1. A. Einstein, Autobiographical Notes, inAlbert Einstein?Philosopher-Scientist, P. A. Schilpp, ed. (Library of Living Philosophers, Evanston, 1949).
2. T. S. Kuhn,The Structure of Scientific Revolutions, 2nd ed. (University of Chicago, 1970).
3. P. A. M. Dirac,The Principle of Quantum Mechanics, 4th ed. (Cambridge, 1958), Section 81.
4. G. Källen,Kongelige Danske Videnskabernes Selskab 27, No. 12 (1953); reprinted inQuantum Electrodynamics, J. Schwinger, ed. (Dover, 1958), p. 398.
5. R. P. Feynman and A. R. Hibbs,Quantum Mechanics and the Path Integral (McGraw-Hill, 1970), Chapter 9 and p. 260.