  Another time I venture on stage with a book that belongs only half to my professional field of mathematics, the other half to physics. The  external reason is not very different from that which led some time  ago to the origin of the book Raum Zeit Materie. In the  winter term 1927/28 Zürich was suddenly deprived of all  theoretical physics by the simultaneous departures of Debye and  Schrödinger. I tried to fill the gap by changing an already  announced lecture course on group theory into one on group theory and quantum mechanics.   …  Since I have for some years been deeply occupied with the theory of the representation of continuous groups, it appeared to me at this point to be a fitting and useful project, to provide an organically coherent account of the knowledge in this field won by mathematicians, on such a scale and in such a form, that is suitable for the requirements of quantum physics.