Logic Matters 2008-04-02

Oh, bother ...

Adil Sanaulla has emailed to point out that the Proof Sketch at the bottom of p. 178 of IGT is flawed. I cheerfully say [with an obvious change slight of notation] that "it will be easy to prove that the Sigma_1 wff NPrf(w, x) is equivalent to the Pi_1 wff (Az)(NPrf(w, z) --> z = x)", which is just false. Suppose w is such that NPrf(w, z) is false for any z. Ouch. I guess I must have been bamboozled by Theorem 13.1, unthinkly applying the same argument while forgetting that NPrf(w, x) is not functional.I wonder what the minimum mutilation repair is ...

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