 Most of Bohr’s writings on the correspondence principle are collected in Volume 3 of Niels Bohr Collected Works (abbreviated here BCW), which is titled The Correspondence Principle (1918–1923). As Bohr himself reports (Bohr 1922), the first germ of the correspondence principle can be found in his 1913 lecture “On the constitution of molecules and atoms,” although the term does not appear in his writings until 1920. In the years before Bohr adopted the expression “correspondence principle,” he described the relation as an analogy between classical and quantum mechanics. For example, in his 1918 paper “On the Quantum Theory of Line Spectra.” (which he refers to as “Q.o.L.”) Bohr writes, “It seems possible to throw some light on the outstanding difficulties by trying to trace the analogy between the quantum theory and the ordinary theory of radiation as closely as possible” (Bohr 1918, p. 4; BCW 3, p. 70). In his later writings, however, Bohr explicitly rejects this view that the correspondence principle can be thought of as an analogy. He writes,