Now I spent most of my time in Göttingen, two years, in explaining to others what I, myself, have learned in Leipzig. There was one man to whom I needn't explain it, he had been in from the beginning, he knew absolutely everything, and that was Max Born. He was not very much interested in me, nor in a way I in, I in him. He did everything in an exceedingly mathematical manner. Franck and Eucken were much more interested in the connection with a variety of experiments and so I had the opportunity to apply what I learned, what I learned in Leipzig. All this, of course, was a natural thing for me to do because I had started from chemistry. I had started from the very fact whose physics explanation has now become possible. This is, in my opinion, what at least the early parts of quantum mechanics really consisted in. They made out of physics and chemistry one simple unified science. Now, this unity, this connection, between two ways of looking at the same thing, of course, goes back to the very beginning of Bohr's ideas. The name for it is the Correspondence Principle. The law in classical physics and the law in quantum mechanics don't sound the same, they sound quite different. But there is a correspondence between them, so that the laws of quantum mechanics, when many quanta are there, become the laws of physics, of classical physics. Now, what I was con- concerned with was a certain aspect of all this. I came to begin with from a study of chemistry and in chemistry we are, of course, concerned with molecules. And in the molecules, one is concerned with the position of the nuclei of atoms and the connection between- with the glue between these nuclei, the electrons. And here now is a problem whose most simple, practical solution is not classical physics which can't explain everything, not quantum mechanics either, which can explain everything but in a complicated, indeed, in an unnecessarily complicated way. Here, the correspondence principle has a different and very practical meaning. You make most progress if you explain half the problem relating to the behavior of a nuclei, the motion of the center of the atoms. Do that in classical physics. But explain the behavior of the glue that holds the atoms together - the behavior of the electrons - explain that in quantum mechanics. You apply the correspondence principle as the practical means to explain each part of your problem as it can be best explained.
