“My friend is my other self.” (Pythagoras)

Philos is Greek for friend, and comes from the word Philia, which means friendship. Philia is a major topic in Greek ethics, for it is taken as a kind of virtue and as a necessary condition for happiness. Aristotle chiefly distinguishes three types of friendship: for pleasure, for usefulness, and for goodness. While the first two kinds are incidental, the last kind is complete and perfect, especially between virtuous men, each wishes the other to be good for the good of the other party himself. The root of the perfect friendship is to take the friend as another self.
The just person, Socrates says in the Republic, puts himself in order [kosmêsanta], is his own friend [philon], and harmonizes [sunarmosanta] the three parts of his soul or mind (reason, ambition, and appetite):