Nietzsche introduces the eternal return in The Gay Science in a deliberately strange way: as a thought experiment delivered by a demon. Suppose, the demon says, that you must live your life again, in exactly the same way, infinitely many times. Every joy, every pain, every moment of boredom, every regret — again and again, unchanged. How does that make you feel? Nietzsche's point is not that this will happen. The point is that the feeling of your response tells you something true about how you are living.
Review each moment on the turning wheel and answer the question Nietzsche's thought experiment poses: would you live it again, exactly as it was? Joy, pain, and ordinary time all count. The test is not whether you approve of the past, but whether you can affirm the whole life that contains it.
Nietzsche presents the eternal return not as a metaphysical claim but as a thought experiment with maximum stakes. Suppose a demon tells you that you must live this life again, in exactly the same sequence, infinitely. How do you react? The reaction, Nietzsche suggests, is diagnostic. If the thought is crushing, something is wrong with how you are living. If the thought fills you with joy — if you can say 'da capo' to your life — then you are living as you should.
The weight of the past
In Thus Spoke Zarathustra, Nietzsche introduces the idea through the concept of the will to power — not power over others, but the capacity to affirm. The central question is whether you can affirm not just the good things but the whole — the suffering, the humiliation, the loss, the parts you would rewrite if you could. This affirmation is what he calls amor fati: love of fate. Not resignation to it. Not acceptance of it. Love of it, as the specific thing that produced you.
My formula for greatness in a human being is amor fati: that one wants nothing to be different, not forward, not backward, not in all eternity. Not merely bear what is necessary, still less conceal it — but love it. — Friedrich Nietzsche, Ecce Homo
The practical version
The eternal return as a decision heuristic: before acting, ask whether you would be willing to do this thing infinitely many times. This is stricter than 'would this be good for me?' and even stricter than 'would I regret this?' It asks: could I affirm this completely enough to choose it again, without reservation, forever? Most decisions look different under this test. Some things that seemed acceptable become clearly wrong. Some things that seemed small become clearly important.
Nietzsche's own life makes this idea uncomfortable: he spent most of it in pain, socially isolated, increasingly mentally deteriorating. He entered permanent insanity in 1889 and died in 1900 after eleven years of incapacity. The eternal return, as a test he himself set, asks: could he affirm all of this? There is evidence that in his most lucid periods he believed he could. This is either delusional or the most extraordinary act of self-creation in the history of philosophy.
Against regret as a way of life
The eternal return is partly an argument against the habit of retrospective self-punishment — the replaying of mistakes, the wishing things had gone differently, the carrying of the past as a weight rather than a resource. Nietzsche is not saying the past was good. He is saying that your relationship to it determines whether it makes you stronger or simply smaller. The past as something to affirm is a different relationship than the past as something to escape.