This contribution is focused on the comparison between book I and book X of Aristotle's Nicomachean Ethics, whrein two different interpretations of the notion of happiness are provided. One can find into book I, in fact, a general account of happiness and of the relations between its own composing parts. Into book X, instead, the content of happiness is specified by a concrete model of complete life, with regard to the problematic implications between political life and contemplative life.