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« Feynman was a truly great teacher. He prided himself on being able to devise ways to explain even the most profound ideas to beginning students. Once, I said to him, Dick, explain to me, so that I can uderstand it, why spin one-half particles obey Fermi-Dirac statistics. Sizing up his audience perfectly, Feynman said, I'll prepare a freshman lecture on it. But he came back a few days later to say, I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it. »

D. L. Goodstein, J. R. Goodstein, Feynman's Lost Lecture: The Motions of Planets Around the Sun, 2014.