The sage of Sils-Maria tells us that this revelation came to him midway between tears and ecstasy, as a fantasy with a real meaning. The crux of his idea is a finite world built up of a finite number of atoms. From the fact that the present state emerges out of the immediately preceding one, the latter from the one just before, and so on, he concludes that the present state exhibits repetition both forwards and backwards. All becoming recurs and moves in a multiple cycle of absolutely identical states. Let us discard for the moment all philosophical objections, above all this, that the recurrence of the same disposition of atoms may not necessarily entail the recurrence of the same psychical states. Furthermore, let us suppress the cynical thought that in the return to the same state the world would have reason to enjoy extreme happiness only for moments, but to lament for aeons. Then we are left with the comparatively simple question: Is this repetition, from the point of view of physics, conceivable and possible? It would be the death-knell of Nietzsche's idea if the answer of a great physical research scientist were entirely in the negative. But Einstein still allows it a small measure of life. "Eternal repetition," so he expressed himself, "cannot be denied by science with absolute certainty." The disciples of Nietzsche will have to rest satisfied with this very small concession. For what, in Nietzsche's eyes, is a logical necessity becomes transformed by Einstein's supplementary remark into a vague assumption, the product of fantasy. From the point of view of physics the recurrence of the same condition is to be regarded as "enormously improbable." This statement is founded chiefly on the famous second Law of Thermodynamics, according to which the processes of Nature are in the main irreversible, so that a one-sided tendency is expressed in natural phenomena. The fact that the course of phenomena is in only one sense or direction speaks in favour of the view that the events of the world are to be regarded as occurring only once. So that when Nietzsche, in contradistinction to this, vigorously supported the doctrine of repetition, he contradicted at least one important recognized theorem of physics. The fact that he did not become conscious of this contradiction, but that, on the contrary, he regarded his idea as the most important event in the development of his intellect, may be regarded as an example of a _docta ignorantia_. But it is allowable, too, that philosophic fantasies that complete the poetical picture of the universe should be given expression. And Nietzsche would presumably have been deprived of a degree of pleasure if he had been aware of this second law. "Truth is the most expedient error"; this statement may be traced back to a sequence of thought developed by Nietzsche. But the Eternal Repetition is shattered by just this remark, for judged by its consequences it would be a very inexpedient error.