[FOM] Interpretability in Q

[Daniel Mehkeri]

Richard Heck writes:
 > As I understand him, Nelson is prepared to accept (i) Q and (ii) as
 > much induction as is interpretable in Q

Later:

 > In particular, I\Delta_0 + \omega_1 proves that it can itself be
 > interpreted in Q

But then, that can't be a reason for accepting I\Delta_0 + \Omega_1.

Is there something weaker than I\Delta_0 + \Omega_1 that proves 
I\Delta_0 + \Omega_1 to be interpretable in Q? (And what proves _that_ 
to be interpretable in Q, and so on?)

I scanned _Predicative Arithmetic_ and didn't see this answered. The 
meta-theorems do seem to require some meta-theory like the one 
mentioned, which, for finitary reasons, we know to be interpretable in 
Q. But what is Nelson's reason for accepting it?

Daniel Mehkeri