 In order to understand the substance of the correspondence principle, it is necessary to first understand the basic framework of the old quantum theory. Niels Bohr first articulated the postulates of the old quantum theory in 1913, in a three-part paper titled “On the Constitution of Atoms and Molecules” (Bohr 1913). Bohr had adopted Ernest Rutherford’s model of the atom, according to which most of the mass of the atom is concentrated in a small central nucleus, while the electrons orbit the nucleus in planetary trajectories. The key challenge facing Rutherford’s model was that it was unstable: according to classical electrodynamics, the electron, which is an accelerated charged body, should radiate energy and so rapidly collapse into the nucleus. Bohr’s solution was to incorporate Max Planck’s theory of radiation, which postulates that “the energy radiation from an atomic system does not take place in the continuous way assumed in ordinary electrodynamics, but that it, on the contrary, takes place in distinctly separated emissions” (Bohr 1913, p. 4; BCW 2, p.  164).[1]