[FOM] Proof "from the book" of the incompleteness theorem

[Torkel Franzen]

Arnon Avron says:

 >This is indeed the first proof (out of 3) that Smullyan presents
 >in his book on the incompleteness theorems. The trouble with this 
 >proof is that it misses one of the most important aspects of Godel's
 >proof: the actual construction of a *true* sentence which the system
 >fails to prove, and a *proof* that it is true.

  Godel's proof does not establish that the Godel sentence of a theory
is true. It would be unfortunate if it did, since the Godel sentence
of a theory is sometimes false.