Fermi was not the one who discovered the neutrons. He was the one who made the first reasonably large scale nutr- use of them. I want to tell you the occasion when I first heard about neutrons, a little earlier than what I just told you. That was connected with another paper that, to my mind, was very amusing and was connected with a controversy between Heisenberg and an excellent Russian physicist whom I had known in Leipzig and later in Copenhagen, Lev Landau. And I'm anxious to try to tell you, in a qualitative way, what the controversy was and how I got into it. Landau calculated, in a formal way, which I don't want to explain here, that electrons that move freely, as they do in metals, tend to exclude magnetic fields, weaken the penetration of the magnetic fields into the metal. That is called diamagnetism, incidentally, a phenomenon which in its extreme form in superconductivity is known by now very well - the electric fields being- the magnetic fields being completely excluded from the interior of infinitely conducting materials. But Landau had a theory, mathematical theory, calculating how magnetic fields would be weakened as they tried to penetrate into a metal. Heisenberg wouldn't believe it, that cannot happen. Electrons, free electrons, or electrons moving freely, will not weaken magnetic fields, cannot have an effect on magnetic fields. The only way they can have an effect- they could have an effect was through currents. But in statistical physics, inside a metal, if there is a magnetic field, the electrons won't move straight, they will move in circles. But at each place the motion will be due to electrons of two circles, this circle and this circle. And the two circles will give opposite currents at each point. The statistics over all the electrons will not permit the flowing of any circle- or of any current and therefore will not produce, cannot produce, any magnetic effect. I knew Landau, I knew Heisenberg. I told them I will try to understand this and they agreed that it might be a nice thing to look at and I did. And I found Landau was right, Heisenberg was wrong. Yet Heisenberg's argument was a very simple argument. What could- how could it have been possibly wrong?