In seeing his friend in danger of losing himself to passion, Elliot might have heard an echo of Hume’s own philosophical precepts. In his “Treatise of Human Nature,” Hume argued that “reason alone can never be a motive to any action of the will.” Desire, for example, “arises not from reason.” And yet it can (and ought to be) “directed by it.”

As Elliot foresaw, his friend’s bliss was soon shattered. The comtesse’s husband died; she was free to try to convince the Prince de Conti to marry her, and focused her formidable energy on doing so. A distressed Hume was transformed into her platonic adviser and confidant.

Yet he acquitted himself with dignity. When it became clear to everyone except Boufflers that the prince would not marry her, Hume urged her to be reasonable.

In effect, Hume did for her as Elliot had done for him. He reminded her that, insofar as it never causes or creates our desires, reason is indeed passion’s slave. But it is a most useful slave, for it helps us understand and guide our competing passions.

The “chief triumph of art and philosophy,” he wrote years before meeting Boufflers, is that it “refines the temper” and “points out to us those dispositions which we should endeavor to attain, by a constant bent of mind and by repeated habit.”

Those lines sound as if they came from a philosopher whose life reflects his convictions and intends to offer us a model for our own lives. Scholars of the urbane and portly Hume typically see him as an unlikely candidate to place alongside, say, Socrates as a philosopher of this “art of living.” So it’s worth remembering that Hume proved himself equal to his philosophy in his relationship with Boufflers.

He corresponded with her until the end of his life. In fact, he was on his own deathbed when news of the Prince de Conti’s death reached him. Yet he took up his pen to commiserate with the greatest love of his life.

And at the letter’s end he said goodbye: “I see death approach gradually without any anxiety or regret. I salute you, with great affection and regard, for the last time.”