"Common friendships can be shared. In one friend one can love beauty; in another, affability; in another, generosity; in another, a fatherly affection; in another a brotherly one; and so on. But this frienship love takes possession of the soul and reigns there with full sovereign sway: that cannot possibly be duplicated. If two friends asked you to help them at the same time, witch of them would you dash to? If they asked for conflicting flavours who would have the priority? If one entrusted to your silence something which it was useful for the other to know, how would you get out of that? The unique, highest friendship loosens all other bonds. That secret witch I have sworn to reveal to no other, I can reveal without perjury to him who is not another: he is me. It is a great enough miracle for oneself to be redoubled (...)"
"For the perfect friendship which I am talking about is indivisible: each gives himself so entirely to his friend that he has nothing left to share with another."
Michel de Montaigne, "On Friendship", (1580 )