 Among those who seemed to correctly understand the core of Bohr’s correspondence principle, even fewer embraced it. Indeed many of Bohr’s contemporaries were distrustful of the principle, despite their willingness to opportunistically make use of it to obtain concrete results when other methods failed. Into this third category fall three of the most important contemporaneous critics of the correspondence principle: Arnold Sommerfeld, Wolfgang Pauli, and Werner  Heisenberg.[7]