He had discovered a great law of human action, without knowing it—namely, that in order to make a man or a boy covet a thing, it is only necessary to make the thing difficult to attain.
Our note: After Tom tricks his friends into paying him for the chance to whitewash a fence.
Verified in the full text of The Adventures of Tom Sawyer on Project Gutenberg · Project Gutenberg ebook #74
Painted the same decade: Joan of Arc, Jules Bastien-Lepage, 1879