“Be joyful because it is humanly possible.”

Page 210.
Stepping Westward (1965)

Sir Edmund Hillary : King Of The World
Context: We didn’t know if it was humanly possible to reach the top of Mt. Everest. And even using oxygen as we were, if we did get to the top, we weren’t at all sure whether we wouldn’t drop dead or something of that nature.

As quoted in "What's Possible" in Fast Company (19 December 2007) http://www.fastcompany.com/articles/2001/04/al0401.html
Unsourced variant: Being a champion is redefining what's humanly possible.

Homage to Catalonia (1938)

‘The Monster Squad’ Was Released 25 Years Ago Today: Fred Dekker Talks About the Film’s Reception and Anniversary https://www.slashfilm.com/the-monster-squad-was-released-25-years-ago-fred-dekker-talks/2/ (August 14, 2012)

From Critique of Everyday Life: Volume 1 (1947/1991)
Context: Everything great and splendid is founded on power and wealth. They are the basis of beauty. This is why the rebel and the anarchic protester who decries all of history and all the works of past centuries because he sees in them only the skills and the threat of domination is making a mistake. He sees alienated forms, but not the greatness within. The rebel can only see to the end of his own ‘private’ consciousness, which he levels against everything human, confusing the oppressors with the oppressed masses, who were nevertheless the basis and the meaning of history and past works. Castles, palaces, cathedrals, fortresses, all speak in their various ways of the greatness and the strength of the people who built them and against whom they were built. This real greatness shines through the fake grandeur of rulers and endows these buildings with a lasting ‘beauty’. The bourgeoisie is alone in having given its buildings a single, over-obvious meaning, impoverished, deprived of reality: that meaning is abstract wealth and brutal domination; that is why it has succeeded in producing perfect ugliness and perfect vulgarity. The man who denigrates the past, and who nearly always denigrates the present and the future as well, cannot understand this dialectic of art, this dual character of works and of history. He does not even sense it. Protesting against bourgeois stupidity and oppression, the anarchic individualist is enclosed in ‘private’ consciousness, itself a product of the bourgeois era, and no longer understands human power and the community upon which that power is founded. The historical forms of this community, from the village to the nation, escape him. He is, and only wants to be, a human atom (in the scientifically archaic sense of the word, where ‘atom’ meant the lowest isolatable reality). By following alienation to its very extremes he is merely playing into the hands of the bourgeoisie. Embryonic and unconscious, this kind of anarchism is very widespread. There is a kind of revolt, a kind of criticism of life, that implies and results in the acceptance of this life as the only one possible. As a direct consequence this attitude precludes any understanding of what is humanly possible.