Fifty years ago, John Kenneth Galbraith’s “The Affluent Society” soared to the top of the best-seller list. The book has ideas and phrases, like “conventional wisdom,” that have embedded themselves deeply in our culture. Much of “The Affluent Society” is rooted in the 1950s, but the book’s central question remains central today: Should a rich society embrace free-market capitalism and private wealth, or should that society use its wealth for more public purposes like fighting poverty and improving infrastructure?

“The Affluent Society” reflects both the economy and the culture of 1958. The book’s main observation was that America has become unbelievably prosperous. In the 1930s, America wasn’t so rich, and by the 1970s, America’s wealth wasn’t so remarkable anymore. Galbraith beautifully captured that moment in the late 1950s when rising prosperity freed the median American from having real fears about basic necessities.

The book’s tone also reflects the style of the well-educated liberal outsiders of the 1950s, who were themselves the intellectual heirs of pre-war social critics. When Galbraith wittily mocks the conventional wisdom, he has put on Mencken’s mantle. Just as the Sage of Baltimore attracted readers in the 1920s by letting them feel superior to boobs in Tennessee, Galbraith invites readers to look down on the conservatives of his day. He was one of the cadre of public intellectuals of the 1950s, like Richard Hofstadter, and intellectual politicians, like Adlai Stevenson, who offered a seemingly sophisticated alternative to Ike’s military mien and prayer breakfasts.

But while “The Affluent Society” reflects American society in the 1950s, it was quite detached from postwar trends in economics, which is why Galbraith has rarely been embraced by economists. In the 1940s, cutting-edge economists turned to mathematical models and statistics. Galbraith did not. His books are essentially number-free. Even an otherwise glowing review in the New York Times pointed out Galbraith’s tendency to favor witty ripostes over economic rigor.

“The Affluent Society” is better seen as eloquent moral suasion rather than expert economic analysis. Galbraith sees two possible paths for an affluent America, and he strongly favors one of them. “The Affluent Society” argues that America can put its faith in free enterprise, which might create more and more stuff, or it can follow the more morally uplifting path of trying to reduce poverty and improve the quality of life with the help of a more robust public sector. Galbraith is at his most memorable when he denigrates the American combination of private wealth and public poverty: “the family which takes its mauve and cerise, air-conditioned, power-steered and power-braked automobile out for a tour passes through cities that are badly paved, made hideous by litter, blighted buildings, billboards and posts for wires that should have long since been put underground.” Galbraith attacks the relentless pursuit of output, which, as he sees it, only satisfies unnecessary desires ginned up by clever advertisers.

Galbraith’s advocacy of public spending aimed at reducing inequality and improving infrastructure helped usher in the 1960s. Lyndon Johnson’s war on poverty was decidedly Galbraithian. While the New Deal social programs were born of economic desperation, Johnson’s social spending reflected the confidence of prosperity, just as Galbraith had foreseen. But after 1969, the American public gradually turned against Galbraithian social policy. By 1980, Galbraith’s arch-nemesis, Milton Friedman, had found an intellectual home in the White House. In the 1990s, even Democrats embraced private wealth over public spending. But in 2008, “The Affluent Society” seems relevant once more. As the political pendulum swings left, candidates once again call for a more vibrant state to right social wrongs. The excesses of the 1960s are forgotten and once again, the government is seen as society’s savior. For people of all political stripes, it is worthwhile returning to “The Affluent Society,” and pondering what Galbraith got right and what he got wrong.

While I am a staunch supporter of free markets, I agree with Galbraith that there is much the public sector needs to do. Private firms do not automatically provide safe streets, good roads, and clean water. Even more important, Galbraith was dead right in arguing that we need more effective schools. Human capital is our best tool against poverty and economic stagnation.

Galbraith’s great failure was that he never really understood how much society is strengthened by a free and competitive private sector. “The Affluent Society” argues that a lack of regulation made American homes inferior to those in European social democracies. That view was wrong in 1958 and is completely untenable today. American housing is the best in the world, and the weaknesses of the housing market reflect too much, not too little, regulation, especially those rules that stymie construction and make housing unaffordable. While Galbraith was right that some social problems do need a stronger public sector, his analysis would read better today if he had also appreciated the tremendous vitality that comes with economic freedom.

Mr. Glaeser is the Glimp Professor of Economics at Harvard, director of the Taubman Center for State and Local Government, and a senior fellow at the Manhattan Institute.