Psy-Kosh comments on Belief in Belief - Less Wrong

66 Post author: Eliezer_Yudkowsky 29 July 2007 05:49PM

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Comment author: Quarkster 29 November 2009 02:50:06AM 4 points [-]

"Those who find this confusing may find it helpful to study mathematical logic, which trains one to make very sharp distinctions between the proposition P, a proof of P, and a proof that P is provable"

This is a bit of a side question, but wouldn't a proof that P is provable be a proof of P? In fact, it sounds like a particularly elegant form of proof.

Comment author: Psy-Kosh 29 November 2009 03:11:07AM *  2 points [-]

Hrm... if the system is isn't necessarily trustworthy, then that the system proves that it can prove P doesn't mean that it's actually true that it can prove P, I guess.

EDIT: actually, having it as an explicit axiom "If this proves P, then P" runs you into trouble in any system that has something like Lob's theorem.

("if some specific subset of the rest of this system, (ie, other than this axiom) proves P, then P" can potentially be okay, though)