Aristotle's identification of happiness with excellent rational activity makes it difficult to see how others could be valuable to the individual in a way that does not reduce them to a mere means to his own flourishing. In Nicomachean Ethics IX.9 Aristotle faces this challenge head on by addressing the question of why the happy and seemingly self-sufficient person needs friends. My dissertation provides a detailed examination of the chapter and establishes three main theses. First, while the friend is a means to the good person's well-being, he is also "another self," one who is in some real sense identified with the good person. Second, Aristotle employs two related notions to account for the identification in question: that of living together (suzēn), and that of a sunaisthetic perception (sunaisthēsis), a special form of perception. When Aristotle speaks of friends living together he refers to their shared participation in excellent practical and theoretical activities, a participation in which the contribution of each individual is informed by that of the other and, thereby, enriched. Each friend becomes, in a sense, identified with the well-being of the other, and is valued by the other as something that is both good and also part of his own flourishing. The cognitive appreciation of the friend as part of one's own flourishing occurs by means of a sunaisthetic perception, a synoptic perception that simultaneously grasps something as both good and also proper to or belonging to oneself. Third, the relationship between suzēn and sunaisthēsis turns out to be a special instance of the activity-pleasure relationship that is central to Aristotle's account of human flourishing.