Nevertheless, liberalism has a core, and that is the right of the individual to stand apart. John Stuart Mill’s “On Liberty” is the closest thing liberalism has to a founding tract. Mill set out to explain why it was in the interest of society in general to give individuals the greatest possible right to speak and act as they wish. Individuals, that is, do not have some kind of “natural right” to free speech independent from its social value. Rather, he wrote, mankind is fallible; our saving grace is that our errors are “corrigible.” We acknowledge our fallibility by listening to those with whom we disagree, and testing our ideas against the strongest possible counter-argument. Only thus do we have a chance of approximating, if not actually reaching, the truth.

Read today, this passage sounds as archaic as the chivalric code. In our own world, after all, free speech abounds while the intellectual habits that make free speech actually matter degenerate. The rhetoric of “fake news” turns different sides of the political debate into rival camps, each encased in its own cognitive bubble. In The Open Society, written in the heyday of Nazi Germany, Karl Popper described irrationalism as the sine qua non of the totalitarian state. Popper and Mill compel us to ask an epistemological question: How can the quintessentially rationalist faith of liberalism flourish in an age that systematically demeans rationality?

Whether one begins an account of liberalism with Mill, or Locke, or the Founding Fathers, it is fair to say that all early liberals would have accepted Adam Smith’s proposition that prosperity will be best served if men are given free rein to pursue their self-interest. Yet by the end of the 19th century, as the industrial economy both raised living standards and plunged workers—now equipped with the vote—into appalling conditions in factories and mines, the doctrine of laissez-faire became both politically and morally unsustainable to liberals themselves. In 1909, Herbert Croly published The Promise of American Life, an immensely influential book that argued that Jeffersonian individualism no longer offered a real guarantee of freedom. “The democratic principle requires an equal start in the race,” Croly wrote, but so long as private property was sacred, equal rights could not guarantee equal opportunity to citizens not born to privilege. Liberalism could not be satisfied merely with the promise of equal rights.

The trunk of liberalism now separated into two boughs. One revived the free-market tradition, arguing that political freedom could not flourish absent full economic freedom. This point of view, associated with Austrian economists like Ludwig von Mises and Friedrich Hayek, flourished in the 1920s, but was discredited—or was certainly seen to have been discredited—by the calamity of the Great Depression. It would not reemerge for decades. The other liberalism was buoyed up by FDR’s New Deal and then sustained as the bulwark against totalitarianism by mid-century thinkers like Popper, Isaiah Berlin and George Orwell. This was the moderately interventionist, secular, empirical, pragmatic doctrine that became something like a civic religion in the United States after World War II. The “vital center,” as Arthur Schlesinger called it, occupied a spot midway between the strict individualism of 19th-century England and the collectivist social democracy of post-war Europe.