It’s one thing to know a great deal about history, economics, literature, or science. It’s another to be able to communicate that knowledge effectively to a mass audience.

Daniel J. Flynn’s recent book, Blue Collar Intellectuals: When the Enlightened and the Everyman Elevated America, celebrates five cases of 20th-century figures who rose from humble origins to become first-rate thinkers, then focused on sharing their expertise with people from similar backgrounds.

Among those Flynn profiles is the late economist Milton Friedman, who made money early in his life by scooping ice cream, selling fireworks, clerking at a department store, waiting tables, and selling encyclopedias door to door.

Rather than breed resentments, Friedman’s experiences in the free market helped make him one of its most effective exponents. He trusted people to make their own decisions, and mistrusted elites to make decisions for them, because he knew both well. People were not the masses — a glob of humanity to be pushed in a singular direction by their superiors — but individuals with a multitude of interests unmanageable by remote. Friedman’s economics never floated in the ether but instead remained grounded. He lectured his fellow economists that “the only relevant test of the validity of a hypothesis is comparison of its predictions with experience.”

Economics wasn’t a parlor game. He infused common sense into his field — for example, pointing out that flooding the market with money, just as with widgets or apples, leads to depreciation. Friedman understood that economics wasn’t merely about numbers. It was about people. Numbers are predictable; people — not so much. The variables, then, played havoc with the hubristic visions of planners.