Depiction of Louis XVI being led to the guillotine at the time of his execution, Place de la Concord, 1793. Created or published between 1890 and 1920. (Library of Congress Prints and Photographs Division)

In her recent “The Formula for a Richer World? Equality, Liberty, Justice,” Deirdre McCloskey counsels, “The world is rich and will become still richer. Quit worrying.”

Economists such as Thomas Piketty and politicians such as Bernie Sanders have been stressing the dangers of economic inequality. But McCloskey argues that the old maxim is true, and that “a rising tide lifts all boats.”

As the ideas of liberalism and the market-based economy have spread around the globe, she argues, they have greatly improved life for the poor. As she writes, “the problem is poverty, not inequality as such — not how many yachts the L’Oréal heiress Liliane Bettencourt has, but whether the average Frenchwoman has enough to eat.”

This view isn’t an outlier. It’s one that has been increasingly embraced by philosophers, libertarians and economists. And there’s good reason to question it.

Political philosophers from Plato to Adam Smith worried about inequality

McCloskey builds on the ideas of Adam Smith to make her case, emphasizing the role and importance of his ideas in combating poverty over time.

As a historian of political thought, I welcome her emphasis on the ideas of the “great thinkers.” But Smith and other thinkers across the Western tradition understood economic inequality as disruptive on its own terms — above and beyond the problem of poverty.

Consider Smith, whom McCloskey identifies as perhaps the most crucial source for Western ideas about political economy. Smith is indeed concerned about the problem of poverty. In his “Wealth of Nations,” for example, he cautions, “No society can surely be flourishing and happy, of which the far greater part of the members are poor and miserable.” But he is arguably just as concerned about the problem of the rich. In his “Theory of Moral Sentiments,” Smith laments the effects of great wealth on both individuals and society. The more wealth individuals acquire, he writes, the more they succumb to pride and vanity, inflaming their “natural selfishness and rapacity.”

Such personal vices, to his mind, lead to social maladies. The pride and vanity that come with enormous wealth fuel a sense of being above the rules and indifference to those not of their class. It also engenders a perverse worship of the wealthy by the poor, which Smith characterizes as “the great and most universal cause of the corruption of our moral sentiments,” as the poor begin to think of vanity and pride as virtues rather than vices.

Smith is just one figure in a long tradition of concern about economic inequality that begins with the Western tradition’s first systematic political philosopher: Plato. While Plato is commonly associated with the elitism of the “Republic’s” philosopher-rulers, by contemporary standards, he is a radical economic egalitarian. In his “Republic,” Plato cautions against building two parallel cities, “the city of the poor and that of the rich,” where the poor struggle to be productive citizens and plot against the rich — while the rich become luxurious, idle and plot against the poor.

In his later, but less familiar dialogue, “The Laws,” he more thoroughly develops his thoughts on inequality. “It is impossible that those who become very rich become also good,” he stresses repeatedly. He said he believes that wealthy citizens start to feel that their wealth is more valuable and important than their virtue — so they don’t need to follow the laws or other norms that create social harmony. This makes inequality, to his mind, “the greatest of all plagues.” Plato’s prescription is to cap wealth for the richest citizens at four times the wealth of the poorest ones.

Plato’s concern with inequality grew from the experiences of ancient Athens and Sparta. In his “Lives,” Plutarch notes that extreme inequality burdened these city-states. In Athens, “disparity between the rich and the poor had culminated, as it were, and the city was in an altogether perilous condition”; and in Sparta, there persisted a “dreadful inequality,” where “the city was heavily burdened with indigent and helpless people, and wealth was wholly concentrated in the hands of a few.” Celebrated lawgivers Solon, in Athens, and Lycurgus, in Sparta, introduced measures ranging from the cancellation of debts to the redistribution of property. One such law included mandatory common meals: the wealthy had to eat the same foods as the poorest citizens. Because they’d be cut off from the privileges of their relative wealth, they wouldn’t begin to assume their moral superiority.

Contemporary scholarship shows that inequality — not just poverty — has high costs

Contemporary psychologists, public health scholars and political scientists have begun to demonstrate empirically what figures like Solon, Lycurgus, Plato, Smith and Jean-Jacques Rousseau knew all along: Extreme economic inequality and the great wealth it fosters hurt societies, independent of poverty.

Psychology professor Paul K. Piff, for example, has shown that drivers of expensive cars are far less likely to follow traffic rules than those driving less expensive cars. Epidemiologists Richard Wilkinson and Kate Pickett have outlined various ways in which inequality — independent of poverty itself — leads to greater crime rates, worsened physical and mental health, and less social mobility. Political scientists Jacob S. Hacker and Paul Pierson, among others, have argued that great economic inequality has created a political system skewed in favor moneyed interests.

So even if McCloskey were right that we are seeing a significant improvement in the absolute economic status of poor people (though that’s disputed), it would not solve the larger problems associated with substantial inequality. In looking to solve these problems in the future, we would be better off by first understanding the past.

David Lay Williams is professor of political science at DePaul University, and author of “Rousseau’s Platonic Enlightenment” and “Rousseau’s Social Contract: An Introduction.” He is writing “The Greatest of All Plagues: Economic Inequality in Western Political Thought,” which is under contract with Princeton University Press.