1. C. N. Yang andR. L. Millis:Phys. Rev.,96, 191, (1954).
2. C. N. Yang:Phys. Today (June, 1980), p. 43.
3. C. N. Yang:Ann. N. Y. Acad. Sci.,294, 86 (1977).
4. See, for example,V. F. Weisskopf:Phys. Today (November, 1981), p. 69.
5. P. A. M. Dirac: inThe Physicist’s Conception of Nature, edited byJ. Mehra, (Boston, Mass., 1973), p. 52; see alsoR. J. Adler, M. J. Bazin andM. Schiffer:Introduction to General Relativity (New York, N. Y., 1975), p. 491.