details the course of each process by the principle that among all the conceivable processes which can change the state of a given physical structure into another state during a certain time interval, the process which actually takes place is always ‘one for which the integral over that time interval of a certain magnitude, the so-alled Lagrange function, has the smallest value, Therefore, if we know the value of the Lagrange function, we ean fully specify the course of the process actually taking place. It is certainly no wonder that the discovery of this law—the so-called least-action principle, after which the elementary quantum of action was’ later also named—made its discoverer Leibniz, and soon aiter him also his follower Maupertuis, 80 boundlesly enthusiastic; for these scientists believed themselves to have found in it a tangible evidence for a ubiquitous higher reason ruling all nature, In fact, the leastaction principle introduces a completely new idea into the concept of causality: ‘The causa effciens, which operates from the pres- ent into the future and makes fucure situations appear as determined by earlier ones, is joined by