A generation ago, the philosopher Jeremy Waldron argued that only religious dogma could furnish such a belief. Sagar worries that everyone else (unbelievers as well as Christians who didn’t get the memo) needs to understand why they are committed to equality. But rather than looking for some enduring feature of humanity—our rational faculty, or immortal soul, or physical vulnerability—to justify equality, Sagar argues that it is far better to think of it as a social fiction that humans came to believe, first in Christian form and later in a secular guise. And, he continues, we should not disguise how recently this fiction became popular. (Some might doubt that it ever fully did.)

As brilliant as Sagar’s case is, his focus on the basic equality of human beings says nothing about the rise of the contentious politics of equalization in modern times. After all, the Christian and later secular belief that everyone is in some sense morally equal has generally been compatible with massive inequalities in standing, treatment, and wealth. In fact, humans have had to learn over and over again the idea that they are equal (which Sagar thinks is worth doing). But that doesn’t explain when and why they did, or how they could treat one another so unequally despite such a belief.

Much like McMahon, Teresa Bejan is careful to distinguish between how thinkers conceptualized equality in the ancient world and how they saw it in modern times, focusing on the rise of “parity”: the idea that people in a society ought to be treated as on par. Where notions of equality presuppose sameness, treating people in a “paritarian” spirit presupposes difference. In her discussion of 17th-century England in “What Was the Point of Equality?,” for example, Bejan shows how almost everyone accepted the idea of equality in a post-feudal world: Everyone (at least every white man) was created equal in some sense. But radical groups like the Diggers and the Levellers aimed for something more than equality; they wanted parity between different groups of equal men. According to Bejan, they and their successors in egalitarian movements today speak in a language of equality, but what they want is something more specific: the treatment that some already have and that others should also receive. Equality was and is compatible with disparate treatment, Bejan notes. Parity, on the other hand, insists on placing previously subordinated people on the higher level of the privileged.

Oriented more toward concerns about basic status, neither Bejan nor Sagar examines the left-wing anger toward economic hierarchy and why it, in particular, became pivotal not just for small groups like the Levellers but also for millions in modern times—or why Occupy, “Pikettymania,” and Sanders helped revive a consciousness of it. This question sits at the center of David Lay Williams’s excellent new survey, The Greatest of All Plagues.

Williams says his goal in the book is to draw attention to something that has long been missed: that the West’s canonical political thinkers, not just its lowly workers, have been persistent critics of accumulated wealth. While he starts with Plato and the New Testament, it is his discussion of Rousseau, Marx, and John Stuart Mill that helps Williams register the continuities and changes in reasoning about economic inequality. Plato may have been committed to notions of natural difference, but he was also anxious, Williams observes, about the consequences of too much money concentrated in too few hands and the threats posed by too much poverty to political stability. Rousseau vividly stressed the political costs of economic inequality—especially wealth passed from generation to generation, which established a permanent form of privilege. Even to Plato, it was obvious that envy, if allowed to become supercharged, would fuel a dangerous unrest. But for Rousseau, the dangers went further: The economic inequality of the 18th century’s new era of commerce and opulence also threatened other forms of equality.

Like McMahon, Williams discusses Marx at length in his study. It might still surprise some that Marx made so little of equality—certainly when compared with Rousseau, let alone Gracchus Babeuf, who is often credited as the first communist and called for a “conspiracy of equals” to rescue the French Revolution’s ideals. As McMahon observes, the word “equality” appears just once in The Communist Manifesto, and in a derogatory way. Whereas Rousseau focused constantly on economic inequality as an enormous problem of modern commercial societies, Marx did not. Rather, his concern was unfreedom, especially the alienation of our powers to others in an extractive labor process over which workers have no control.

Williams concedes that freedom is Marx’s preeminent goal, but he also suggests that “Marx’s concern with the problem of extreme economic inequality is a powerful animating principle underlying his critical and constructive political philosophy.” Fair enough: As the contemporary analyst of global inequality Branko Milanović discusses in his own recent survey of modern economists, Visions of Inequality, the North Atlantic of Marx’s era saw a greater and greater disparity in wealth—with the top 1 percent owning 60 percent of wealth in the United Kingdom, for example—and it is natural that economic inequality would serve as a backdrop for his thinking. But this makes it only more important that Marx considered inequality a problem not in itself but because it signaled and symbolized unfreedom: An unequal society is likely to be an unfree one too. But ultimately, for Marx, exploitation—not inequality—is the worst thing that humans can do to one another.

No wonder, then, that Marx’s theory of political economy encompassed a full-scale interpretation of capitalism. He didn’t fixate single-mindedly on the 1 percent, but instead dedicated Capital to the study of everything from the labor process to the systemic dynamics that he thought would unleash political revolution. By the same token, he presumed that communist emancipation might well allow for some inequalities in what people have and get, so long as this remained consistent with free social relations. As McMahon observes, both Vladimir Lenin and Joseph Stalin, faithful to their master, dismissed “equality mongering,” especially if it meant a bourgeois prattling about unfairness that was not willing to entertain the abolition of classes.

Marxist or not, Marx’s point is momentous for closing our decade of concern with inequality. McMahon’s big conclusions from his millennial survey are disarming and depressing. Telling the grand story of equality’s formation over human history, he prefers to stress that equality and inequality are necessarily intertwined—most frighteningly in his chapter on fascism, which sought an equality within the Aryan “race” while exterminating outcasts. (“The uniform makes all men equal,” proclaimed Der Angriff, the National Socialist newspaper.)

McMahon contends that some forms of inequality will persist no matter what kind of egalitarian society a community seeks; humans have evolved with an ambivalence toward equal standing, he argues, precisely because so much turns on competitive preeminence. Whatever the equalities established in a given society, it will always contain some hierarchies, too. If people are never going to be equal in every respect, McMahon continues, then various forms of inequality will persist and perhaps worsen, even as other forms are minimized.

These conclusions of our greatest historian of equality so far are arresting, but what really matters, as Marx said, is eliminating the inequalities that matter. Equality both of status and distribution were breakthroughs, and anger when they are reversed is one of the most promising developments of late. This doesn’t mean that society can or should reach full equality along all dimensions, whatever that might mean. But, as Marx concluded, it must involve getting clear about when and in what forms inequality is an eradicable injustice, something to mobilize against in the name of emancipation and freedom. A few months ago, the historian Quinn Slobodian dismissed the whole concern with inequality since Piketty. It is, Slobodian argued, a diversionary obsession of what the French economist himself called the “Brahmin left”—that is, “highly educated people willing to consume critiques of the system while remaining too materially committed to risk its transformation.”

I am a little more forgiving about the spate of books on inequality—I wrote one myself. But there’s another reason too: The concern with inequality is like a stepping stone to a higher perspective, which means welcoming these histories of inequality for helping readers move beyond their own terms. After all, they force us to decide what deeper problems—including unfreedom on a local and global scale—are even more important.