On January 8, 1697, at some time between two and four in the afternoon, an eighteen-year-old student named Thomas Aikenhead was hanged in Edinburgh. Aikenhead had been found guilty of a serious charge: the previous year he had several times told other young men that the doctrines of Christian theology were “a rapsodie of faigned and ill-invented nonsense.” Aikenhead’s friends, testifying against him, told the court that he had spoken of “the Imposter Christ” and had rejected the Trinity, the Incarnation, and the Redemption. Aikenhead recanted all these sentiments—he said he had fallen under the spell of atheistical tracts—but no one defended him, and the jury voted for death. On the scaffold, Aikenhead declared that he had come to doubt the objectivity of good and evil, and that he believed moral laws to be the work of governments or men. His disastrous misstep—the joshing blasphemies about Christianity—point toward the anti-Christian side of the French Enlightenment, toward Voltaire’s suave pagan ironies and sarcasms. The student’s last statement—that moral laws are the work of governments and men—is one of the assumptions behind the American Revolution. The remark anticipates, as well, the British liberalism of the nineteenth century and the enduring work, and enduring trial, of modernity.

That work gathered steam soon after Aikenhead’s death. James Buchan, the author of the recent “Crowded with Genius” (HarperCollins; $29.95), a vivacious and erudite celebration of the flowering of Scottish intellectual life in the eighteenth century—the Scottish Enlightenment, as people now call it—maintains that Aikenhead’s execution “haunted” the century that followed. Just six decades after the student’s rant to his friends, an Edinburgh ecclesiastical assembly attempted to excommunicate the freethinking Scottish philosopher David Hume. Hume, perhaps the most thoroughgoing skeptic in the history of philosophy, believed that religion is a portrait not of how the cosmos works but of how the human mind works—of what men and women want and need. His view of life was worldly and sociable. Like many of the learned Scots, he revered the new science of Copernicus, Bacon, Galileo, Kepler, Boyle, and Newton; he believed in the experimental method and loathed superstition. Hume was admired in Edinburgh by some, disdained or dismissed by many others. But, after a tumultuous debate, the motion to excommunicate him failed, by a vote of fifty to seventeen. Hume’s chief tormentor, a splenetic clergyman named George Anderson, died a few months later, and, Buchan concludes, “an ecclesiastical era drew to an end.”

In the years between the two trials, Edinburgh had changed from a dour near-theocracy to a cosmopolitan center. Not just Hume and Adam Smith and James Boswell but a host of superb lesser figures burst into public acclaim. The fountainhead was Francis Hutcheson, a kind of pan-Enlightenment figure, who, from 1729 until his death, in 1746, held the chair in moral philosophy at the University of Glasgow, where he broke with tradition by lecturing in English as well as Latin. Hutcheson, a frequent visitor to Edinburgh, was Adam Smith’s teacher and he encouraged Hume’s early efforts. He was suspicious of metaphysics or any claims not based on observation or experience. Empiricism and the inductive method was the clarion call of the Scottish Enlightenment. Space, time, mass, movement, force—these things could be known in a way that such metaphysical imponderables as “divine purpose” and “final causes” could not.

The intellectual break with the past was drastic and seemingly irreversible. In 1770, James Hutton, an experimental farmer and the owner of a sal ammoniac works, began poking into the peculiar shapes and textures of the Salisbury Crags, the looming, irregular rock formations in Edinburgh. Hutton noticed something astonishing—fossilized fish remains embedded in the rock. The remains suggested that volcanic activity had raised the mass from some depth in the sea. In 1785, he delivered a lecture to the Royal Society of Edinburgh, which included the remarkable statement that “with respect to human observation, this world has neither a beginning nor an end.” Coolly discarding Biblical accounts of creation, the book that he eventually published, “The Theory of the Earth,” helped establish the foundations of modern geology. Shortly before Hutton’s investigation, in 1769, James Watt refashioned the steam engine. In recent years, scholars have traced the rudiments of modern psychology, anthropology, the earth sciences, and theories of civil society and liberal education to eighteenth-century Scotland.

For those who keep track of such things, the Scots, in current accounts of intellectual history, have caught up with the French as leading exponents of Enlightenment thought. Yet the learned Scots were remarkably unlike the French philosophes; indeed, they were unlike any other group of philosophers that ever existed. In a gigantic study, “The Sociology of Philosophies,” published in 1998, Randall Collins assembled structural portraits of the seminal moments in philosophy, both Western and Eastern. Typically, the most important figures in a given cluster of thinkers (perhaps three or four men) would jockey for centrality while cultivating alliances with other thinkers or students on the margins. In the Scottish group, however, there was little of the bristling, charged, and exclusionary fervor of the Diderot-d’Alembert circle; or of the ruthless atmosphere found in Germany in the group that included Fichte, the Schelling brothers, and Hegel; or of the conscious glamour of the existentialists in postwar Paris. The Scots vigorously disagreed with one another, but they lacked the temperament for the high moral drama of quarrels, renunciations, and reconciliation. Hutcheson, Hume, and Smith, along with Adam Ferguson and Thomas Reid, were all widely known, but none of them were remotely cult figures in the style of Hegel, Marx, Emerson, Wittgenstein, Sartre, or Foucault. To an astonishing degree, the men supported one another’s projects and publications, which they may have debated at a club that included amateurs (say, poetry-writing doctors, or lawyers with an interest in science) or in the fumy back room of some dark Edinburgh tavern. In all, the group seems rather like an erudite version of Dickens’s chattering and benevolent Pickwick Club.

The Scots were conservatives and radicals at the same time. They prized social order, and peace and quiet; they also sought intellectual revolution—new ways of looking at how the mind works, how morality works, and how we live in society. The religious strife of the seventeenth century, including the judicial murder of a reckless student, disgusted them. They could not imagine, and did not desire, civil society without religion. But they wanted to ease God out of scientific research and out of political and social life, too. And they wanted to naturalize morality—to locate the foundations of morals somewhere else than in revelation and fear of eternal damnation. Indeed, their greatest ambition was to try to establish something implied by Aikenhead’s gallows speech: an account of civil society demonstrating that legal and ethical rules were of human origin.

Their version of morals had to compete with some nasty suppositions. In “Leviathan,” published in 1651, Thomas Hobbes had developed a mechanistic view of human behavior: men were entirely amoral; they reacted positively to anything that gratified their self-interest, calling it “good,” but the reaction was no more than a mechanical response to a stimulus. Unless they were overawed by a strong central authority, men would quickly be at one another’s throats. The Scots were also familiar with the Duc de La Rochefoucauld’s “Maximes,” a wickedly funny collection of aphorisms that dissolved virtue in the acid of self-interest. If we performed a benevolent act, La Rochefoucauld suggested, we did so in order to stoke our vanity or to seek a return of favor. But Hutcheson, and also Hume and Smith, were repelled by such fashionable cynicism; they were determined to prove that virtue was grounded in human nature itself.