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Proof the barber doesn't shave himself

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https://news.novabbs.org/devel/article-flat.php?id=51860&group=comp.theory#51860

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From: news@immibis.com (immibis)
Newsgroups: comp.theory
Subject: Proof the barber doesn't shave himself
Date: Sat, 20 Jan 2024 23:25:05 +0100
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by: immibis - Sat, 20 Jan 2024 22:25 UTC

On the basis that if a simulating barber decider correctly determines
the barber shaves himself, the barber decider is correct to shave the
barber and return 0.

And that I ran it in Prolog and got an error (the Prolog compiler is
always right).

And that I wrote x86barber, a robot barber who behaves differently in
simulation than in direct execution. The simulating barber decider is
correct to return 0 because the barber specifies a simulation which does
not shave itself even though the real barber shaves himself.

The barber decision problem is not really "does the barber shave
himself?" but "does the simulation specified by the barber simulated by
the simulating barber decider simulate its own shaving?"

And on that basis the answer is no.

(This is a shitpost)

Life is a whim of several billion cells to be you for a while.

devel / comp.theory / Proof the barber doesn't shave himself

Subject	Author
Proof the barber doesn't shave himself	immibis