Problems of Naturalness: Some Lessons from String Theory
  We consider some questions of naturalness which arise when one considers conventional field theories in the presence of gravitation: the problem of global symmetries, the strong CP problem, and the cosmological constant problem. Using string theory as a model, we argue that it is reasonable to postulate weakly broken global discrete symmetries. We review the arguments that gravity is likely to spoil the Peccei-Quinn solution of the strong CP problem, and update earlier analyses showing that discrete symmetries can lead to axions with suitable properties. Even if there are not suitable axions, we note that string theory is a theory in which CP is spontaneously broken and $\theta$ in principle calculable. $\theta$ thus might turn out to be small along lines suggested some time ago by Nelson and by Barr. 
Michael Dine