical developmental work should not listen to me.

John Mikes

On 1/14/07, Wei Dai <[EMAIL PROTECTED]> wrote:

Jason wrote:
> If that is true then my underlying assumptions were flawed.  My
> argument assumed that a non-reversible universe could not be simulated
> by a computer with bounded memory and using only reversible
> computations.  The way I arrived at this assumption was imagining a
> non-reversible universe, such as the John Conway's game of life.  If
> the computer that implements this simulation has limited memory then in
> order for the simulation to continue forever, prior states cannot be
> saved in memory and instead old states would have to be overwritten.
> This destruction of information which cannot be undone would be
> logically irreversible as I understand it.  However if the simulation
> were one where each state has a 1 to 1 mapping, overwritting old states
> does not destroy them forever because previous states could always be
> computed from the current state.

Ok, I understand your argument more clearly now. But, why do you assume a
computer with bounded memory? Even with a finite amount of energy, we can
(theoretically) obtain unbounded memory by spreading it over an unbounded
volume of space. I'd guess that in practice this has approximately the
level of difficulty as achieving an unbounded number of computations from
finite amount of energy.

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