The discussions in Chap.  2 showed that the distinct fundamental concepts of a particle and a wave from classical physicsClassical physics were challenged by new experimental results at the beginning of the twentieth century. There followed the emergence of new ideas, leading to the unification of these concepts, and to the need to reconstruct a formal framework involving new postulates. This approach led to the postulates of non-relativistic quantum mechanics stated in Chap. 4 .