In September 1961, Richard Feynman gave the first in a series of lectures on basic physics at the California Institute of Technology. At the start of the first day of class, he described the foundations of his subject to almost 200 aspiring scientists (and more than a few of his colleagues): 'The principle of science, the definition, almost, is the following: The test of all knowledge is experiment.' This, he declared, 'is the sole judge of scientific "truth".' Over the next two years, Feynman distilled more than two centuries of knowledge into his lectures. He was a better physicist than he was either a mentor or a man (Feynman was wretched to women and disdainful of his students), yet he remains the greatest teacher most physicists never had. And though he taught his introductory course only once, the magisterial book that derived from it, The Feynman Lectures on Physics (1963-65), still cultivates physicists today.