For Aristotle, a friend is "another self," and thus one's own self-conscious awareness is deepened by seeing oneself reflected in one's friend as a mirror. The most profound form of friendship might be called "existential friendship." This is suggested in those passages of the Nicomachean Ethics where Aristotle uses the term to einai for "existence." "Existence is desirable and lovable for all" (1168a1-10). We love existing, and consequently we love those activities through which we exist, and we love other human beings in whom we can see our existence at work. Love of others is an extension of one's self-loving existence. Mothers love their children as extensions of their own self-loving existence, and this mother-child bond is at the origin of all social bonding.
The experience of existential friendship is most fully depicted in the following passage, where the term "existence" (to einai) appears four times (1170a25-b19):
It appears then that life in the ruling sense is sensation or thought. Now if living itself is good and pleasant (and it seems to be so from the fact that all desire it, and those who are decent and blessed most of all, since the life they lead is most choiceworthy and their living is most blessed), and if one who sees is aware that he sees, and one who hears is aware that he hears, and one who walks is aware that he walks, and similarly in the other cases, there is something in us that is aware that we are at work, so that whenever we perceive, we are aware that we perceive, and whenever we think, we are aware that we think, and if being aware that we are perceiving or thinking is being aware that we are (since our existence is a good thing by nature, and it is pleasant to be aware of the the good that is present in oneself), and if being alive is choiceworthy, and especially so for good people, because their existence is good and pleasant for them (since people are pleased by being additionally aware of something that is good in itself), and if a serious person is the same way toward a friend as he is toward himself (since the friend is another self), then just as one's own existence is choiceworthy for each person, so too, or very nearly so, is that of a friend.
But one's existence is choiceworthy on account of the awareness of oneself as being good, and such awareness is pleasant in itself. Therefore, one also ought to share in a friend's awareness that he is, and this would come about through living together and sharing conversation and thinking; for this would seem to be what living together means in the case of human beings, not feeding in the same place like fatted cattle. So if existence is choiceworthy in itself to a blessed person, since it is good and pleasant by nature, and that of one's friend is very nearly the same, then a friend would also be something choiceworthy. But that which is choiceworthy for him ought to be present to him, or he will be deficient in that respect. Therefore, for someone who is going to be happy, there will be a need for friends of serious worth.
Thus, as Aristotle says in the EUDEMIAN ETHICS: "To perceive and to know one's friend is somehow necessarily to perceive and somehow know one's self" (1245a35-38).
This full self-awareness of one's personal existence through activities of sensing and thinking shared with one's friends is said to be a "blessed" (makarios) state, the Greek term for the "blessed ones"--the gods--or for human beings enjoying a fully happy life of unimpeded pleasure in existence. This existential friendship is for Aristotle what the beatific vision of God in Heaven is for Christians (see Augustine, City of God, xxii, 27-30). In Thomas Aquinas's commentary on this passage, he feels compelled to add a qualification that is not found in Aristotle's text: "Here he is discussing the kind of happiness that is possible in this life" (sec. 1911). Obviously, Aquinas is worried that Aristotle's readers might conclude that this existential friendship brings the deepest happiness simply, and so they might fail to see the need for the beatitude of Heaven. Repeatedly, Aquinas has to tell his readers that Aristotle's account of happiness is restricted to earthly happiness, and is therefore inferior to the transcendent happiness of Heaven (secs. 113, 129). Of course, Aristotle never says this. On the contrary, Aristotle suggests that existential friendship achieves the deepest happiness of which human beings are capable, which comes through the full and unimpeded activity of sensing, thinking, and desiring.
Moreover, the happiness of existential friendship does not come from a solitary life of contemplation, which casts doubt on the claim in book 10 of the Ethics that the life of solitary contemplation is the highest. This also denies the famous teaching of Descartes--derived from Augustine--that becoming fully aware of our existence requires a withdrawal from social life into a purely inward experience of one's existence as pure thought thinking itself. For Aristotle, each person's self-conscious existence is a social activity. I think with my friends, therefore I am.
The natural sociality of human intellectual existence is confirmed by the modern Darwinian idea of the "social brain" in neuroscience and evolutionary psychology. The evolution of the primate brain was probably driven, not by the need to understand the complexity of the physical world, but by the need to navigate through the intricacies of the social world, which required the ability to read the minds of one's fellow primates in negotiating the terms of social cooperation.
The discovery of "mirror neurons" indicates that the primate brain has been designed so that a primate individual can enter the minds of other primates by mentally simulating their conscious experiences. The need of primate offspring for prolonged parental care, which included many years of social learning, created evolutionary pressures for the evolution of primate brains capable of what Aristotle describes as existential friendship.
A previous post on sympathy and mirror neurons can be found here.