Nietsche, as others before and after him (its roots, I am told, are in the West Asian tradition) , espoused the idea of an ever repeating clockwork universe, in which all lives are led over and over; each time identically. In choosing this view, one is compelled to a love of Fate, to -in the language of Zarathustra- choose one's 'downgoing', and indeed much of 'Thus Spoke...' follows from it as a premise (although it is not mentioned until the closing chapters).
Nietsche, not fool enough to believe it could be proved to be reality, presented it (out of Zarathustra's voice) as a thought experiment:
What, if some day or night a demon were to steal after you into your loneliest loneliness and say to you: 'This life as you now live it and have lived it, you will have to live once more and innumerable times more' ... Would you not throw yourself down and gnash your teeth and curse the demon who spoke thus? Or have you once experienced a tremendous moment when you would have answered him: 'You are a god and never have I heard anything more divine.' (The gay science, via wikipedia)
But as a thought experiment, what are its consequences? Certainly they are not the same as those of the premise itself. And has anyone attempted to rehabilitate this notion, in a thought experiment or otherwise, in a manner other than the (fairly obviously futile) direct argument toward the concrete reality of the eternal return?