What is enlightenment? In a 1784 essay with that question as its title, Immanuel Kant answered that it consists of “humankind’s emergence from its self-incurred immaturity”, its “lazy and cowardly” submission to the “dogmas and formulas” of religious or political authority. Enlightenment’s motto, he proclaimed, is: “Dare to understand!” and its foundational demand is freedom of thought and speech.

What is the Enlightenment? There is no official answer, because the era named by Kant’s essay was never demarcated by opening and closing ceremonies like the Olympics, nor are its tenets stipulated in an oath or creed. The Enlightenment is conventionally placed in the last two thirds of the 18th century, though it flowed out of the Scientific Revolution and the Age of Reason in the 17th century and spilled into the heyday of classical liberalism of the first half of the 19th. Provoked by challenges to conventional wisdom from science and exploration, mindful of the bloodshed of recent wars of religion, and abetted by the easy movement of ideas and people, the thinkers of the Enlightenment sought a new understanding of the human condition. The era was a cornucopia of ideas, some of them contradictory, but four themes tie them together: reason, science, humanism and progress.

Foremost is reason. Reason is non-negotiable. As soon as you show up to discuss the question of what we should believe (or any other question), as long as you insist that your answers, whatever they are, are reasonable or justified or true and that therefore other people ought to believe them too, then you have committed yourself to reason, and to holding your beliefs accountable to objective standards.

War was no longer thought of as a divine punishment to be endured and deplored

If there’s anything the Enlightenment thinkers had in common, it was an insistence that we energetically apply the standard of reason to understanding our world, and not fall back on generators of delusion like faith, dogma, revelation, authority, charisma, mysticism, divination, visions, gut feelings or the hermeneutic parsing of sacred texts.

Many writers today confuse the Enlightenment endorsement of reason with the implausible claim that humans are perfectly rational agents. Nothing could be further from historical reality. Thinkers such as Kant, Baruch Spinoza, Thomas Hobbes, David Hume and Adam Smith were inquisitive psychologists and all too aware of our irrational passions and foibles. They insisted that it was only by calling out the common sources of folly that we could hope to overcome them. The deliberate application of reason was necessary precisely because our common habits of thought are not particularly reasonable.

That leads to the second ideal, science, the refining of reason to understand the world. That includes an understanding of ourselves. The Scientific Revolution was revolutionary in a way that is hard to appreciate today, now that its discoveries have become second nature to most of us.

The need for a “science of man” was a theme that tied together Enlightenment thinkers who disagreed about much else, including Montesquieu, Hume, Smith, Kant, Nicolas de Condorcet, Denis Diderot, Jean-Baptiste d’Alembert, Jean-Jacques Rousseau, and Giambattista Vico. Their belief that there was such a thing as universal human nature, and that it could be studied scientifically, made them precocious practitioners of sciences that would be named only centuries later. They were cognitive neuroscientists, who tried to explain thought, emotion and psychopathology in terms of physical mechanisms of the brain. They were evolutionary psychologists, who sought to characterise life in a state of nature and to identify the animal instincts that are “infused into our bosoms”. They were social psychologists, who wrote of the moral sentiments that draw us together, the selfish passions that divide us and the foibles of shortsightedness that confound our best-laid plans. And they were cultural anthropologists, who mined the accounts of travellers and explorers for data both on human universals and on the diversity of customs and mores across the world’s cultures.

The idea of a universal human nature brings us to a third theme, humanism. The thinkers of the Age of Reason and the Enlightenment saw an urgent need for a secular foundation for morality, because they were haunted by a historical memory of centuries of religious carnage: the Crusades, the Inquisition, witch-hunts, the European wars of religion.

They laid that foundation in what we now call humanism, which privileges the wellbeing of individual men, women, and children over the glory of the tribe, race, nation or religion. It is individuals, not groups, who are sentient – who feel pleasure and pain, fulfillment and anguish. Whether it is framed as the goal of providing the greatest happiness for the greatest number or as a categorical imperative to treat people as ends rather than means, it was the universal capacity of a person to suffer and flourish, they said, that called on our moral concern.

Steven Pinker. Photograph: Scott Nobles

Fortunately, human nature prepares us to answer that call. That is because we are endowed with the sentiment of sympathy, which they also called benevolence, pity and commiseration. Given that we are equipped with the capacity to sympathise with others, nothing can prevent the circle of sympathy from expanding from the family and tribe to embrace all of humankind, particularly as reason goads us into realising that there can be nothing uniquely deserving about ourselves or any of the groups to which we belong. We are forced into cosmopolitanism: accepting our citizenship in the world.

A humanistic sensibility impelled the Enlightenment thinkers to condemn not just religious violence but also the secular cruelties of their age, including slavery, despotism, executions for frivolous offences such as shoplifting and poaching and sadistic punishments such as flogging, amputation, impalement, disembowelment, breaking on the wheel and burning at the stake. The Enlightenment is sometimes called the Humanitarian Revolution, because it led to the abolition of barbaric practices that had been commonplace across civilisations for millennia.

If the abolition of slavery and cruel punishment is not progress, nothing is, which brings us to the fourth Enlightenment ideal. With our understanding of the world advanced by science and our circle of sympathy expanded through reason and cosmopolitanism, humanity could make intellectual and moral progress. It need not resign itself to the miseries and irrationalities of the present, nor try to turn back the clock to a lost golden age.

The ideal of progress also should not be confused with the 20th-century movement to re-engineer society for the convenience of technocrats and planners, which the political scientist James Scott calls Authoritarian High Modernism. The movement denied the existence of human nature, with its messy needs for beauty, nature, tradition and social intimacy. Starting from a “clean tablecloth”, the modernists designed urban renewal projects that replaced vibrant neighbourhoods with freeways, high-rises, windswept plazas and brutalist architecture.

“Mankind will be reborn,” they theorised, and “live in an ordered relation to the whole.” Though these developments were sometimes linked to the word progress, the usage was ironic: “progress” unguided by humanism is not progress.

Rather than trying to shape human nature, the Enlightenment hope for progress was concentrated on human institutions. Human-made systems like governments, laws, schools, markets and international bodies are a natural target for the application of reason to human betterment.

In this way of thinking, government is not a divine fiat to reign, a synonym for “society”, or an avatar of the national, religious or racial soul. It is a human invention, tacitly agreed to in a social contract, designed to enhance the welfare of citizens by coordinating their behaviour and discouraging selfish acts that may be tempting to every individual but leave every one worse off. As the most famous product of the Enlightenment, the US Declaration of Independence, put it, in order to secure the right to life, liberty and the pursuit of happiness, governments are instituted among people, deriving their just powers from the consent of the governed.

The Enlightenment also saw the first rational analysis of prosperity. Its starting point was not how wealth is distributed but the prior question of how wealth comes to exist in the first place. Specialisation works only in a market that allows the specialists to exchange their goods and services and Smith explained that economic activity was a form of mutually beneficial cooperation (a positive-sum game, in today’s lingo): each gets back something that is more valuable to him than what he gives up. Through voluntary exchange, people benefit others by benefiting themselves; as he wrote: “It is not from the benevolence of the butcher, the brewer, or the baker that we expect our dinner, but from their regard to their own interest. We address ourselves, not to their humanity but to their self-love.” Smith was not saying that people are ruthlessly selfish, or that they ought to be; he was one of history’s keenest commentators on human sympathy. He only said that in a market, whatever tendency people have to care for their families and themselves can work to the good of all.

Exchange can make an entire society not just richer but nicer, because in an effective market it is cheaper to buy things than to steal them and other people are more valuable to you alive than dead. (As the economist Ludwig von Mises put it centuries later: “If the tailor goes to war against the baker, he must henceforth bake his own bread.”) Many Enlightenment thinkers, including Montesquieu, Kant, Voltaire, Diderot and the Abbé de Saint-Pierre endorsed the ideal of doux commerce, gentle commerce. The American founders – George Washington, James Madison and especially Alexander Hamilton – designed the institutions of the young nation to nurture it.

This brings us to another Enlightenment ideal, peace. War was so common in history that it was natural to see it as a permanent part of the human condition and to think peace could come only in a messianic age. But now war was no longer thought of as a divine punishment to be endured and deplored, or a glorious contest to be won and celebrated, but a practical problem to be mitigated and someday solved. In Perpetual Peace, Kant laid out measures that would discourage leaders from dragging their countries into war. Together with international commerce, he recommended representative republics (what we would call democracies), mutual transparency, norms against conquest and internal interference, freedom of travel and immigration and a federation of states that would adjudicate disputes between them.

For all the prescience of the founders, framers and philosophers, Enlightenment Now is not a book of Enlighten-olatry. The Enlightenment thinkers were men and women of their age, the 18th century. Some were racists, sexists, antisemites, slaveholders or duellists. Some of the questions they worried about are almost incomprehensible to us, and they came up with plenty of daffy ideas together with the brilliant ones. More to the point, they were born too soon to appreciate some of the keystones of our modern understanding of reality, including entropy, evolution, and information.

They of all people would have been the first to concede this. If you extol reason, then what matters is the integrity of the thoughts, not the personalities of the thinkers. And if you’re committed to progress, you can’t very well claim to have it all figured out. It takes nothing away from the Enlightenment thinkers to identify some critical ideas about the human condition and the nature of progress that we know and they didn’t.

• Enlightenment Now: The Case for Science, Reason, Humanism and Progress is published by Allen Lane. To order a copy for £21.25 (RRP £25) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.

