The definition of quantum mechanical variables can only be made with the aid of classical physical concepts. These are identical — except for refinements — with the concepts of everyday life. Heisenberg has written: "The concepts of classical physics will remain the basis of any exact and objective science. Because we demand of the results of science that they can be objectively proved (i. e. by measurements, registered on suitable apparatus) we are forced to express these results in the language of classical physics... Thus while the laws of classical physics... appear only as limiting cases of more general and abstract connections, the concepts associated with these laws remain an indispensable part of the language of science without which it would not be possible even to speak of scientific results' 1.