Re: Fitch's paradox and OWA

From: Daryl McCullough <>
Date: 31 Dec 2009 11:22:33 -0800
Message-ID: <>

Marshall says...
>On Dec 31, 7:10=A0am, (Daryl McCullough)

>> I don't see a rule saying f |- []f. Where did you see that?
>He didn't say that there was an explicitly stated rule of
>that form. He said that in step 8 of the derivation, they
>use a rule that was explicitly stated as
> If |- f then |- []f
>but they use it *as if* the rule was
> f |- []f

No, I don't think they did that. What they did was to assume K(p & ~K(p)), and show that that leads to a contradiction. That's a proof of ~K(p & ~K(p)). So we have |- ~K(p & ~K(p)). Then we can apply the rule "If |- f, then |- [] f" to conclude []~K(p & ~K(p))

Daryl McCullough
Ithaca, NY
Received on Thu Dec 31 2009 - 20:22:33 CET

Original text of this message