 Between 1913 and 1925 Bohr, Arnold Sommerfeld and others were able to improve Bohr’s model, and together with the introduction of spin and Wolfgang Pauli’s exclusion principle it gave a reasonably good description of the basic chemical elements. The model ran into problems, nonetheless, when one tried to apply it to spectra other than that of hydrogen. So there was a general feeling among all leading physicists that Bohr’s model had to be replaced by a more radical theory. In 1925 Werner Heisenberg, at that time Bohr’s assistant in Copenhagen, laid down the basic principles of a complete quantum mechanics. In his new matrix theory he replaced classical commuting variables with non-commuting ones. The following year, Erwin Schrödinger gave a simpler formulation of the theory in which he introduced a second-order differential equation for a wave function. He himself attempted a largely classical interpretation of the wave function. However, already the same year Max Born proposed a consistent statistical interpretation in which the square of the absolute value of this wave function expresses a probability amplitude for the outcome of a measurement.