Crossword puzzle clue "he wrote no human thing is of serious importance"

He wrote "No human thing is of serious importance"

Most serious or least serious

Thing of little importance

It could be spread human-to-human

"The only serious thing in the world," per Oscar Wilde

"Much too serious a thing to be left to the military": Clemenceau

The artist ___ wrote a four-book treatise on a) human proportions; b) botany; c) choral music; d) theology

To whom Einstein wrote, "Not often in life has a human being caused me such joy by his mere presence as you did"