When one looks back over the development of physics, one sees that it can be pictured as a rather steady development with many small steps and superposed on that a number of big jumps. Of course it is these big jumps which are the most interesting feature of this development. The background of steady development is largely logical, people are working out the ideas which follow from the previous set-up according to standard methods. But then, when we have a big jump, it means that something entirely new has to be introduced.