"Purpose" is a scientific and educational book. Now these two concepts are used so often that their original meaning is lost in the fog of excessive servility and mystery. Like most serious scientists, I think that science has nothing to do with the secrets of nature, nor with immutable truths. This is simply a method by which we operate with a minimal set of assumptions that can be used to directly substantiate the existence of many natural phenomena through direct logical constructions. The physical law of conservation of energy is by no means the truth. This is just an assumption, effectively explaining a huge number of natural phenomena. To prove the universal applicability of this assumption is impossible, no matter how concrete examples of its fairness you cite. But to refute this assumption can be a single fact that it would not be able to explain. But even such a refutation does not deprive the assumption of justice. It testifies to the need for another, more general assumption. It is so that the law of conservation of energy was replaced by a more general Einstein postulate of conservation of energy and mass. Einstein's law is not true to the same extent as his predecessor.