The origins of Nietzsche's eternal recurrence of the same.
The thought of eternal return came to Nietzsche in the landscape of the Oberengadin, which Nietzsche visited for the first time during that summer of 1881. The landscape of the Engadin seemed to him one of life's greatest gifts; from that point on it became one of his principal places of work. The thought of eternal return was not discovered in or calculated from other doctrines. It simply came. But like all great thoughts it came only because, surreptitiously, its way had been paved by long labors and great travail. What Nietzsche here calls a “thought” is—grasped in a provisional way—a projection upon beings as a whole, with a view to the question of how being is what it is. Such a projection opens up beings in a way that alters their countenance and importance. Truly to think an essential thought of this sort means to enter into the novel lucidity opened up by the thought; it means to see all things in its light and to find oneself totally ready and willing to face all the decisions implicated in the thought. Of course, we are inclined to take such thoughts as “mere” thoughts, as something unreal and ineffectual. In truth, this thought of eternal return of the same has a shattering impact on all Being. The span of the thinker’s vision no longer ends at the horizon of his “personal experiences.” Something other than he himself looms there, abiding beneath, above, and beyond him, something that no longer pertains to him, the thinker, but to which he can only devote himself. This characteristic of the event [Ereignis] is not contradicted by the fact that the thinker at first and for a long time preserves the insight as totally his own, inasmuch as he must become the site of its development. That is the reason why Nietzsche initially says so little concerning his insight into the “eternal return of the same.”