Aristotle argued that there were three kinds of friendship – those motivated by utility, by pleasure and by virtue. According to the political scientist Dennis C Rasmussen, the bond between David Hume and Adam Smith was a ‘textbook model’ of the last kind, ‘a stable, enduring, reciprocal bond that arises not just from serving one another’s interests or from taking pleasure in one another’s company, but also from the shared pursuit of a noble end – in their case, philosophical understanding’.

It is hard to think of another friendship involving intellectuals of such calibre. Hume and Smith were the leading figures of the 18th-century Scottish Enlightenment, by common consent, respectively, the greatest philosopher and economist Britain has produced, and arguably the greatest in their fields in the world. Given that, it is perhaps surprising that Rasmussen’s The Infidel and the Professor is the first book to try to tell the story of their relationship.

When the reader discovers the paucity of sources for information about their friendship, however, this book’s uniqueness becomes more understandable. Smith and Hume are known to have exchanged at least 170 letters, but only fifty-six survive, the majority, forty-one, from Hume to Smith. There is little else to fill the