Information is much more portable in the modern world than it used to be. So are people… There are three things which have revolutionized academic life in the last twenty years… : jet travel, direct-dialing telephones and the Xerox machine… As long as you have access to a telephone, a Xerox machine, and a conference grant fund, you're OK, you're plugged into the only university that really matters -- the global campus.

In David Lodge's 1988 novel Small World, a professor explains why he believes that the elite university has become obsolete:

Morris Zapp had a point, but he overemphasized the technologies of the 1980s. Two decades after his words were written, they have been superseded by e-mail, digital documents, Web sites, blogs, teleconferencing, Skype, and smartphones. And two centuries before they were written, the technologies of the day -- the sailing ship, the printed book, and the postal service -- had already made information and people portable. The result was the same: a global campus, a public sphere, or as it was called in the 17th and 18th centuries, the Republic of Letters.

Any 21st-century reader who dips into intellectual history can't help but be impressed by the blogosphere of the 18th. No sooner did a book appear than it would sell out, get reprinted, get translated into half a dozen languages, and spawn a flurry of commentary in pamphlets, correspondence, and additional books. Thinkers like Locke and Newton exchanged tens of thousands of letters; Voltaire alone wrote more than eighteen thousand, which now fill fifteen volumes. Of course this colloquy unfolded on a scale that by today's standards was glacial -- weeks, sometimes even months -- but it was rapid enough that ideas could be broached, criticized, amalgamated, refined, and brought to the attention of people in power. A signature example is Beccaria's On Crimes and Punishments, which became an instant sensation and the impetus for the abolition of cruel punishments throughout Europe.

Given enough time and purveyors, a marketplace of ideas can not only disseminate ideas but change their composition. No one is smart enough to figure out anything worthwhile from scratch. As Newton (hardly a humble man) conceded in a 1675 letter to fellow scientist Robert Hooke, "If I have seen further it is by standing on the shoulders of giants." The human mind is adept at packaging a complicated idea into a chunk, combining it with other ideas into a more complex assembly, packaging that assembly into a still bigger contrivance, combining it with still other ideas, and so on. But to do so it needs a steady supply of plug-ins and subassemblies, which can come only from a network of other minds.

A global campus increases not only the complexity of ideas but their quality. In hermetic isolation, all kinds of bizarre and toxic ideas can fester. Sunlight is the best disinfectant, and exposing a bad idea to the critical glare of other minds provides at least a chance that it will wither and die. Superstitions, dogmas, and legends ought to have a shorter half-life in a Republic of Letters, together with bad ideas about how to control crime or run a country. Setting fire to a person and seeing whether he burns is a dumb way to determine his guilt. Executing a woman for copulating with devils and turning them into cats is equally inane. And unless you are a hereditary absolutist monarch, you are unlikely to be persuaded that hereditary absolutist monarchy is the optimal form of government.

The jet airplane is the only technology of Lodge's small world of 1988 that has not been made obsolete by the Internet, and that reminds us that sometimes there is no substitute for face-to-face communication. Airplanes can bring people together, but people who live in a city are already together, so cities have long been crucibles of ideas. Cosmopolitan cities can bring together a critical mass of diverse minds, and their nooks and crannies can offer places for mavericks to seek refuge. The Age of Reason and the Enlightenment were also an age of urbanization. London, Paris, and Amsterdam became intellectual bazaars, and thinkers congregated in their salons, coffeehouses, and bookstores to hash out the ideas of the day.

Amsterdam played a special role as an arena of ideas. During the Dutch Golden Age in the 17th century it became a bustling port, open to the flow of goods, ideas, money, and people. It accommodated Catholics, Anabaptists, Protestants of various denominations, and Jews whose ancestors had been expelled from Portugal. It housed many book publishers, who did a brisk business printing controversial books and exporting them to the countries in which they had been banned. One Amsterdammer, Spinoza, subjected the Bible to literary analysis and developed a theory of everything that left no room for an animate God. In 1656 he was excommunicated by his Jewish community, who, with memories of the Inquisition still fresh, were nervous about making waves among the surrounding Christians. It was no tragedy for Spinoza, as it might have been if he had lived in an isolated village, because he just picked up and moved to a new neighborhood and from there to another tolerant Dutch city, Leiden. In both places he was welcomed into the community of writers, thinkers, and artists. John Locke used Amsterdam as a safe haven in 1683 after he had been suspected of taking part in a plot against King Charles II in England. René Descartes also changed addresses frequently, bouncing around Holland and Sweden whenever things got too hot.

The economist Edward Glaeser has credited the rise of cities with the emergence of liberal democracy. Oppressive autocrats can remain in power even when their citizens despise them because of a conundrum that economists call the social dilemma or free-rider problem. In a dictatorship, the autocrat and his henchmen have a strong incentive to stay in power, but no individual citizen has an incentive to depose him, because the rebel would assume all the risks of the dictator's reprisals while the benefits of democracy would flow diffusely to everyone in the country. The crucible of a city, however, can bring together financiers, lawyers, writers, publishers, and well-connected merchants who can collude in pubs and guild halls to challenge the current leadership, dividing the labor and diffusing the risk. Classical Athens, Renaissance Venice, revolutionary Boston and Philadelphia, and the cities of the Low Countries are examples of cities where new democracies were gestated, and today urbanization and democracy tend to go together.

The subversive power of the flow of information and people has never been lost on political and religious tyrants. That is why they suppress speech, writing, and association, and why democracies protect these channels in their bills of rights. Before the rise of cities and literacy, liberating ideas had a harder time being conceived and amalgamated, and so the rise of cosmopolitanism in the 17th and 18th centuries deserves part of the credit for the Humanitarian Revolution.

My own view is that the two developments really are linked. When a large enough community of free, rational agents confers on how a society should run its affairs, steered by logical consistency and feedback from the world, their consensus will veer in certain directions. Just as we don't have to explain why molecular biologists discovered that DNA has four bases -- given that they were doing their biology properly, and given that DNA really does have four bases, in the long run they could hardly have discovered anything else -- we may not have to explain why enlightened thinkers would eventually argue against African slavery, cruel punishments, despotic monarchs, and the execution of witches and heretics. With enough scrutiny by disinterested, rational, and informed thinkers, these practices cannot be justified indefinitely. The universe of ideas, in which one idea entails others, is itself an exogenous force, and once a community of thinkers enters that universe, they will be forced in certain directions regardless of their material surroundings. I think this process of moral discovery was a significant cause of the Humanitarian Revolution.

Bringing people and ideas together, of course, does not determine how those ideas will evolve. The rise of the Republic of Letters and the cosmopolitan city cannot, by themselves, explain why a humanitarian ethics arose in the 18th century, rather than ever-more-ingenious rationales for torture, slavery, despotism, and war.

It begins with skepticism. The history of human folly, and our own susceptibility to illusions and fallacies, tell us that men and women are fallible. One therefore ought to seek good reasons for believing something. Faith, revelation, tradition, dogma, authority, the ecstatic glow of subjective certainty -- all are recipes for error, and should be dismissed as sources of knowledge.

I am prepared to take this line of explanation a step further. The reason so many violent institutions succumbed within so short a span of time was that the arguments that slew them belong to a coherent philosophy that emerged during the Age of Reason and the Enlightenment. The ideas of thinkers like Hobbes, Spinoza, Descartes, Locke, David Hume, Mary Astell, Kant, Beccaria, Smith, Mary Wollstonecraft, Madison, Jefferson, Hamilton, and John Stuart Mill coalesced into a worldview that we can call Enlightenment humanism. (It is also sometimes called classical liberalism, though since the 1960s the word liberalism has acquired other meanings as well.) Here is a potted account of this philosophy -- a rough but more or less coherent composite of the views of these Enlightenment thinkers.

Is there anything we can be certain of? Descartes gave as good an answer as any: our own consciousness. I know that I am conscious, by the very fact of wondering what I can know, and I can also know that my consciousness comprises several kinds of experience. These include the perception of an external world and of other people, and various pleasures and pains, both sensual (such as food, comfort, and sex) and spiritual (such as love, knowledge, and an appreciation of beauty).

We are also committed to reason. If we are asking a question, evaluating possible answers, and trying to persuade others of the value of those answers, then we are reasoning, and therefore have tacitly signed on to the validity of reason. We are also committed to whatever conclusions follow from the careful application of reason, such as the theorems of mathematics and logic.

Though we cannot logically prove anything about the physical world, we are entitled to have confidence in certain beliefs about it. The application of reason and observation to discover tentative generalizations about the world is what we call science. The progress of science, with its dazzling success at explaining and manipulating the world, shows that knowledge of the universe is possible, albeit always probabilistic and subject to revision. Science is thus a paradigm for how we ought to gain knowledge -- not the particular methods or institutions of science but its value system, namely to seek to explain the world, to evaluate candidate explanations objectively, and to be cognizant of the tentativeness and uncertainty of our understanding at any time.

Among the beliefs about the world of which we can be highly confident is that other people are conscious in the same way that we are. Other people are made of the same stuff, seek the same kinds of goals, and react with external signs of pleasure and pain to the kinds of events that cause pain and pleasure in each of us.

The indispensability of reason does not imply that individual people are always rational or are unswayed by passion and illusion. It only means that people are capable of reason, and that a community of people who choose to perfect this faculty and to exercise it openly and fairly can collectively reason their way to sounder conclusions in the long run. As Lincoln observed, you can fool all of the people some of the time, and you can fool some of the people all of the time, but you can't fool all of the people all of the time.

By the same reasoning, we can infer that people who are different from us in many superficial ways -- their gender, their race, their culture -- are like us in fundamental ways. As Shakespeare's Shylock asks:

The commonality of basic human responses across cultures has profound implications. One is that there is a universal human nature. It encompasses our common pleasures and pains, our common methods of reasoning, and our common vulnerability to folly (not least the desire for revenge). Human nature may be studied, just as anything else in the world may be. And our decisions on how to organize our lives can take the facts of human nature into account -- including the discounting of our own intuitions when a scientific understanding casts them in doubt.

Hath not a Jew eyes? hath not a Jew hands, organs, dimensions, senses, affections, passions? fed with the same food, hurt with the same weapons, subject to the same diseases, healed by the same means, warmed and cooled by the same winter and summer, as a Christian is? If you prick us, do we not bleed? if you tickle us, do we not laugh? if you poison us, do we not die? and if you wrong us, shall we not revenge?

The other implication of our psychological commonality is that however much people differ, there can be, in principle, a meeting of the minds. I can appeal to your reason and try to persuade you, applying standards of logic and evidence that both of us are committed to by the very fact that we are both reasoning beings.

You and I ought to reach this moral understanding not just so we can have a logically consistent conversation but because mutual unselfishness is the only way we can simultaneously pursue our interests. You and I are both better off if we share our surpluses, rescue each other's children when they get into trouble, and refrain from knifing each other than we would be if we hoarded our surpluses while they rotted, let each other's children drown, and feuded incessantly. Granted, I might be a bit better off if I acted selfishly at your expense and you played the sucker, but the same is true for you with me, so if each of us tried for these advantages, we'd both end up worse off. Any neutral observer, and you and I if we could talk it over rationally, would have to conclude that the state we should aim for is the one where we both are unselfish.

The universality of reason is a momentous realization, because it defines a place for morality. If I appeal to you to do something that affects me -- to get off my foot, or not to stab me for the fun of it, or to save my child from drowning -- then I can't do it in a way that privileges my interests over yours if I want you to take me seriously (say, by retaining my right to stand on your foot, or to stab you, or to let your children drown). I have to state my case in a way that would force me to treat you in kind. I can't act as if my interests are special just because I'm me and you're not, any more than I can persuade you that the spot I am standing on is a special place in the universe just because I happen to be standing on it.

Morality, then, is not a set of arbitrary regulations dictated by a vengeful deity and written down in a book; nor is it the custom of a particular culture or tribe. It is a consequence of the interchangeability of perspectives and the opportunity the world provides for positive-sum games. This foundation of morality may be seen in the many versions of the Golden Rule that have been discovered by the world's major religions, and also in Spinoza's Viewpoint of Eternity, Kant's Categorical Imperative, Hobbes and Rousseau's Social Contract, and Locke and Jefferson's self-evident truth that all people are created equal.

From the factual knowledge that there is a universal human nature, and the moral principle that no person has grounds for privileging his or her interests over others', we can deduce a great deal about how we ought to run our affairs. A government is a good thing to have, because in a state of anarchy people's self-interest, self-deception, and fear of these shortcomings in others would lead to constant strife. People are better off abjuring violence, if everyone else agrees to do so, and vesting authority in a disinterested third party. But since that third party will consist of human beings, not angels, their power must be checked by the power of other people, to force them to govern with the consent of the governed. They may not use violence against their citizens beyond the minimum necessary to prevent greater violence. And they should foster arrangements that allow people to flourish from cooperation and voluntary exchange.

This line of reasoning may be called humanism because the value that it recognizes is the flourishing of humans, the only value that cannot be denied. I experience pleasures and pains, and pursue goals in service of them, so I cannot reasonably deny the right of other sentient agents to do the same.

If all this sounds banal and obvious, then you are a child of the Enlightenment, and have absorbed its humanist philosophy. As a matter of historical fact, there is nothing banal or obvious about it. Though not necessarily atheistic (it is compatible with a deism in which God is identified with the nature of the universe), Enlightenment humanism makes no use of scripture, Jesus, ritual, religious law, divine purpose, immortal souls, an afterlife, a messianic age, or a God who responds to individual people. It sweeps aside many secular sources of value as well, if they cannot be shown to be necessary for the enhancement of human flourishing. These include the prestige of the nation, race, or class; fetishized virtues such as manliness, dignity, heroism, glory, and honor; and other mystical forces, quests, destinies, dialectics, and struggles.

For all that, Enlightenment humanism did not, at first, carry the day. Though it helped to eliminate many barbaric practices and established beachheads in the first liberal democracies, its full implications were roundly rejected in much of the world. One objection arose from a tension between the forces of enlightenment we have been exploring in this chapter and the forces of civilization we explored in the previous one -- though as we shall see, it is not difficult to reconcile the two. The other objection was more foundational, and its consequences more fateful.

I would argue that Enlightenment humanism, whether invoked explicitly or implicitly, underlay the diverse humanitarian reforms of the 18th and 19th centuries. The philosophy was explicitly invoked in the design of the first liberal democracies, most transparently in the "self-evident truths' in the American Declaration of Independence. Later it would spread to other parts of the world, blended with humanistic arguments that had arisen independently in those civilizations. And as we shall see in chapter 7, it regained momentum during the Rights Revolutions of the present era.