As a body which has been set in motion continues to move as long as no external agency interferes with it, the idea was conceived long ago of constructing an apparatus which should illustrate perpetual motion. The fact was overlooked that every movement meets with external impediments and gradually ceases, unless a new impetus is given to it from without and a new force is introduced to counteract the impediments. Thus, for instance, a pendulum would swing backward and forward for an eternity at the same speed if the resistance of the atmosphere and the friction at the point it hangs from did not gradually deprive it of the mechanical kinetic energy of its motion and convert it into heat. We have to furnish it with fresh mechanical energy by a spring (or, as in the pendulum-clock, by the drag of a weight). Hence it is impossible to construct a machine that would produce, without external aid, a surplus of energy by which it could keep itself going. Every attempt to make such a _perpetuum mobile_ must necessarily fail; the discovery of the law of substance showed, in addition, the theoretical impossibility of it. The case is different, however, when we turn to the world at large, the boundless universe that is in eternal movement. The infinite matter, which fills it objectively, is what we call _space_ in our subjective impression of it; _time_ is our subjective conception of its eternal movement, which is, objectively, a periodic, cyclic evolution. These two "forms of perception" teach us the infinity and eternity of the universe. That is, moreover, equal to saying that the universe itself is a _perpetuum mobile_. This infinite and eternal "machine of the universe" sustains itself in eternal and uninterrupted movement, because every impediment is compensated by an "equivalence of energy," and the unlimited sum of kinetic and potential energy remains always the same. The law of the persistence of force proves also that the idea of a _perpetuum mobile_ is just as applicable to, and as significant for, the cosmos as a whole as it is impossible for the isolated action of any part of it. Hence the theory of _entropy_ is likewise untenable.