In Die Metaphysik der Sitten of 1797, Immanuel Kant writes about friendship. A person feels a strong need to open up to others. But fear of misuse forces him to keep a good part of his judgments, above all about other people, locked inside himself. Ein jeder Mensch hat Geheimnisse und darf sich nicht blindlings Anderen anvertrauen. Every person has secrets and may not confide in others blindly. Moral friendship, Kant writes, ist das völlige Vertrauen zweier Personen in wechselseitiger Eröffnung ihrer geheimen Urteile und Empfindungen, is the complete trust of two persons in opening their secret judgments and feelings to each other. A confidant with these qualities is rare, Kant says, as rare as a black swan. Yet such moral friendship does exist now and then.