  We shall formulate the postulates, from which the wave equation can be
obtained. In the end it will be understandable, that it is the most natural law
describing the motion of a system. The postulates themselves are simple and
apparent: the first and the fifth postulate set the definition of the
coordinate and the definition of its evolution. The second and the fourth
postulate define the momentum and define its evolution. The third postulate
sets the superposition principle. On the basis of these statements it is
possible to obtain the wave equation.
