Re: Can you find anything wrong with this solution to the Halting Problem?

From: Peter Olcott (olcott_at_att.net)
Date: 07/15/04 

Date: Wed, 14 Jul 2004 23:57:24 GMT

> Now suppose you created such a WillHalt program, call it P1. Suppose I then
> take a copy of P1, and make a small change to it so that its output _is_
> accessible to other programs -- I return the output rather than writing it
> to the screen. Call this new program P2. Obviously P2 is susceptible to
> Turing's proof, so P2 cannot exist. But your claim is that P1 can exist, so
> it must be the case that it's impossible to modify P1 as I've described.

No you can go ahead and modify it, and all you succeed in doing
is making another modified version that will no longer work correctly.
This does not change the fact that the original unmodified version
will continue to produce correct results every time.

> But how could this be the case? To change P1 to P2, all I need to do is
> replace a few statements that write a value to the screen with a few
> statements that return that value instead. What could possibly prevent me
> from doing this?
>
> Note that your attempts to make P1 "protected" while it is executing are
> quite beside the point. I'm not claiming that I need to write a program to
> modify P1, or write to P1's memory, or anything like that. I just need to
> you provide me with a copy of P1's source, and I'll change a couple of lines
> in it to produce P2.
>
> --Mark
>
>