  Weyl’s book is one of the first textbooks on the new theory of  quantum mechanics. As Weyl indicates in the preface it was necessary  for him to include a short account of the foundation of quantum  theory in order to be able to show how the theory of groups finds its application in that theory. If the book fulfils its purpose, Weyl  suggests, then the reader should be able to learn from it the  essentials of both the theory of groups and quantum theory. Weyl’s  aim was to explain the mathematics to the physicists and the physics  to the mathematicians. However, as Yang (1986, 10) points out,  referring to Weyl’s book: 