Well, the leaders of theoretical physics in America at that time were Gregory Breit and Robert Oppenheimer and John Van Vleck. Oppenheimer had written me a letter saying he would invite me, if I got a National Research Council Fellowship. But Breit attracted me more, he was more, I felt, in touch with the great currents of physics. And he was a slow decider, like me. Oppenheimer was all for a quick decision, after, without considering all the sides of a question, as more my spirit. So I picked Breit in preference to Oppenheimer. And Van Vleck, was he a serious contender? And I did not really seriously consider Van Vleck. Just to get into a little bit more substance of physics; during that first year with Breit, you heard a December, 1933, lecture by Oppenheimer on Cosmic Rays and Elementary Particles. That was I think an important one in your thinking and development. Could you discuss that? Yes, Oppenheimer claimed that the existing theory would go wrong at energies of 137 times the mass of the electron. And this packed lecture, one evening, where he made this claim, simply turned me off, because I couldn't see that that reasoning was justified. And, in fact, in the end, it wasn't. The behavior predicted by Quantum Theory at that time turned out to hold at very high energies. And to be an absolute central clue in unraveling the behavior of cosmic rays. Cosmic rays, that was a romantic word in those days. The two great men of experimental physics, Robert Millikan and Arthur Compton, had diametrically opposite views. Millikan thought that the cosmic rays were some kind of electromagnetic or other radiation. And Compton felt that they were particles. Every Physical Society meeting where the two men were present produced, at some point, after somebody's paper, a debate between Compton and Millikan on this point.