Hume then considers the process of causal inference, and in so doing he introduces the concept of belief. When people see a glass fall, they not only think of its breaking but expect and believe that it will break. Or, starting from an effect, when they see the ground to be generally wet, they not only think of rain but believe that there has been rain. Thus belief is a significant component in the process of causal inference. Hume then proceeds to investigate the nature of belief, claiming that he was the first to do so. He uses the term, however, in the narrow sense of belief regarding matters of fact. He defines belief as a sort of liveliness or vividness that accompanies the perception of an idea. A belief, in other words, is a vivid or lively idea. This vividness is originally possessed by some of the objects of awareness—by impressions and by the simple memory-images of them. By association it comes to belong to certain ideas as well. In the process of causal inference, then, an observer passes from an impression to an idea regularly associated with it. In the process the aspect of liveliness proper to the impression infects the idea, Hume asserts. And it is this aspect of liveliness that Hume defines as the essence of belief.

Hume does not claim to prove that events themselves are not causally related or that they will not be related in the future in the same ways as they were in the past. Indeed, he firmly believes the contrary and insists that everybody else does as well. Belief in causality and in the resemblance of the future to the past are natural beliefs, inextinguishable propensities of human nature (madness apart), and even necessary for human survival. Rather, what Hume claims to prove is that such natural beliefs are not obtained from, and cannot be demonstrated by, either empirical observation or reason, whether intuitive or inferential. Although reflection shows that there is no evidence for them, it also shows that humans are bound to have them and that it is sensible and sane to do so. This is Hume’s skepticism: it is an affirmation of that tension, a denial not of belief but of certainty.

Morals and historical writing

The Enquiry Concerning the Principles of Morals is a refinement of Hume’s thinking on morality, in which he views sympathy as the fact of human nature lying at the basis of all social life and personal happiness. Defining morality as those qualities that are approved (1) in whomsoever they happen to be and (2) by virtually everybody, he sets himself to discover the broadest grounds of the approvals. He finds them, as he found the grounds of belief, in “feelings,” not in “knowings.” Moral decisions are grounded in moral sentiment. Qualities are valued either for their utility or for their agreeableness, in each case either to their owners or to others. Hume’s moral system aims at the happiness of others (without any such formula as “the greatest happiness of the greatest number”) and at the happiness of self. But regard for others accounts for the greater part of morality. His emphasis is on altruism: the moral sentiments that he claims to find in human beings, he traces, for the most part, to a sentiment for and a sympathy with one’s fellows. It is human nature, he holds, to laugh with the laughing and to grieve with the grieved and to seek the good of others as well as one’s own. Two years after the Enquiry was published, Hume confessed, “I have a partiality for that work”; and at the end of his life he judged it “of all my writings incomparably the best.” Such statements, along with other indications in his later writings, make it possible to suspect that he regarded his moral doctrine as his major work. He here writes as a person having the same commitment to duty as others. The traditional view that he was a detached scoffer is deeply wrong: he was skeptical not of morality but of much theorizing about it.

Following the publication of these works, Hume spent several years (1751–63) in Edinburgh, with two breaks in London. An attempt was made to get him appointed as successor to Adam Smith, the Scottish economist (later to be his close friend), in the chair of logic at Glasgow, but the rumour of atheism prevailed again. In 1752, however, Hume was made keeper of the Advocates’ Library at Edinburgh. There, “master of 30,000 volumes,” he could indulge a desire of some years to turn to historical writing. His History of England, extending from Caesar’s invasion to 1688, came out in six quarto volumes between 1754 and 1762, preceded by Political Discourses (1752). His recent writings had begun to make him known, but these two brought him fame, abroad as well as at home. He also wrote Four Dissertations (1757), which he regarded as a trifle, although it included a rewriting of Book II of the Treatise (completing his purged restatement of this work) and a brilliant study of “the natural history of religion.” In 1762 James Boswell, the biographer of Samuel Johnson, called Hume “the greatest writer in Britain,” and the Roman Catholic Church, in 1761, recognized his philosophical and literary contributions by putting all his writings on the Index Librorum Prohibitorum, its list of forbidden books.

The most colourful episode of his life ensued: in 1763 he left England to become secretary to the British embassy in Paris under the Earl of Hertford. The society of Paris accepted him, despite his ungainly figure and gauche manner. He was honoured as eminent in breadth of learning, in acuteness of thought, and in elegance of pen and was taken to heart for his simple goodness and cheerfulness. The salons threw open their doors to him, and he was warmly welcomed by all. For four months in 1765 he acted as chargé d’affaires at the embassy. When he returned to London at the beginning of 1766 (to become, a year later, undersecretary of state), he brought Jean-Jacques Rousseau, the Swiss-born philosopher connected with the Encyclopédie of Denis Diderot and d’Alembert, with him and found him a refuge from persecution in a country house at Wootton in Staffordshire. This tormented genius suspected a plot, took secret flight back to France, and spread a report of Hume’s bad faith. Hume was partly stung and partly persuaded into publishing the relevant correspondence between them with a connecting narrative (A Concise and Genuine Account of the Dispute Between Mr. Hume and Mr. Rousseau, 1766).

In 1769, somewhat tired of public life and of England too, he again established a residence in his beloved Edinburgh, deeply enjoying the company—at once intellectual and convivial—of friends old and new (he never married), as well as revising the text of his writings. He issued five further editions of his History between 1762 and 1773 as well as eight editions of his collected writings (omitting the Treatise, History, and ephemera) under the title Essays and Treatises between 1753 and 1772, besides preparing the final edition of this collection, which appeared posthumously (1777), and Dialogues Concerning Natural Religion, in which he refuted the cosmological and teleological arguments for the existence of God (held back under pressure from friends, it was published posthumously in 1779). His curiously detached autobiography, The Life of David Hume, Esquire, Written by Himself (1777; the title is his own), is dated April 18, 1776. He died in his Edinburgh house after a long illness and was buried on Calton Hill.

Adam Smith, his literary executor, added to the Life a letter that concludes with his judgment on his friend as “approaching as nearly to the idea of a perfectly wise and virtuous man as perhaps the nature of human frailty will permit.” His distinguished friends, with ministers of religion among them, certainly admired and loved him, and there were younger men indebted either to his influence or to his pocket. The mob had heard only that he was an atheist and simply wondered how such an ogre would manage his dying. Yet Boswell has recounted, in a passage in his Private Papers, that, when he visited Hume in his last illness, the philosopher put up a lively, cheerful defense of his disbelief in immortality.