 An alternative interpretation of Bohr’s correspondence principle has been defended by Olivier Darrigol in his excellent and highly readable history of quantum theory From \(c\)-Numbers to \(q\)-Numbers. Instead of viewing the correspondence principle as a statement about quantum and classical frequencies, he interprets it as what was earlier called the intensity correspondence: The quantum transition probability between two stationary states separated by \(\tau\) is proportional to the squared modulus of the classical amplitude of the \(\tau\)th harmonic vibration, which classically is a measure of the intensity (see Equation 5).