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Some quotes from The Unbearable Lightness of Being by Milan Kundera:

We can never know what to want, because, living only one life, we can neither compare it with our previoius lives nor perfect it in our lives to come.

There is no means of testing which decision is better, because there is no basis for comparison. We live everything as it comes, without warning, like an actor going on cold. And what can life be worth if the first rehearsal for life is life itself? That is why life is always like a sketch. No, "sketch" is not quite the word, because the sketch is an outline of something, the groundwork for a picture, whereas the sketch that is our life is a sketch for nothing, an outline with no picture.