 Reading between the lines, Pauli seems to understand Bohr’s correspondence principle as a selection rule about transitions between stationary states that are connected with harmonics in the classical motion. His objection is not to this correspondence principle, but rather with attempts at extending this principle to explain the closing of electron groups in the atom—the sort of extension of the correspondence principle that Pauli elsewhere referred to as “correspondence principle imperialism” (Imperialismus des Korrespondenzprinzips) (Pauli to Heisenberg, February 28th, 1925; quoted in Serwer 1977, p.233). The closing of electron shells would ultimately be explained by Pauli’s exclusion principle, though initially Pauli was dissatisfied with such an explanation insofar as his exclusion rule could not be given a more general grounding and justification. When Bohr suggested that the correspondence principle might provide such a grounding for the exclusion principle, Pauli replied,