While studying in Utrecht in 1764, the trainee lawyer and diarist James Boswell met a young woman called Belle de Zuylen – known as Zélide in Boswell’s journal – a novelist, religious doubter and amorous adventurer, with a lightning mind which “flashes with so much brilliance [it] may scorch”. Boswell was in search of a wife, and Belle, he assumed, would be in need of a husband.

Despite being rebuffed, he persisted in his attentions, finally applying, not to Belle herself, but to her father. The “terms of the treaty”, as Robert Zaretsky puts it in Boswell’s Enlightenment, were “as onerous as they were outlandish”. As Mrs Boswell, Belle would swear never to see, or write to, another man, not to publish any literary works without her husband’s approval and, in the words of the proposal, “never to speak against the established religion or customs of the country she might find herself in”, which was most likely to be Scotland. It appears that Belle’s father passed on the invitation because when Boswell tried again a year later, Belle herself replied that all she knew of Scotland was that it produced “decidedly despotic husbands and humble, simple wives who blushed and looked at their lords before opening their mouths”.

Belle and Boswell were both Protestants, but while he had been raised in the fearful mood of Ayrshire Calvinism, where the brightest rays were those cast by the fires of damnation, Holland’s Protestantism was milder, “marked not by the harsh vision of Knox”, as Zaretsky writes, “but instead by the humane spirit of Grotius, the 17th-century legal philosopher and statesman”. There would be no danger of metaphysical debate between the pair, however. Had she accepted, Belle would have signed a document agreeing not to argue about religion with her husband. His views were settled. Speculation on the afterlife was “absurd in a man, but in a woman … more absurd than I can express”.

Boswell was scarcely alone among contemporaries in his ways of thinking about love and belief, but his illiberalism seems paradoxical. Not only did he set down in writing the fullest portrait imaginable of a liberal and free-thinking man, Samuel Johnson, but in his journal he left behind a unique record of the daily activities, doubts and dreads, vanities and self-vauntings, of another man: himself. His attraction to unconventional, forward-looking types was not due to any philosophical affinity, but rather sprang from temperament. Zaretsky opens his book with a scene of Boswell and his friend Temple climbing to the top of Arthur’s Seat in Edinburgh and shouting “with all the abandon of their 16 years: ‘Voltaire, Rousseau, immortal names!’” Shortly afterwards, he fell under the spell of David Hume, a great name in European philosophy. It was the high point of the Scottish Enlightenment, the time of Hume, Adam Smith and others, paving the way for Robert Burns, Sir Walter Scott and Henry Cockburn. In 1694, a young man had been hanged for blasphemy in Scotland’s capital. Now, just over half a century later, students climbed to the highest point in the city to bawl out the names of free thinkers, the way their modern descendants might celebrate famous sportsmen.

Boswell offers the perfect specimen for an old-and-new experiment of the kind that Zaretsky performs: a man at home in the salons of the age of Enlightenment, but one who was scarcely an enlightened thinker. His title is two-sided: while the bulk of the narrative is devoted to placing Boswell in his historical and cultural context during the period of what is now recognised as the European Enlightenment (the Scottish dimension takes a disappointing second place), he is also concerned with Boswell’s personal development. The evidence of the journal suggests that, despite Boswell’s acquaintance with some of the most forward-thinking people of his time – including Voltaire and Rousseau, both of whom he sought out – there was to be no redemption from the established customs of a life largely guided by the markers of class, money and religion.

Boswell never conquered his ample faculty of self-deception, not least when it came to matters of the heart and libido, nor his overwhelming fear of God – or, perhaps more accurately, of death: what Boswell sought from religion was not so much a guide to living but a form of reassurance against the possibility of oblivion. Whereas others, such as Hume, insisted that belief in the afterlife was tantamount to wishful thinking, Boswell lived in hope of hearing the unbelievers say that they had got it wrong – this in itself would prove something. His final visit to Hume, to see the great infidel on his deathbed, was driven primarily not by compassion or even duty. Just as Boswell attended public executions in order to observe the behaviour of the condemned in the instant before being whisked off to an infernal hereafter, so he wanted to see if Hume would be driven by cowardice back to the religious faith of his early years. “Did not the thought of annihilation, he demanded, make Hume uneasy? … Hume responded that it made him no less easy than the thought that he had never been. Once again, Boswell sought to hear the cry of anguish and declaration of faith, but he instead heard only the urbane reassurance that neither was necessary.” When asked if he was not seduced by the thought of seeing his friends again in eternity, Hume pointed out that his enemies were likely to be there, too. It was a polite way of trying to shake Boswell out of his childlike notions of heaven and hell.

Boswell remained unenlightened in other ways, too, and not only as judged by today’s standards. While he clung to and recorded Johnson’s every word, he brushed aside an equally great figure who happened to be his neighbour in Ayrshire, but one with barely a pick and shovel to his name: Burns. When he wrote to Boswell in 1788, Burns was, as the biographer Peter Martin writes in his Life of James Boswell (1999), “no obscure poet”. But he was poor. Zaretsky doesn’t mention Burns, but Martin points out that Boswell, the ninth Laird of Auchinleck, “felt that literary distinction was the privilege of the educated and aristocratic. Burns did not qualify”.

Though only 288 pages long, Boswell’s Enlightenment has the feeling of being padded out. Zaretsky’s academic background obliges him to insert references to colleagues in the field, as if to give scholarly backing to assertions verging on the obvious. “As the historian Linda Colley argues, Great Britain in the mid-18th century simmered with debates over the nature and extent of liberty” – as if no one had argued it before. His animadversions on Frederick the Great, the libertine Scotophobe John Wilkes, the Corsican freedom fighter Pasquale di Paoli, the appealing Belle and others are amusing and instructive, but readers might ask themselves why they are not reading Boswell’s journal instead. What captivates us in his own writing is not the philosophical reaction to his age, but the vivid and startling picture he paints of it. He was not an original thinker, but as he himself put it: “I am in reality an original character. Let me moderate & cultivate my Originality. God would not have formed such a diversity of men if he had intended that they should all come up to a certain standard … Let me then be Boswell and render him as fine a fellow as possible.”