 Hans Radder has also argued that Bohr’s understanding of the correspondence principle evolved over his career. Radder (1991) identifies three different phases. In the first phase (from about 1913 to 1915) the correspondence principle was merely a “numerical correspondence”; that is, it concerned a numerical agreement of the values of certain quantities in classical mechanics (and electrodynamics) and the old quantum theory. In the second phase, lasting between 1916 and 1922, the correspondence principle was not just a numerical agreement but involved a conceptual continuity as well: “The same fundamental concepts \(\omega_{n,\tau}\) and \(C_{n,\tau}\), which govern the motion of the electrons in their orbits, and the same function \(f\) determining the transition probabilities are claimed to underlie both kinds of theory” (Radder 1991, p. 206). In the third phase from 1923 until 1925 when the new matrix (quantum) mechanics was introduced, the conceptual continuity was abandoned in favor of a “formal” correspondence. Radder means something different than Darrigol when he describes the correspondence principle as “formal.” He explains,