 Although in hindsight, Pauli claims that both methodologies influenced him, in his correspondence dating from 1924 and 1925, one finds Pauli almost exclusively defending a Sommerfeldian approach, dismissing both the correspondence principle and model-based reasoning more generally. Pauli’s critical remarks about Bohr’s correspondence principle center on two issues. First, Pauli objected to the use of even a “formal” notion of an electron orbit, which is central to the application of the correspondence principle (e.g., Pauli to Bohr, February 21st 1924). Second, Pauli doubted that the correspondence principle would be able to explain the closing of the electron shells as Bohr had hoped. In 1924 Pauli wrote to Bohr: