Well, that was not the only thing I learned from Heisenberg. The main thing that there was to learn was something about which I want to talk and to talk and to talk at the risk that maybe nobody will understand because the number of people who don't understand it right greatly exceeds the number of people that do, and that is Heisenberg's main work, his wonderful work on the Uncertainty Principle. The Theory of Waves and Particles is so constructed that if you know that the hydrogen atom is in its lower state, then you can calculate, without any doubt, the energy. But if you ask another question: the hydrogen atom is in its lower state, where is the electron? Is it at such a distance from the nucleus or at such a distance or at such a distance. The only answer that is furnished by the standard theory is: maybe this, maybe this, maybe this distance, with probabilities that we can predict. Some very important people, like Einstein, did not like this. His remark was- I can believe that God governs the world by any set of laws, but I cannot believe that he's playing a dice. Now, this is an important question. And it is perhaps the most interesting thing I learned in my life. And I want to talk about it. They are abstract ideas, but not complicated ideas. Einstein's statement, his remark, was really something to the effect- I can not imagine any science without causality. What the rules of the causality is, that we can develop, that we can learn. We don't know all of them. But without cause and effect there can be no science. To that question, in the theory not only of Heisenberg, but of his teacher, Niels Bohr, there are two answers. The first answer is this: if you look at a situation and ask all possible questions about it's past, you will find that these questions, without any exception, are consistent with the principles of causality. Ask the same question, not about the past but about the future and the answer about it is- the answer to it is: the future is uncertain. Heisenberg's uncertainty principle. There are many situations, not all but many, where it is impossible to know the present sufficiently accurately to predict the future in a complete manner. The discussion of this uncertainty principle by Heisenberg involves ideas of this kind.