John von Neumann was one of the great scientists of this century. It is a particular pleasure for me to comment on his contribution to mathematical physics since I had the privilege to know him personally. My wife and myself had the pleasure of being invited to one of his splendid dinner parties in his house in Princeton. So I first got to know him as a charming host and only gradually became familiar with some of his many ideas which he spewed like a volcano. Since they contain a legion of facets I shall focus on four points where I think he laid his fingers onto the heart of physics. For a long time his importance for physics was underrated, Pauli once told me that he had said to von Neumann: "If a mathematical proof is what matters in physics you would be a great physicist". I disagree with this statement, I think he had the right vision of what will become important in physics.