I don't think that the multiverse account implies that it's irrational to pursue life goals or irrational to believe that pursuing life goals is meaningful or valuable. For even if the multiverse theory is true, I take it that you yourself are confined to one universe, our universe. The beings very similar to you who inhabit other universes are at best "counterparts" of you, which leaves open the question "What will you do with your life?" It may be well and good if one of your counterparts works hard to achieve wisdom, promote justice, or whatever, in some other universe. But his/her hard work isn't yours and doesn't occur in your universe. We need your shoulder to the wheel here!
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Assuming that the multiverse account of the universe is true -- and every possible reality is being simultaneously played out in an infinite number of parallel universes -- am I logically forced into accepting a nihilistic outlook on life? Or is it still possible to accept the truth of the multiverse account and still rationally believe that the pursuit of life goals is both meaningful and valuable, despite the fact that every possible outcome -- or potential reality -- is unfolding somewhere in another parallel universe?