“To this end we now need many preparatory courageous human beings who cannot very well leap out of nothing — any more than out of the sand and slime of present-day civilization and metropolitanism: human beings who know how to be silent, lonely, resolute, and content and constant in invisible activities; human beings who are bent on seeking in all things for what in them must be overcome; human beings distinguished as much by cheerfulness, patience, unpretentiousness, and contempt for all great vanities as by magnanimity in victory and forbearance regarding the small vanities of the vanquished; human beings whose judgment concerning all victors and the share of chance in every victory and fame is sharp and free; human beings with their own festivals, their own working days, and their own periods of mourning, accustomed to command with assurance but instantly ready to obey when that is called for, equally proud, equally serving their own cause in both cases; more endangered human beings, more fruitful human beings, happier beings!”

1980s
Context: If you are not at all concerned with the world but only with your personal salvation, following certain beliefs and superstitions, following gurus, then I am afraid it will be impossible for you and the speaker to communicate with each other. …We are not concerned at all with private personal salvation but we are concerned, earnestly, seriously, with what the human mind has become, what humanity is facing. We are concerned as human beings, human beings who are not labelled with any nationality. We are concerned at looking at this world and what a human being living in this world has to do, what is his role?

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.

We Are Doomed: Reclaiming Conservative Pessimism (2009)

Source: We Are All Completely Beside Ourselves

Henry Jekyll's Full Statement of the Case.
Strange Case of Dr Jekyll and Mr Hyde (1886)

Yanni in Words. Miramax Books. Co-author David Rensin

Source: The Sirens of Titan (1959), Chapter 2 “Cheers in the Wirehouse” (p. 52)

Reported in Proceedings in honor of Mr. Justice Frankfurter and distinguished alumni at the meeting of the Council, Harvard Law School Association in Cambridge, April 30, 1960.
Other writings

As quoted in Ingmar Bergman Directs (1972) by John Simon