“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.”

The Labyrinth of Solitude (1950)
Variant: Solitude is the profoundest fact of the human condition. Man is the only being who knows he is alone.
Context: 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.