The concept of eigenvalues comes from linear algebra rather than quantum mechanics, and came into the latter through the use of linear algebra and matrices. Arnold Sommerfeld felt that Niels Bohr’s atom could only be approximately correct in that it really had only one quantum condition, other than the use of Planck’s model. Paul Dirac had made significant advances in the foundations of quantum mechanics, and particularly in the mathematical basis of the theory, while the Gottingen group was waiting for their paper to appear in print. “In the present theory, the fundamental principles of quantum theory and the principle of conservation of energy follow mathematically from the quantum-mechanical equations.” Paul Ehrenfest was determined to try to calm the water between Bohr and A. Einstein. The rise of the new quantum mechanics had only served to stir up the debate once again, but Ehrenfest would not give up the task.