“What if some day or night a demon were to steal into your loneliest loneliness and say to you: ‘This life as you now live and have lived it you will have to live once again and innumerable times again; and there will be nothing new in it, but every pain and every joy and every thought and sigh and everything unspeakably small or great in your life must return to you, all in the same succession and sequence––even this spider and this moonlight between the trees, and even this moment and I myself. The eternal hourglass of existence is turned upside down again and again, and you with it, speck of dust!’”

That idea comes to us in the shape of a thought experiment, under Aphorism 341, “The Greatest Weight,” from Friedrich Nietzsche’s The Gay Science; it is an idea that has come to be called “eternal recurrence.”