A lot of people are selling Enlightenment these days. After the Brexit vote and the election of President Trump, David Brooks published a paean to the “Enlightenment project,” declaring it under attack and calling on readers to “rise up” and save it. Commentary magazine sent me a letter asking for a donation to provide readers “with the enlightenment we all so desperately crave.” And now there’s Steven Pinker’s impressive new book Enlightenment Now, which may be the definitive statement of the neo-Enlightenment movement that is fighting the tide of nationalist thinking in America, Britain and beyond.

Do we all crave enlightenment? I don’t. I like and respect Mr. Pinker, Mr. Brooks and others in their camp. But Enlightenment philosophy didn’t achieve a fraction of the good they claim, and it has done much harm.

Boosters of the Enlightenment make an attractive case. Science, medicine, free political institutions, the market economy—these things have dramatically improved our lives. They are all, Mr. Pinker writes, the result of “a process set in motion by the Enlightenment in the late 18th century,” when philosophers “replaced dogma, tradition and authority with reason, debate and institutions of truth-seeking.” Mr. Brooks concurs, assuring his readers that “the Enlightenment project gave us the modern world.” So give thanks for “thinkers like John Locke and Immanuel Kant who argued that people should stop deferring blindly to authority” and instead “think things through from the ground up.”

As Mr. Pinker sums it up: “Progress is a gift of the ideals of the Enlightenment, and will continue to the extent that we rededicate ourselves to those ideals.”

Very little of this is true. Consider the claim that the U.S. Constitution was a product of Enlightenment thought, derived by throwing out the political traditions of the past and applying unfettered human reason. Disproving this idea requires only reading earlier writers on the English constitution. The widely circulated 15th-century treatise In Praise of the Laws of England, written by the jurist and statesman John Fortescue, clearly explains due process and the theory now called “checks and balances.” The English constitution, Fortescue wrote, establishes personal liberty and economic prosperity by shielding the individual and his property from the government. The protections that appear in the U.S. Bill of Rights were mostly set down in the 1600s by those drafting England’s constitutional documents—men such as John Selden, Edward Hyde and Matthew Hale.

These statesmen and philosophers articulated the principles of modern Anglo-American constitutionalism centuries before the U.S. was created. Yet they were not Enlightenment men. They were religious, English nationalists and political conservatives. They were familiar with the claim that unfettered reason should remake society, but they rejected it in favor of developing a traditional constitution that had proved itself. When Washington, Jay, Hamilton and Madison initiated a national government for the U.S., they primarily turned to this conservative tradition, adapting it to local conditions.

Nor is there much truth in the assertion that we owe modern science and medicine to Enlightenment thought. A more serious claim of origin can be made by the Renaissance, the period between the 15th and 17th centuries, particularly in Italy, Holland and England. Tradition-bound English kings, for example, sponsored pathbreaking scientific institutions such as the Royal College of Physicians, founded in 1518. One of its members, William Harvey, discovered the circulation of the blood in the early 17th century. The Royal Society of London for Improving Natural Knowledge, founded in 1660, was led by such men as Robert Boyle and Isaac Newton, decisive figures in physics and chemistry. Again, these were politically and religiously conservative figures. They knew the arguments, later associated with the Enlightenment, for overthrowing political, moral and religious tradition, but mostly they rejected them.

In short, the principal advances that today’s Enlightenment enthusiasts want to claim were “set in motion” much earlier. And it isn’t at all clear how helpful the Enlightenment was once it arrived.

What, then, was “the Enlightenment”? This term was promoted, first and foremost, by the late-18th-century philosopher Immanuel Kant. Mr. Pinker opens his first chapter by endorsing Kant’s declaration that only reason allows human beings to emerge from their “self-incurred immaturity” by casting aside the “dogmas and formulas” of authority and tradition.

For Kant, reason is universal, infallible and a priori—meaning independent of experience. As far as reason is concerned, there is one eternally valid, unassailably correct answer to every question in science, morality and politics. Man is rational only to the extent that he recognizes this and spends his time trying to arrive at that one correct answer.

This astonishing arrogance is based on a powerful idea: that mathematics can produce universal truths by beginning with self-evident premises—or, as Rene Descartes had put it, “clear and distinct ideas”—and then proceeding by means of infallible deductions to what Kant called “apodictic certainty.” Since this method worked in mathematics, Descartes had insisted, it could be applied to all other disciplines. The idea was subsequently taken up and refined by Thomas Hobbes, Baruch Spinoza, John Locke and Jean-Jacques Rousseau as well as Kant.

This view of “reason”—and of its power, freed from the shackles of history, tradition and experience—is what Kant called “Enlightenment.” It is completely wrong. Human reason is incapable of reaching universally valid, unassailably correct answers to the problems of science, morality and politics by applying the methods of mathematics.

The first warning of this was Descartes’s 1644 magnum opus, The Principles of Philosophy, which claimed to reach a final determination of the nature of the universe by moving from self-evident premises through infallible deductions. This voluminous work is so scandalously absurd that no unabridged English version is in print today. Yet Descartes’s masterpiece took Europe by storm and for decades was the main textbook of the Cartesian school of science. Kant followed this dubious example with his Metaphysical Foundations of Natural Science (1786), in which he claimed to have deduced Newton’s laws of motion using pure reason, without empirical evidence.

It was once well understood that much of the modern world’s success grew out of conservative traditions that were openly skeptical of reason. When I was a graduate student at Rutgers in the 1980s, the introductory course in modern political theory had a section called “Critics of the Enlightenment.” These figures included more conservative thinkers such as David Hume, Adam Smith and Edmund Burke. They emphasized the unreliability of “abstract reasoning,” which they believed could end up justifying virtually any idea, no matter how disconnected from reality, as long as it sounded self-evidently true to someone.

One such myth was Locke’s claim that the state was founded on a contract among free and equal individuals—a theory the Enlightenment’s critics understood to be both historically false and dangerous. While the theory did relatively little harm in tradition-bound Britain, it led to catastrophe in Europe. Imported into France by Rousseau, it quickly pulled down the monarchy and the state, producing a series of failed constitutions, the Reign of Terror and finally the Napoleonic Wars—all in the name of infallible and universal reason. Millions died as Napoleon’s armies sought to destroy and rebuild every government in Europe in accordance with the one correct political theory allowed by Enlightenment philosophy. Yet Napoleon was simply trying, in Mr. Brooks’s phrase, to “think things through from the ground up.”

Advocates of the Enlightenment tend to skip this part of the story. Mr. Pinker’s 450-page book doesn’t mention the French Revolution. Mr. Pinker cites Napoleon as an “exponent of martial glory” but says nothing about his launching a universal war in the name of reason. These writers also tend to pass over Karl Marx’s debt to the Enlightenment. Marx saw himself as promoting universal reason, extending the work of the French Revolution by insisting that the workers of the world stop (again in Mr. Brooks’s words) “deferring blindly to authority.” The “science” Marx developed “from the ground up” killed tens of millions in the 20th century.

The Enlightenment also propagated the myth that people’s only moral obligations are those they freely choose by reasoning. That theory has devastated the family, an institution built on moral obligations that many people, it turns out, won’t choose unless guided by tradition. Mr. Pinker’s book is filled with charts showing the improvement in material conditions in recent centuries. He offers us no charts describing the breakdown of marriage or the increase in out-of-wedlock births in “enlightened” societies. Nor is he worried about the destruction of religion or the national state. Kant believed that both were out of conformity with reason, and Mr. Pinker sees no grounds to disagree.

Which brings us to the heart of what’s wrong with the neo-Enlightenment movement. Mr. Pinker praises skepticism as a cornerstone of the Enlightenment’s “paradigm of how to achieve reliable knowledge.” But the principal figures of Enlightenment philosophy weren’t skeptics. Just the opposite: Their aim was to create their own system of universal, certain truths, and in that pursuit they were as rigid as the most dogmatic medievals.

Anglo-Scottish conservatives, from Richard Hooker and Selden to Smith and Burke, were after something very different. They defended national and religious custom even as they cultivated a “moderate skepticism”—a combination the English-speaking world called “common sense.” If old institutions weren’t in evident need of repair, a common-sense view favored leaving them unmolested, since there was always the risk of making things much worse. But it also saw the potential in attempts to improve mankind’s knowledge, so long as the weakness and unreliability of human reason were kept firmly in view. As Newton wrote in his Opticks: “Arguing from experiments and observations by induction be no demonstration of general conclusions, yet it is the best way of arguing which the nature of things admits of.”

I think of these moderate, skeptical words frequently these days, as I follow the political and cultural transformation of the English-speaking world. American and British elites, once committed to a blend of tradition and skepticism, now clamor for Enlightenment. They insist that they have attained universal certainties. They display contempt worthy of Kant himself toward those who decline to embrace their dogmas—branding them “unenlightened,” “immature,” “illiberal,” “backward-looking,” “deplorable” and worse.

If these elites still had access to common sense, they wouldn’t talk this way. Enlightenment overconfidence has gone badly wrong often enough to warrant serious doubts about claims made in the name of reason—just as doubt is valuable in approaching other systems of dogma. Such doubts would counsel toleration for different ways of thinking. National and religious institutions may not fit with the Enlightenment, but they may have important things to teach us nonetheless.

The most important political truth of our generation may be this: You can’t have both Enlightenment and skepticism. You have to choose.

A version of this essay appeared in the Wall Street Journal on April 6, 2018. You can read the original here.