Re: Proof by construction, again (was Re: [sl4] prove your source code)

From: Tim Freeman (
Date: Sun Jul 20 2008 - 08:30:42 MDT

--- On Sat, 7/19/08, Tim Freeman <> wrote:
> Entity A could prove to entity B that it has source code S by
> consenting to be replaced by a new entity A' that was constructed by
> a manufacturing process jointly monitored by A and B. During this
> process, both A and B observe that A' is constructed to run source
> code S. After A' is constructed, A shuts down and gives all of its
> resources to A'.

From: Matt Mahoney <>
>But A cannot know if S is its own source code. (I assume that S
>includes current state information needed to make a copy of
>itself). If it could know, then A could simulate itself (with
>infinite recursion, which is impossible).

--- On Sat, 7/19/08, Tim Freeman <> wrote:
> The above proof technique also leads to the conclusion that backing
> up my computer is impossible. My computer cannot possibly know all
> of the bits it uses, since if it could, it could simulate itself,
> with infinite recursion, which is impossible.

Then I explained how backups normally happen, why they aren't
really impossible, and what stops the infinite recursion.

From: Matt Mahoney <>
>No. The backup disk is part of the computer. Its original contents are lost.

So you stand by your bold assertion that backing up computers is
impossible. The backup media are defined to be part of the computer
that's being backed up, so the loss of previous contents of the backup
media is a failure of the backup. Odd.

I back up to blank CD's that are not part of the computer. Offsite
backups are both important and possible. If you don't have a regular
and personal reminder that the backup media aren't part of the
computer being backed up, you should improve your backup processes.

The analogy with the original problem is valid -- if raw materials
cannot be found to construct a new A', then there's no analogue of
blank backup media available. Someone would have to reduce their
scope to make room to construct a new A'. This implies that the
process is not free, but it doesn't imply that the process is

>More generally, two machines cannot know each other's states because
>then each machine would have more information than the other (as
>measured by Kolmogorov complexity).

The state of a machine is a function of the machine and the
observation time. Your statement is incoherent because you're
omitting the observation times.

I do not intend to reply to you on this thread any more unless the
quality of your arguments dramatically improves.

Tim Freeman      

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