 Nonetheless, as in 1920 when he first introduced the term correspondence principle, Bohr is here in 1928 (after the replacement of the old quantum theory by modern quantum mechanics) interpreting the correspondence principle as a connection between transition processes and harmonics in the classical motion of the particles. Moreover, just as in 1920, he is explicitly tying the correspondence principle to his view that quantum mechanics is a rational generalization of classical mechanics. The correspondence principle thus provided a conceptual link between classical mechanics (and electrodynamics), the old quantum theory, and modern quantum mechanics. This rational generalization thesis in turn was the foundation of Bohr’s views on complementarity, and more broadly, his Copenhagen interpretation of quantum mechanics. Thus the correspondence principle was not only technically important in the discovery of modern quantum mechanics, as Jammer (1966, p. 118) has emphasized, but it was also was the very foundation of Bohr’s philosophy.