Were the law of the conservation of energy really a law abstracted from experiences and absolutely valid, it would be proven by the complete passing over, without any remainder, of one phenomenon of life into another; as, for instance, by the transformation of a process of heat into a process of motion; and physics would have a right to draw the conclusion of an analogy between this and other processes. The play of world-events as pure relation-values, its potential comprehensibility, would be proven by a single transformation without residue, of heat into motion and motion back into heat--that is, by a single _completely reversible process_. But the idea of reversible processes has practical and theoretical possibility only in an absolutely closed system. Such a thing, however, is not to be had in the world of actuality. All things here, without exception, stand in relation to one another, and these mutual relations do not admit of total suspension even for a single moment of time. Thus at no time can one get anything but approximately closed systems; therefore at no time can one attain to anything but approximately correct results. Every attempt to demonstrate practically a completely reversible process works with minimum losses, which the physicist, to be sure, lays to the charge of the procedure adopted, but which the thinker is equally justified in interpreting as a _loss_ of energy. No matter what the exactitude with which the experiment is carried out, no matter how small in value the loss, it is always there; _there is no such thing as a completely reversible process!_ One can only derive a law of the conservation of energy from the facts, if for thought the same is already given. From experiments alone, inductively, it would be as impossible to arrive at a law of the conservation of energy as it would be to arrive at the concept of the circle solely from the concept of the polygon. The circle must be given beforehand as ultimate concept (_Grenzbegriff_); and in exactly the same way the law of the conservation of energy must be given beforehand as ultimate concept (_Grenzbegriff_), if the experiments are to lead up to it. Thus it was with Robert Mayer’s great intuition: it was a thing given. And this intuition was taken up by science and worked out, because here was given it a means of proving with scientific appliances the impossibility of a _perpetuum mobile_. Perpetual motion, however, is the violation of the law of adequate cause, transferred to the domain of the physical. That is one side of the matter. The other is that the law of the conservation of energy conformable with its nature, can only possess validity in the domain of processes reversible and not dependent upon time, for in a non-reversible process there would lie no possibility whatever of its proof.