Solitude is the profoundest fact of the human condition. Man is the only being who knows he is alone, and the only one who seeks out another. His nature – if that word can be used in reference to man, who has 'invented' himself by saying 'no' to nature – consists in his longing to realize himself in another. Man is nostalgia and a search for communion. Therefore, when he is aware of himself he is aware of his lack of another, that is, of his solitude.
The Labyrinth of Solitude (1950)