creased or diminished by any process in nature, All changes in nature are in reality simply the transformations of one form of energy into an- other. For instance, when kinetic energy is lost by friction, an equivalent quantity of thermal ‘energy results, “The law of the conservation of energy is valid in every branch and field of physics, both ac cording to the classical theory and to quantum mechanics. To be sure, there have often been at- tempts to challenge its precise validity in connec- tion with the processes taking place within a sin- gle atom, and to assign to it a mere statistical significance for such processes. But every single experiment thus far undertaken has shown that such attempts are futile, and there is no reason to deny that the law of the conservation of energy is an absolutely and universally valid law of nature, ‘Those with positivistic leanings counter fre- quently with the critical objection that there is nothing extraordinary about the universal validity of such a law. According to them the explanation ‘of the mystery is simply that after all it is man himself who prescribes the laws for nature, And