The Gay Science by Friedrich Nietzsche (1882)

Imagine a demon slipping into your loneliest hour to tell you that this life, every pain and joy and unremarkable afternoon of it, you must live again and again in the same sequence, forever. Would you curse the messenger, or fall down grateful? Nietzsche calls this the greatest weight, and he sets it down here for the first time, the eternal recurrence posed not as cosmology but as the hardest question a person can be asked: could you love your existence so completely you would will every detail of it back? The book that holds this question is the one where the madman runs into the marketplace at dawn, lantern lit, howling that we have wiped away the horizon and killed God, and have not yet seen the blood on our hands. And yet its title means joyful knowledge, gai saber, the troubadour's art. Nietzsche wants thought stripped of every comfort to still come out singing. It ends on three Latin words, Incipit tragoedia, and the tragedy that begins is _Thus Spoke Zarathustra_.