Over the past 50 years, the philosopher who’s played the most significant role in the cultural life of the United States isn’t Richard Rorty, Jerry Fodor, or Martha Nussbaum, but rather Immanuel Kant. This is largely because of his impact on postwar trends in moral philosophy. Kant’s influence during this period coincided with the rise of ethics as a dominant concern in political theory. John Rawls, Ronald Dworkin, and Thomas Nagel are among the most prominent examples of this widespread tendency. It is possible to exaggerate the novelty of this development; after all, questions of morality have always had a place in reflective debate about politics. Yet the strict subordination of politics to morals in recent decades does represent some kind of departure, or at least a return to a style of Kantian thought that had long been out of favor. Compared with current tendencies in political philosophy, normative inquiry was of marginal importance to the leading political thinkers in the first half of the 20th century, among them Max Weber, Otto Bauer, Michael Oakeshott, John Maynard Keynes, Friedrich Hayek, Raymond Aron, and Hannah Arendt.

I don’t mean to suggest that these thinkers took no interest in human values. Because politics is a product of the struggle over values, political thinking can hardly avoid such a pivotal issue. But there’s a difference between the study of human preferences and attachments and a preoccupation with an ideal theory of value. An ideal view of politics begins with a normative approach to power; it operates with a purely moral conception of how social and political relations ought to be organized. From this perspective, as Kant proposed in an essay he wrote in 1793, the rules of prudence ought to be sacrificed to the dictates of moral law. Dishonesty in politics might well be advantageous, to take one example, but from where Kant was standing, it could never be morally right.

The normative turn in political philosophy has overlapped with the rise of moralism in certain areas of US policy, most obviously in international affairs. This tendency came to prominence in the early years of the new millennium, as Dick Cheney and George W. Bush set about reconceiving the international order in terms of ideal values. Because democracy was now regarded as a “universal” norm, it was thought that it might systematically replace dictatorships across the globe: With the aid of strategic military intervention, authoritarian regimes would fall like dominoes throughout the Middle East. In line with this approach, practical obstacles were wished away under the influence of a crusading righteousness.

In general terms, the retreat of political philosophy onto the terrain of abstract morals has exacted a heavy cost. The loss has been twofold. First, a narrow focus on the absolute obligations of duty is prone to disregard the tangible desires of human beings. Second, an exclusive interest in what is unconditionally justified can lead to the suspension of all concern with what is beneficial. In the decades before Kant set about converting political theory into a branch of moral inquiry, the great Scottish philosopher and historian David Hume had been cultivating what amounted to the opposite perspective. Hume’s program had two core features: It examined what actually motivated people instead of what they ought to do in principle; and it calculated the advantages that emerged from a course of action as opposed to evaluating its intrinsic merits. This approach might be seen as a criticism of Kantianism before Kant. More accurately, it can be viewed as a rejection of Stoicism, elements of which were later revived by Kant in combination with strands of Lutheran pietism. It is right, in Kant’s analysis, that the Stoics were ultimately deemed deficient, but they were to be praised for equating happiness with “purely moral perfection.”

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Remarkably, James Harris’s intellectual biography of Hume is the first to have been attempted. As such, it covers the full trajectory of Hume’s intellectual career—from his earliest experiments in epistemology and ethics, through his views on religion, economics, and politics, to his mature efforts to complete his classic History of England. The result is an engrossing reconstruction of his ideas along with his position in 18th-century intellectual life. A significant place is given to Hume’s “anatomy” of human nature, and thus to the criticism of Stoicism which he developed in that context.

Much of Harris’s achievement derives from his capacity for cogent synthesis. For the first time, the various advances made by two generations of innovative scholarship about Hume are brought together in a comprehensive treatment. At the same time, Harris’s book brings its own distinctive analysis to bear on the full range of Hume’s output. Harris emphasizes three main points: Hume’s ambition as a writer, the diversity of his pursuits, and the implications of his skepticism. It was above all as a skeptic that Hume took issue with attempts to revivify the philosophy of Stoicism.

Each of these claims is subtly implicated in the others. As Harris shows, it was partly Hume’s yearning for fame as an author that drove him to pursue a variety of literary ventures. This diversity, in turn, was held together by a general skepticism about the mental and moral aptitudes of human beings. In his 20s, Hume was determined to improve his “talents in literature.” At the end of his life, he recollected the landmarks in his career in terms of a sequence of “literary occupations.” Throughout, he resolved to experiment in a variety of genres—historical narrative, philosophical argument, the essay, and the dialogue. As Harris emphasizes, Hume’s craft in each was a constant preoccupation and a source of pride. The goals of elegance and precision guided everything he wrote, and whatever he undertook was elaborately revised. The desire for recognition encouraged literary experimentation, increasing the range of subjects to which he could apply his skepticism.

At certain points after his death, Hume was regarded primarily as a philosopher, while at other times he was seen as a historian. For the Whig politician Henry Brougham, he was a sagacious chronicler; for John Stuart Mill, his only accomplishment was in philosophy. Harris argues that instead of trying to compartmentalize Hume in accordance with a modern academic division of labor, we should see him as a “man of letters” who contributed to a variety of fields. It becomes clear that the copious range of Hume’s assignments is best explained in terms of his vocation as a writer. His self-conscious attachment to the role of author is partly explained by his relative financial independence: He was not forced to survive by the “trade” of writing, nor to make his way in one of the professions. At the same time, he was able to remain aloof from the entanglements of patronage.

The image of the philosopher has been transformed over the past 200 years, especially since the rise of a professional class of faculty members, which began around the end of the 19th century. In the 17th and 18th centuries, the role was less clear-cut. Locke’s Two Treatises of Government was a political polemic rather than an academic tract. Rousseau chose to write fiction and autobiography as well as works on natural law and political economy. Adam Smith saw his Wealth of Nations as an exercise in philosophy and regarded himself in a Humean vein as a “philosopher” in the broad sense. Although Harris makes plain that Hume was bent on succeeding as a man of letters, he also shows that he pursued this task in what he termed a “philosophical” mode. What Hume meant was that his philosophy would be a vehicle for his skepticism. As he embraced the intellectual modesty that he thought characterized a skeptical outlook, he also reconceptualized what skepticism could achieve.

After an early crisis of confidence, Hume spent his mid-20s studying philosophy in France, moving from Paris to Rheims in 1734 before settling in La Flèche in the Loire Valley the following year. Over the next two years, he came to terms with the writings of French luminaries like Nicolas Malebranche and Pierre Bayle. All the while, he was gestating a series of major works that would transform our understanding of epistemology, politics, and morals by subjecting many of the verities of ancient and modern philosophy to forms of doubt that were paralyzing and liberating at the same time.

In his first book, A Treatise of Human Nature, an astonishingly precocious work of philosophical genius published in 1739 but drafted two years earlier, we are invited to consider the limitations of forms of dogmatism that pretend to reach beyond the confines of our understanding to uncover the underlying principles of the universe—extending to the nature of God, his attitude toward his progeny, and the nature of our obligations to his will. For Hume, fundamental principles in metaphysics, such as final causes in nature or the ultimate grounds for human psychology, are as inaccessible to reason as the basic questions in natural religion: “Any hypothesis,” as he put it, “that pretends to discover the ultimate original qualities of human nature, ought at first to be rejected as presumptuous and chimerical.” But while Hume regarded it as impossible to penetrate the hidden machinery of the universe, it was possible, he insisted, to improve our “knowledge of man.” Explicitly following in the footsteps of Locke and the Third Earl of Shaftesbury, among others, Hume hoped to establish the “science of man” on the same foundations on which Bacon and Newton had advanced our knowledge of nature: by experimental observation of the empirical world, carried out with due attention and circumspection.

The science of human nature would therefore have to begin with individuals as they took part in the common affairs of life. Yet this proposed anatomy of social psychology was preceded by a critique of the powers of reason to explain the world outside the bounds of experience. This critique was directed against improbable hypotheses that had been generated by earlier systems of philosophy, from the ancients to Descartes and beyond. Hume’s skepticism about earlier attempts to fathom the unfathomable bred in him a melancholy mixture of doubt and despair: “Where am I, or what? From what causes do I derive my existence, and to what condition shall I return? Whose favour shall I court? And whose anger must I dread?” Skepticism, Hume was indicating, unsettles the precepts of religion and ultimately destroys all trust and belief. Luckily, our nature soon distracts us from these uncertainties and prompts us to take comfort in the truths of custom and common sense.

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For Hume, skepticism about metaphysical subjects ended in an indolence born of seclusion. The only solution was to transfer the skeptical impulse in philosophy from the solitude of the study to the wider social world. Under these circumstances, skepticism fostered equanimity rather than discontent. In society, the true skeptic acknowledged the value of common sense without submitting slavishly to its whims. Skepticism in this context was profitable and enabling; it criticized without destroying the conditions of criticism, which depended on the existence of society and government. The positive results of criticism could be seen in society, politics, and morals. Philosophy could expose damaging ideas in ethics, unsocial attitudes in religion, and dangerous postures in politics. We might usefully think of Hume as having tackled each of these in turn.

Harris adroitly captures the extent to which Hume was doubtful about all reigning systems of moral philosophy—including theories that based ethical conduct on the precepts of reason, those that traced social harmony to a shared sentiment of justice, and those that grounded moral judgment on the appetite for pleasure. In exposing each of these doctrines, Hume’s skepticism was rewarding rather than incapacitating. It offered incisive correctives instead of a descent into abysmal uncertainty. In this spirit, Hume took issue with what he called “the selfish system of morals,” associated in his 1751 Enquiry Concerning the Principles of Morals with Hobbes and Locke, whom he interpreted as seeking to reduce every human motive to self-interest or self-regard. Plainly, Hume believed, benevolence was all around us—evident in families, friendships, and clubs. Yet he also criticized the view, popularized by Francis Hutcheson, that moral sentiments were sufficient to guarantee the peace of society. Benevolence was real but not extensive, Hume concluded. It could flourish under favorable circumstances, including prosperity and benign government, but it would never simply be implemented at the behest of reason.

Hume’s skepticism about the effectiveness of our rational faculties in moral judgment was partly directed against divines like Samuel Clarke, who tried to discover the criteria for virtue in an immutable measure of rightness accessible to the mind. At the same time, it was directed against latter-day Stoics like Shaftesbury, for whom the idea of virtue was sufficient to motivate good conduct. In fact, Hume believed, morality had its origins in moral feelings, not rational principles. Incitement to action could only be sparked by a driving passion in the individual, not purely by the notion of what they ought to do. As Hume put it in a letter to Hutcheson in 1739, “virtue can never be the sole motive to any action.”

On the basis of this insight, Hume reconceived the task of philosophy. It ought not to be championed, as the ancient schools had done, as a “medicine for the mind.” Nor was it a source of rules for action that would guarantee righteousness. Its role was critical reflection rather than exhortation or consolation. Accordingly, such Stoic precepts as were to be found in Seneca or Epictetus were deemed to be misbegotten aspirations. Mere arguments could never move us without engaging our affections. Ideas of the good, in order to be effective, had to be rooted in what was agreeable. Philosophy for this reason had to educate through taste, by appeal to active psychological preferences. It was idle to prescribe what ideally ought to obtain. More than this, enforcing ideals that had no traction in existing mores would undermine the fabric of society.

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Because philosophy, for Hume, was charged with intervening in social affairs, it was obliged to understand the current state of society as well as its processes of change. In that sense, it was a skeptical commentary on its own time, guided by historical understanding. This perspective makes sense of Hume’s choice of projects after the Treatise: His Essays, written and revised from the 1730s through the 1770s, were a series of attempts to expose the prejudices of the age, while his History set those prejudices in a longer-term perspective by elucidating the reasons for their emergence. History, like philosophy, could not transform human behavior, but it could deepen our grasp of the circumstances in which we operated.

During his final 13 years, until his death in 1776, Hume extended his social repertoire and cultivated new friendships. He became acquainted with the leading French philosophes of the day and gained some knowledge of the rhythms of popular politics in London. However, his labors as a philosopher and historian were largely over, leaving him to refine his political ideas against the backdrop of metropolitan strife in the 1760s and then rebellion in the American colonies. As befitted a historically grounded social philosophy, the object of his skepticism shifted as circumstances changed.

Skepticism for Hume did not mean cynicism; it did not entail deriding cherished attitudes and opinions. As Harris notes, the outright atheism broadcast in Parisian salons seemed dogmatic and rebarbative to Hume. Even the “paradoxes” of Rousseau appeared pointlessly provocative. Rather than expose harmless prejudices to contempt, Hume set about exploring the consequences of belief. His aim was to eradicate counterproductive assumptions rather than dismantle benign customs. Religious dispositions, economic hypotheses, and political presuppositions could usefully be subjected to constructive criticism. An undertaking of this kind would not remodel entrenched “proclivities,” but it could deepen our understanding of the context in which they operated. With this end in view, Hume challenged prevailing ideas about political parties, the contract of government, and the balance of trade. Heated disputes about each of these topics had ensured that they were misunderstood. By comparison, sober investigation revealed systematic confusion, showing how many of our beliefs frustrated our desires.

Harris’s study cumulatively demonstrates how Hume brought history and philosophy together. Since philosophy is powerless to prescribe the terms of its application, to be effective it has to understand the world in which it operates. More recent developments in Anglo-American thought have tended to withdraw from practical analysis into speculative accounts of ideal value. This has resulted in a species of political philosophy that has lost any grasp of the social world on which it pretends to reflect. The sterility of many of the resulting debates serves to recommend Hume’s ambition to make philosophy historical and to render history philosophical.