 The Bohr-Sommerfeld core model of the atomic structure came into trouble in the beginning of the 1920s due to the fact that it couldn’t handle an increasing number of spectroscopic phenomena. In 1924 Wolfgang Pauli introduced a new degree of freedom according to which two electrons with the same known quantum numbers could not be in the same state. A year later, in 1925, Ralph Kronig, Georg Uhlenbeck and Samuel Goudsmit explained this new degree of freedom by introducing the non-classical concept of electron spin. It has been suggested, however, that Pauli’s proposal meant a lethal blow not only to the Bohr-Sommerfeld model, but also to the correspondence principle because “how to reconcile the classical periodic motions presupposed by the correspondence principle with the classically non-describable Zweideutigkeit of the electron’s angular momentum?” (Massimi 2005, p. 73)