  Weyl (1931b, 2 edn) analyzed Dirac’s relativistic theory of the  electron (Dirac (1928a,b)). Although this theory correctly accounted  for the spin of the electron, there was however a problem because in  addition to the positive-energy levels, Dirac’s theory predicted the  existence of an equal number of negative-energy levels. Dirac (1930)  reinterpreted the theory by assuming that all of the negative-energy  levels were normally occupied. The Pauli Exclusion Principle, which  asserts that it is impossible for two electrons to occupy the same  quantum state, would prevent an electron with positive energy from  falling into a negative- energy state. Dirac’s theory also predicted  that one of the negative-energy electrons could be raised to a state  of positive energy, thereby creating a ‘hole’ or  unoccupied negative-energy state. Such a hole would behave like a  particle with a positive energy and a positive charge, that is, like  a positive electron. 