There have been many famous philosophical friendships. Political scientist Dennis C. Rasmussen lists some of the most important: Socrates and Plato; Plato and Aristotle; Bentham and Mill; Erasmus and More; Heidegger and Arendt; Marx and Engels; Sartre and de Beauvoir; Whitehead and Russell; Emerson and Thoreau. (He might have added Rousseau and Diderot, though that one ended badly.) But as Rasmussen points out, the great age gap between Plato and Socrates, and between Plato and Aristotle, made these relationships closer to those of mentor and protégé than to friendships between equals. In terms of influence, depth of thought and true intimacy between the two parties, Rasmussen posits that the alliance between David Hume (1711-76) and Adam Smith (1723-90), the two greatest figures of the Scottish Enlightenment, was the most momentous of all philosophical friendships, and he celebrates the two men’s interconnectedness and the joint impact they made on the modern world in his intelligent and beautifully written new book, The Infidel and the Professor: David Hume, Adam Smith, and the Friendship That Shaped Modern Thought.

The claim implicitly made in the book’s subtitle could be challenged. Hume and Smith helped to shape modern thought, certainly, but did they shape it on their own? Plenty of their contemporaries and near-contemporaries can fairly be said to have had a hand in this shaping, including Locke, Voltaire, Montesquieu, Franklin, Rousseau, Jefferson, and Madison. But Hume and Smith, both individually and through their joint attempt to create a new, all-inclusive “science of man,” can be seen to have lit fires that still kindle vigorously. Jürgen Habermas claimed that the Enlightenment is an unfinished project, and that statement is never more true than when applied to the works of these two thinkers. Hume, widely believed to be the greatest philosopher to have written in English, is still read by many, and with great pleasure—the fact that he wrote before philosophy (thought) gave way to “philosophy” (an academic discipline with a professional jargon that is largely opaque to the general reader) has much to do with his continuing popularity. But his radically skeptical position, when taken to its logical extension, is terrifying; indeed, it terrified even him, rendering him “affrighted and confounded” when he first followed its logic in his youth, while engaged on his Treatise of Human Nature. There exists no “I,” he concluded, no coherent self, certainly no “soul” in the Christian sense of the term; all we can really claim to be is a series of perceptions. Nor is there any reason, other than wishful thinking, to believe that there is a God. While not defining himself as an atheist—he said he was too skeptical to take that dogmatic position—neither was he a theist of any description. Obviously, these two existential absences, the absence of a God and of a self, are frightening. Therefore, while paying lip service to Hume’s intelligence and his mastery of Augustan prose, most of his readers have stopped short of the final abyss; they entertain his speculations while remaining safely within their own belief systems, which tend to include a metaphysical or at least “spiritual” element and a firm belief in the reality, and even the significance, of their own identities.

But in 1776 there was no such discipline as economics. Smith, like Hume, classified himself under the broad rubric of “philosopher,” and like Hume he essayed throughout his career, from his early Principles Which Lead and Direct Philosophical Enquiries (unpublished until after his death) to the 1759 landmark Theory of Moral Sentiments, to Wealth of Nations, to create a wide-ranging science of man, built on empirical Lockean principles. As a professor in Edinburgh he lectured on subjects as various as ethics, jurisprudence, and rhetoric, and wrote essays on the development of language and the history of astronomy; later, moving to the University of Glasgow, he occupied first a chair in logic, then, for twelve years, in moral philosophy. The social sciences as we know them today had not yet been defined; there were no chairs in psychology, sociology, anthropology, or economics—it was all philosophy. The age of specialization was just beginning to dawn, and both Hume and Smith were engines of its arrival. In their own time they were what was then known as moral philosophers. To posterity they would be, respectively, a philosopher and an economist. If Hume had been born a couple of centuries later, indeed, he might have decided to go into psychology rather than academic philosophy, for like John Locke before him he was particularly fascinated by the workings of the human mind.

Smith and Hume were intellectual equals, in other words, and they were like-minded. Hume was the elder by twelve years, and he began his writing career early in life, so that by the time Smith was a student at Oxford (1740-46) he was already a confirmed Humean, though the two men were not to meet until 1749. Like many of his contemporaries, Smith was not impressed by the education on offer at Oxford: universities, he later wrote (always excepting his beloved Glasgow) were often “sanctuaries in which exploded systems and obsolete prejudices found shelter and protection, after they had been hunted out of every other corner of the world.” (I must admit that when I attended graduate school in the late twentieth century, it exactly fit this description.) True to academic narrow-mindedness, outraged dons stormed the young Smith’s room as he sat engrossed in Hume’s Treatise on Human Nature, and confiscated the godless volume.

The work Smith himself composed at Oxford, The Principles Which Lead and Direct Philosophical Enquiries (three related essays), was very much in sympathy with Hume’s ideas as laid out in the Treatise. Like Hume, he downplayed the role of reason in human behavior, and stressed the importance of habit, custom, and the passions. Like Hume, he showed suspicion of intellectual “systems.” And again like Hume, he treated religion and its rise in anthropological rather than metaphysical terms. Like The Wealth of Nations decades later, Principles was an uncommonly secular treatise. “Nothing that Smith says rules out the possibility of there actually being an ordered world or an intelligent designer, of course,” Rasumussen remarks, “but the whole work has a distinctly deflationary character, providing unflattering psychological and sociological explanations for beliefs that were widely assumed to emanate from reason if not from God himself.”

It might be appropriate here to point out the significance of Rasmussen’s title, The Infidel and the Professor. Hume, of course, was the Infidel—“the Great Infidel,” as he came to be called. He quickly recovered from the affright and confoundment of his early glimpse at a godless world, and began to revel in his infidelity, thoroughly enjoying his power to offend the pious, for whom he entertained scant respect. He was also well aware that the success of his Enquiry Concerning Human Understanding, his Essays, Moral, Political, and Literary, and his tremendously popular six-volume History of England—which would stand for more than a century as the standard history of the nation—had something to do with the religious skepticism manifested, and often flaunted, throughout his works. He once told a friend that a “Tincture of Deism” often served to increase sales of a book since “the Clamor, which it raises, commonly excites Curiosity, & quickens the Demand. The Book is much rail’d at and much read.” His chapter on miracles in the Enquiry, his essays “Of Superstition and Enthusiasm” and “The Immortality of the Soul,” The Natural History of Religion—all these became bywords for scandalous impiety.  An attempt was even made to excommunicate Hume from the Scottish Kirk (even though he was not actually a member of it). The History of England had made Hume a rich man, and from the security of his independent position he railed cheerfully at what he considered the intellectual fraud inherent in religious systems of every description. “Examine the religious principles, which have, in fact, prevailed in the world,” he wrote at the end of The Natural History of Religion. “You will scarcely be persuaded, that they are any thing but sick men’s dreams: Or perhaps will regard them more as the playsome whimsies of monkies in human shape, than the serious, positive, dogmatical asseverations of a being, who dignifies himself with the name of rational.”

An atheist in the eighteenth century was regarded with something like the horror that a sexual predator garners in the twenty-first. It was a very brave man who was willing to declare himself an atheist. A number of the radical French philosophes were eager to do so—in those salons, in fact, atheism was quite the fashion, despite the fact that the Catholic Church exercised a much tighter stranglehold on France than the Church of England did on England or the Presbyterian Kirk on Scotland. By contrast, Anglo-Saxons, with hardly any exceptions, were reluctant to destroy their reputation with the public, or their chances at political power, social influence, academic employment and nearly every other good thing in life. While Hume claimed he was not an atheist, it’s clear that he judged the probability of there being a God to be very, very slim. Other closet atheists of the period, I suspect, were Thomas Jefferson and Benjamin Franklin. (Hume got on very well with Franklin, unsurprisingly.) And Adam Smith, the Professor, is another possibility—though he was far too discreet and far too dependent on academic appointments to air such dangerous views publicly. He also, unlike Hume, feared notoriety.

Smith went to considerable pains to protect his private papers from the gaze of posterity. He instructed his executors to burn them after his death, which they did, and he was not a prolific letter-writer in any case: though Hume was his “dearest friend” (an epithet each man used when writing to the other, but to no one else), only fifteen letters from Smith to Hume survive, as opposed to forty-one in the other direction. Nor do any other very personal communications remain: Smith was a bachelor who lived with his mother until her death only six years before his own, and he is not known even to have had any significant amorous interludes. As Rasmussen says, “Smith’s biographers frequently lament that he seems to have gone out of his way to make things difficult for them.” And it’s highly likely that he did just that.

“Long reflection on Smith’s friendship with Hume cannot help but push one’s interpretation [of his religious beliefs] toward the skeptical end of the spectrum,” Rasmussen concludes. For one thing, there’s the frequency with which Hume joked about religion in his letters to Smith, in a tone that would have surely offended Smith if he were likely to be offended by such things; but as there was never any real breach between the friends, he can’t have been offended. Typical of this sort of banter is Hume’s recommendation that Smith read Voltaire’s newly published Candide: it “is full of Sprightliness and Impiety,” he wrote, “and is indeed a Satyre upon Providence, under Pretext of criticizing the Leibnitian System.” As Rasmussen points out, it is “noteworthy that Hume seems to suppose that Candide’s impiety would serve to recommend it to Smith.” Such examples of Hume’s apparent confidence that Smith would agree with him on these matters pop up frequently in his letters. Noting that several Bishops had voiced approval of Smith’s Theory of Moral Sentiments, for instance, Hume quipped that “you may conclude what Opinion true Philosophers will entertain of it, when these Retainers to Superstition praise it so highly.”

And then there is A Theory of Moral Sentiments itself. As Rasmussen notes, the book, though Humean, takes religious positions slightly different from those of Hume. Hume thought that religious belief actually increased people’s fears, while Smith allowed that it might alleviate them. Hume believed that religious belief made people worse, while Smith thought that religion might in fact support morality. In A Theory of Moral Sentiments, Smith “suggests that religious beliefs and hopes often spring from what is best in us rather than what is worst.”

Still, it is a remarkably secular document for its time. In its pages Smith, like Hume before him, decides that morality derives from our senses rather than from reason, and certainly not from some inner, God-given set of absolute standards. Morality is essentially a set of habits that cause society to run smoothly and easily. Justice and peaceableness, to name only two moral virtues, are virtues because they are utilitarian; they promote the well-being of all men in society. “Vices” are not, as a Christian might judge them, sins; they are habits that are detrimental to society, such as theft and violence. The rule of law is established not to impose an abstract moral code but to make life safer and pleasanter. In Wealth of Nations Smith would go further and state that the rule of law was imposed principally to protect those with property from those who had none. According to these standards, religious belief, if it is beneficial (a proposition that of course Hume would challenge) exists because it is beneficial—it improves society.

When Smith referred to the deity in the pages of Theory of Moral Sentiments, he used a standard deist formulation: “the author of Nature.” Such references are infrequent, though, and when he revised the text for its sixth edition just a few months before his death in 1790, he played down any discussion of a deity significantly, even adding a few faintly mocking references to religious belief. Rasmussen wonders whether the changes are due to increased skepticism or diminished caution. The latter reason seems an inevitable conclusion: Smith now was old, a rich man, no longer beholden to universities or patrons. He would soon be dead, and had no further fear of offending the Establishment.

Hume and Smith’s friendship strengthened and deepened throughout Hume’s life, in spite of the fact that they seldom found themselves in the same place at the same time, and that Smith was such a poor correspondent. Hume frequently urged Smith to relocate to Edinburgh, but Smith deemed Edinburgh “a very dissolute town” and spent most of his career in Glasgow, at that time a cleaner and more elegant place; in his native village of Kirkcaldy, where he composed Wealth of Nations; and in London, where he saw his works through the press. He would not settle in Edinburgh, in fact, until after Hume’s death. He visited it often enough, however, to take an active part in its cultural life, and he and Hume were founding members, in 1754, of the famous Select Society, which would include luminaries of the period such as Allan Ramsay, Hugh Blair, Adam Ferguson, Lord Kames, and William Robertson.

The influence of Hume’s ideas on Smith’s masterpiece is evident. Hume had discussed commerce and commercial policy extensively in both his Political Discourses and the History of England. Rasmussen judges the single most important passage in Wealth of Nations to be Smith’s claim that “commerce and manufactures gradually introduced order and good government, and with them, the liberty and security of individuals, among the inhabitants of the country, who had before lived almost in a continual state of war with their neighbors, and of servile dependency on their superiors. This, though it has been the least observed, is by far the most important of all their effects.” Smith went on to add that “Mr. Hume is the only writer who, so far as I know, has hitherto taken notice of it.” Perhaps not quite the first—Voltaire’s Letters on England implicitly make the same claim—but Hume’s examination of the subject went to the heart of what Smith was expressing.

Not that he agreed with Smith on everything. As Rasmussen indicates, Smith saw, much more clearly than Hume did, the drawbacks of commercial society, particularly the inevitable division of society into haves and have-nots. “Whenever there is great property, there is great inequality. For one very rich man, there must be at least five hundred poor, and the affluence of the few supposes the indigence of the many.” Neither did Smith share Hume’s enthusiasm for the merchant class, whose interests, he was quick to point out, were not those of the general public.

Smith’s devastating attack on mercantilism, in which he echoed many points made by Hume in Political Discourses, has much to teach us today. The mercantilist system that had reigned in England for two centuries had operated under the assumption, Smith wrote, that nations’ interest “consisted in beggaring all their neighbors.” “The very fact that he set out to investigate the source of the wealth of nations (in the plural),” Rasmussen comments, “shows just how deeply his entire mind-set diverged from that of the mercantilists. For Smith, trade is not a zero-sum game: France’s gain need not be Britain’s loss. On the contrary, both nations can benefit by trading with one another.”

Hume lived long enough to enjoy his friend’s greatest success—Wealth of Nations was published March 9, 1776—but only just. He was already suffering from the disease (probably colon cancer) that would soon kill him. In April, he saw Smith and made a request that put the discreet Professor in a very uncomfortable position. Some twenty-five years earlier, Hume had penned a masterpiece of impiety, Dialogues Concerning Natural Religion, and had been touching it up ever since. He had been in no hurry to get it to the press, as it was truly unpublishable in the climate of the time, at least under his own name. In it, he attacked the “argument from design,” the idea that the order we see in the world implies a wise and beneficent creator, and the first cause argument, in which we can trace events backwards through a string of causes to a first cause—God. He also tackled the question of theodicy (the paradox of evil existing in a world ruled by a beneficent God) in what can only be called a godless manner, and expressed doubts about the moral benefits of belief.

Of course he had written on these subjects before, but this was something new: “the combination of all these issues,” writes Rasmussen, “into a single devastating—and entertaining—package. While nearly everything Hume wrote bears on religion in one way or another, usually to its detriment, in his published writings he had always refrained from marshalling all of his skeptical challenges at once, thereby appearing to leave some kind of refuge for the devout…The uniqueness of the Dialogues lies in its comprehensiveness, which leaves the pious reader no way out, no safe haven.”

Smith was Hume’s literary executor, and Hume now made the request that Smith publish the Dialogues after his death, which would probably be very soon. This Smith was unwilling to do; he feared the hue and cry that would inevitably break out upon the publication, and the onus that would settle on him were it known he had had anything to do with the publication. He voiced these objections to Hume, who reluctantly assigned the task to others. Later scholars have called Smith’s refusal to perform this task as an act of cowardice, but Rasmussen very sensibly begs to differ; all of Hume’s friends had urged him not to publish, he points out, and of course Hume himself had refrained from doing so for a quarter century, and did not plan for it to hit the presses until he himself was in the grave.

Hume’s death, like that of Voltaire two years later, became something of a test case for whether an infidel could die peacefully. Would he not panic when the end came and return to the bosom of the church? But no; Hume approached death with scarcely a ripple of trepidation. “Poor David Hume is dying very fast,” wrote Smith to a mutual friend, “but with great chearfulness and good humour and with more real resignation to the necessary course of things, than any Whining Christian ever dyed with pretended resignation to the will of God.” (Rasmussen points out that this sentence alone should settle any uncertainty about Smith’s religious views.) One assumes that the “chearfulness” was genuine, but of course Hume had to die cheerfully; if he had not done so, he would more or less have negated all he had written. By dying as cheerfully as he had lived, Hume was demonstrating that religion was unnecessary to a happy and virtuous life.

James Boswell, who himself was terrified at the prospect of death, visited Hume to ascertain whether what he’d heard about his easy resignation was true; it appeared to be, and the younger man spread his account of the visit through the London intelligentsia. Hume did his own part toward publicizing his philosophic death by writing a brief autobiography, a seven-page screed that Rasmussen includes as an appendix. In the last letter he ever wrote, Hume sent it to Smith and asked him to publish it—this piece of writing, he assured his friend, contained nothing objectionable—and gave him “entire liberty to make what Additions you please to the account of my Life.” In this last communication of their long alliance, he signed off with “Adieu My dearest Friend.” Two days later, on August 25, 1776, he was dead.

Smith now did something as brave as publishing the Dialogues on Natural Religion would have been; he composed a supplement to Hume’s My Own Life in the form of a letter to William Strahan, its publisher, in which he described the last four month’s of the philosopher’s life (the autobiography ended in April). This document, now known to us as the Letter to Strahan, was as provocative in its way as the Dialogues. In it Smith confirmed reports of Hume’s dying cheerfulness; he also inserted a facetious sally of Hume’s that offended many readers. Hume had imagined trying to put off Charon, the ferryman to Hades, by telling him about how much he still had to achieve on earth. “I might still urge, ‘Have a little patience, good Charon, I have been endeavouring to open the eyes of the Public. If I live a few years longer, I may have the satisfaction of seeing the downfall of some of the prevailing systems of superstition.’ But Charon would then lose all temper and decency. ‘You loitering rogue, that will not happen these many hundred years. Do you fancy I will grant you a lease for so long a term? Get into the boat this instance, you lazy loitering rogue.’” This harmless jest infuriated Christian readers, but Rasmussen shows us that in fact Smith had already toned it down; Hume’s original plaint to Charon had been “Good Charon, I have been endeavouring to open the eyes of the people; have a little patience only till I have the pleasure of seeing their churches shut up, and the Clergy sent about their business.”

Smith’s greatest tribute to his friend, at the end, was also in the Letter to Strahan, and it also enraged the pious. It is the letter’s conclusion: “Upon the whole, I have always considered him, both in his lifetime and since his death, as approaching as nearly to the idea of a perfectly wise and virtuous man, as perhaps the nature of human frailty will permit.” As Smith’s contemporaries, educated in the classics, would have recognized, this was a nod to Plato’s epitaph on Socrates at the end of the Phaedo, and it was written for similar reasons; to absolve the departed philosopher, who had also been accused of refusing to recognize the gods recognized by the state, from allegations of corrupting the morals of the people. Smith’s apologia created an uproar (one can read an amusing account of the indignation among Samuel Johnson’s circle in Boswell’s Life of Johnson); a vocal reaction against it was led by high churchmen, and one writer on Scottish affairs tells us that the Letter made Smith “henceforth be regarded as an avowed sceptic, to the no small regret of many who revered his character and admired his writings.”

Smith affected surprise at the brouhaha, claiming that “a single, and as, I thought a very harmless Sheet of paper, which I happened to Write concerning the death of our late friend Mr Hume, brought upon me ten times more abuse than the very violent attack I had made upon the whole commercial system of Great Britain [Wealth of Nations].” But he never retracted it, nor made any apology. The letter survives as the ultimate, heartfelt affirmation of one of the most productive friendships the world has ever known.