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Very nice obituary at the Guardian of David Lewis, the Princeton metaphysician famous (for a philosopher) for his stalwart defense of the existence of real, live alternative dimensions. On Lewis's theory, any state of affairs that is possible is actual on some world or other that differs from this one only in not being this one. So there is a possible world with David Lewis still on it (his Lewisian “counterpart” at any rate) and that world is richer philosophically than ours.