I asked this on stackexchange with no answer.


The negation would be the obvious generalization of Gödel's second incompleteness from r.e. extensions of PA to any arithmetically definable extension of PA.


I see that there is a complete consistent $\Sigma^0\_2$ extension of PA. Said theory therefore contains either the sentence "I am consistent" or "I am inconsistent". However, just because it's consistent and complete doesn't mean it contains the sentence "I am consistent" — it could contain the sentence "I am inconsistent", and remain consistent by being such that any model is a nonstandard model in which there exists a strictly nonstandard proof of its inconsistency



Is there a consistent arithmetically definable extension of PA that proves its own consistency?
Surprisingly, the answer is yes! Well, let me say that the answer
is yes for what I find to be a reasonable way to understand what
you've asked.


Specifically, what I claim is that if PA is consistent, then there
is a consistent theory $T$ in the language of arithmetic with the
following properties:


1. The axioms of $T$ are definable in the language of arithmetic.
2. PA proves, of every particular axiom of PA, that it satisfies the defining property of $T$, and so
$T$ extends PA.
3. $T$ proves that the set of axioms satisfying that definition forms a consistent theory. In other words, $T$ proves that $T$ is consistent.


In this sense, the theory $T$ is a positive instance of what you
request.


But actually, a bit more is true about the theory $T$ I have in
mind, and it may lead you to think a little about what exactly you
want.


* Actually, PA proves that $T$ is consistent.
* Furthermore, the theory $T$ has exactly the same axioms as PA.


I believe that this was observed first by Feferman: *S. Feferman*, [**Arithmetization of metamathematics in a general setting**](http://www.ams.org/mathscinet-getitem?mr=147397), *Fund. Math.* **49** (1960-1961), 35--92. (Thanks to Andreas Blass for pointing out the precise reference.)


The idea of the proof is simple. We shall simply describe the
axioms of PA in a different way, rather than enumerating them in
the usual way. Specifically, let $T$ consist of the usual axioms
of PA, added one at a time, except that we add the next axiom only
so long as the resulting theory remains consistent.


Since we assumed that PA is consistent, it follows that actually
all the axioms of PA will actually satisfy the defining property
of $T$, and so PA will be contained in $T$. Furthermore, since PA
proves of any particular finite number of axioms of PA that they
are consistent, it follows that PA proves that any particular
axiom of PA will be in $T$.


Because of how we defined it, however, it is clear that PA and
hence also $T$ proves that $T$ is consistent, since if it weren't,
there would be a first stage where the inconsistency arises, and
then we wouldn't have added the axiom making it inconsistent.
Almost by definition, $T$ is consistent, and PA can prove that. So
$T$ proves that $T$, as defined by the definition we gave for it,
is consistent. So this theory $T$ actually proves its own consistency!


Meanwhile, let me point out that if one makes slightly stronger
requirements on what is wanted, then the question has a negative
answer, essentially by the usual proof of the second incompleteness theorem:


**Theorem.** Suppose that $T$ is a arithmetically definable
theory extending PA, such that if $\sigma$ is an axiom of $T$,
then $T$ proves that $\sigma$ is an axiom of $T$ and furthermore
PA proves these things about $T$. If $T$ is consistent, then it does not prove its own consistency.


**Proof.** By the Gödel fixed-point lemma, let $\psi$ be a
sentence for which PA proves $\psi\leftrightarrow\ \not\vdash\_T\psi$. Thus, PA
proves that $\psi$ asserts its own non-provability in $T$.


I claim, first, that $T$ does not prove $\psi$, since if it did,
then since $T$ proves that its actual axioms are indeed axioms, it follows that $T$ would prove that that proof is indeed a proof, and so $T$ would prove that $\psi$ is provable in $T$, a statement which PA and hence $T$ proves is equivalent to $\neg\psi$, and so $T$ would also prove $\neg\psi$, contrary to
consistency. So $T$ does not prove $\psi$. And this is precisely
what $\psi$ asserts, so $\psi$ is true.


In the previous paragraph, we argued that if $T$ is consistent,
then $\psi$ is true. By formalizing that argument in
arithmetic, then since we assumed that PA proved our hypotheses on $T$, we see that PA proves that $\text{Con}(T)\to\psi$. So
if $T$ were to prove $\text{Con}(T)$, then it would prove $\psi$,
contradicting our earlier observation. So $T$ does not prove
$\text{Con}(T)$.
**QED**


