 It is unclear how well many of Bohr’s contemporaries understood what he meant by the correspondence principle. For example, even Léon Rosenfeld, one of Bohr’s closest students and collaborators, recalls Bohr’s annoyance over his failure to have correctly understood the substance of this principle (Rosenfeld [1973] 1979, p. 690). This is perhaps not entirely surprising given the turbidity of Bohr’s writings. Bohr himself was quite conscious of the fact that his dense writing style often led others to misunderstand him. In a letter to Arnold Sommerfeld, Bohr laments,