 The correspondence Bohr describes here is between allowed transitions from one stationary state to another and the harmonic components of the classical motion. Moreover, he takes this correspondence to be the ground or justification for the view that quantum theory is a rational generalization of classical mechanics (for a discussion of this latter view see Bokulich and Bokulich 2005). A few pages later, in a section titled “The Correspondence Principle,” Bohr goes on to describe both the frequency correspondence and the intensity correspondence: