By way of Maria Popova,1 I came by this fiendish thought experiment proposed by Nietzsche:
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…”
Writer John Kaag2 expands upon this in his new book, Hiking With Nitzsche: Becoming Who You Are:
Are we, in the words of William Butler Yeats, “content to live it all again”? Being content in this sense is not being distracted from, or lulled to sleep by, or resigning oneself to a fate that cannot be avoided. It is to live to your heart’s content with the knowledge that you will do this, and everything, again, forever. We made our last turn into the Waldhaus driveway and came to rest beneath its canopied entryway. Nietzsche suggests that the affirmation of the eternal return is possible only if one is willing and able to become well-adjusted to life and to oneself. To be well-adjusted, for Nietzsche, is to choose, wholeheartedly, what we think and where we find and create meaning. The specter of infinite monotony was for Nietzsche the abiding impetus to assume absolute responsibility: if one’s choices are to be replayed endlessly, they’d better be the “right” ones.
Cuts right through those witless, syrupy admonitions to "live each day as if it were your last" and "be present in your life" and "dance like no one is watching" and "today is the first day of the rest of your life," doesn't it? If you knew you would relive this day as if it were a song on infinite repeat, what would you do with it? If this life was not your one life, but the one life you would live over and over and over and over, how would you live?
Gnaw on that one for a while.
1 Popova's post on Brain Pickings that prompted this entry.
2 I recommend Kaag's American Philosphy: A Love Story.