I've copied and pasted this from this link [ [link to www.thefourprecepts.com] because I couldn't be bothered to type it all up from a book:

"The greatest weight -- What, if some day or night a demon were to steal after you into your loneliest loneliness and say to you: 'This Life as you now live it and have lived it, you will have to live once more and innumerable time more; and there will be nothing new in it, but every pain and every joy and every thought and sigh and everthing unutterable small or great in your life will have to 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!'

Would you not throw yourself down and gnash your teeth and curse the demon who spoke thus? Or have you once experienced a tremendous moment when you would have answered him: 'You are a god and never have I heard anything more divine.' If this thought gained possession of you, it would change you as you are or perhaps crush you. The question in each and every thing, 'Do you desire this once more and inumberable times more?' would lie upon your actions as the greatest weight. Or how well disposed would you have to become to yourself and to life to crave nothing more fervently than this ultimate eternal confirmation and seal?" (#341) [from The Gay Science, by Friedrich Nietzsche]