On the heels of the Enlightenment came the French Revolution: a brief promise of democracy followed by a train of regicides, putsches, fanatics, mobs, terrors, and preemptive wars, culminating in a megalomaniacal emperor and an insane war of conquest. More than a quarter of a million people were killed in the Revolution and its aftermath, and another 2 to 4 million were killed in the Revolutionary and Napoleonic Wars. In reflecting on this catastrophe, it was natural for people to reason, "After this, therefore because of this," and for intellectuals on the right and the left to blame the Enlightenment. This is what you get, they say, when you eat the fruit of the tree of knowledge, steal fire from the gods, and open Pandora's box.

The theory that the Enlightenment was responsible for the Terror and Napoleon is, to put it mildly, dubious. Political murder, massacre, and wars of imperial expansion are as old as civilization, and had long been the everyday stuff of European monarchies, including that of France. Many of the French philosophes from whom the revolutionaries drew their inspiration were intellectual lightweights and did not represent the stream of reasoning that connected Hobbes, Descartes, Spinoza, Locke, Hume, and Kant. The American Revolution, which stuck more closely to the Enlightenment script, gave the world a liberal democracy that has lasted more than two centuries. Toward the end of this book I will argue that the data on the historical decline of violence vindicate Enlightenment humanism and refute its critics on the right and the left. But one of these critics, the Anglo-Irish writer Edmund Burke, deserves our attention, because his argument appeals to the other major explanation for the decline of violence, the civilizing process. The two explanations overlap -- both appeal to an expansion of empathy and to the pacifying effects of positive-sum cooperation -- but they differ in which aspect of human nature they emphasize.

According to Burke, no mortal is smart enough to design a society from first principles. A society is an organic system that develops spontaneously, governed by myriad interactions and adjustments that no human mind can pretend to understand. Just because we cannot capture its workings in verbal propositions does not mean it should be scrapped and reinvented according to the fashionable theories of the day. Such ham-fisted tinkering will only lead to unintended consequences, culminating in violent chaos.

Burke was the father of intellectual secular conservatism, which is based on what the economist Thomas Sowell has called a tragic vision of human nature. In that vision, human beings are permanently saddled with limitations of knowledge, wisdom, and virtue. People are selfish and shortsighted, and if they are left to their own devices, they will plunge into a Hobbesian war of all against all. The only things that keep people from falling into this abyss are the habits of self-control and social harmony they absorb when they conform to the norms of a civilized society. Social customs, religious traditions, sexual mores, family structures, and long-standing political institutions, even if no one can articulate their rationale, are time-tested work-arounds for the shortcomings of an unchanging human nature and are as indispensable today as when they lifted us out of barbarism.

But Burke had a point. Unspoken norms of civilized behavior, both in everyday interactions and in the conduct of government, may be a prerequisite to implementing certain reforms successfully. The development of these norms may be the mysterious "historical forces" that Payne remarked on, such as the spontaneous fading of political murder well before the principles of democracy had been articulated, and the sequence in which some abolition movements gave the coup de grâce to practices that were already in decline. They may explain why today it is so hard to impose liberal democracy on countries in the developing world that have not outgrown their superstitions, warlords, and feuding tribes.

Burke clearly went too far. It would be mad to say that people should never have agitated against torture, witch hunts, and slavery because these were long-standing traditions and that if they were suddenly abolished society would descend into savagery. The practices themselves were savage, and as we have seen, societies find ways to compensate for the disappearance of violent practices that were once thought to be indispensable. Humanitarianism can be the mother of invention.

This to-and-fro may explain why the American Revolution was not as calamitous as its French counterpart. The Founders were products not just of the Enlightenment but of the English Civilizing Process, and self-control and cooperation had become second nature to them. "A decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation," the Declaration politely explains. "Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes." Prudence, indeed.

But their decency and prudence were more than mindless habits. The Founders consciously deliberated about just those limitations of human nature that made Burke so nervous about conscious deliberation. "What is government itself," asked Madison, "but the greatest of all reflections on human nature?" Democracy, in their vision, had to be designed to counteract the vices of human nature, particularly the temptation in leaders to abuse their power. An acknowledgment of human nature may have been the chief difference between the American revolutionaries and their French confrères, who had the romantic conviction that they were rendering human limitations obsolete. In 1794 Maximilien Robespierre, architect of the Terror, wrote, "The French people seem to have outstripped the rest of humanity by two thousand years; one might be tempted to regard them, living amongst them, as a different species."

Civilization and Enlightenment need not be alternatives in explaining declines of violence. In some periods, tacit norms of empathy, self-control, and cooperation may take the lead, and rationally articulated principles of equality, nonviolence, and human rights may follow. In other periods, it may go in the other direction.

In The Blank Slate I argued that two extreme visions of human nature -- a Tragic vision that is resigned to its flaws, and a Utopian vision that denies it exists -- define the great divide between right-wing and left-wing political ideologies. And I suggested that a better understanding of human nature in the light of modern science can point the way to an approach to politics that is more sophisticated than either. The human mind is not a blank slate, and no humane political system should be allowed to deify its leaders or remake its citizens. Yet for all its limitations, human nature includes a recursive, openended, combinatorial system for reasoning, which can take cognizance of its own limitations. That is why the engine of Enlightenment humanism, rationality, can never be refuted by some flaw or error in the reasoning of the people in a given era. Reason can always stand back, take note of the flaw, and revise its rules so as not to succumb to it the next time.