The existing theories of the interaction of elementary particles and fields are all unsatisfactory in one way or another. The imperfections may well arise from the use of wrong dynamical systems to represent atomic phenomena, i.e., wrong Hamiltonians and wrong interaction energies. It thus becomes a matter of great importance to set up new dynamical systems and see if they will better describe the atomic world.

Dirac: A Scientific Biography (Cambridge University Press, 1990), p. 197