This life, as you live it at present, and have lived it, you must live it once more, and also innumerable times; and there will be nothing new in it, but every pain and every joy and every thought and every sigh, and all the unspeakably small and great in your life must come to you again, and all in the same series and sequence - and similarly this spider and this moonlight among the trees, and similarly this moment, and I myself. The eternal sand-glass of existence will ever be turned once more, and you with it, you speck of dust!On initial exposure to this, it seemed to me rather arbitrary and untenable. Why would everything repeat? What purpose would that serve and what does it even mean? But on further reflection, if we think as physicists and consider reality as a four-dimensional closed manifold where time is just another dimension, well then the universe is really just a shape. And that shape, being outside of time, is eternal; we could view our course through time as simply following that shape. I do not want to dwell on this interpretation of the eternal return, but it does bring it into the realm of physical possibility.
Superficially the eternal return seems similar to reincarnation, but it is really quite different. Reincarnation, as it is usually envisioned, takes place within an arrow of time that does not repeat. The "self" returns to life in different form - as a different person or an animal, typically. Other events do not repeat and the self either has no memory of prior lives or if it does, it is implicit memory (someone or something may seem familiar, but no memory of the pertinent events is available).
The eternal return and reincarnation share a puzzle, though. In neither case are we consciously aware of the repetition, the return. Instead, it seems as though we are de novo in our existence. Consequently, we do not learn from our experience of life. In our everyday life, when we see events as "repeating" they are not exactly the same but we also recall the previous instance to which we link them. Our memory of the new event includes the fact that we remember its predecessor. We can choose to act differently (or the same), and in part such a choice is based on knowledge of the outcome of the previous instance. This connection to previous incarnations is not available in eternal return or reincarnation. Thus the notion of "self" is challenged by these ideas. What does it mean that one's self is reincarnated, or repeated, if there is no recollection of the past self? Isn't that primarily or even entirely what it means to be a self?
The section in which the above quote is found is called "The Greatest Weight." And in some ways it does seem heavy; after all, this is the life you will live, always and forever. Yet, in another sense it eliminates the burden entirely. Our experience of heaviness in life comes from choice and its implications. We want to make the right choice, for we will have to live with it. But the eternal return is the deepest kind of determinism. Not only is our life determined, but we have already lived it the same way an infinite number of times, and it must be always the same. We do not have a "choice," as much as it may seem so. Consequently, there is no weight whatever on the choice, even though our actions may have a certain kind of significance. It may even be lighter than its opposite, where we live but once and one way. On that view, our actions are perhaps devoid of substance, but our choices are weighty, for they are genuine choices and we must live - at least the rest of our natural lives - with the consequences. Furthermore, we will remember that we have so chosen and we can tie the consequences back to the choice. This is real weight.