Der schottische Autor, dessen Biographie des Schriftstellers und Lexikographen Samuel Johnson zu den klassischen Werken der englischen Literatur zählt, erwähnt d’Argens verschiedentlich in seinen Tagebüchern und Reiseberichten. Im Juni 1764 gab ihm Lord Marishal in Braunschweig folgenden Bericht über d’Argens: "In the afternoon my Lord was very chatty. He told me that the Marquis d’Argens was a good-natured, amiable man, and much liked by the King of Prussia. He is now old. He has married an actress, whom he keeps in great subjection. He has made her learn Greek, and I don’t know how many things, merely to make her of use to him in studies. He is a miserable being, for he is hypochondriac and terrified for death. He had worn a flannel under-waistcoat four years and durst not take it off for fear of catching cold. The King drove out one fear by another, and told him that if he persisted to wear that waistcoat, his perspiration would be entirely stopped, and he must inevitably die. The marquis agreed to quit his waistcoat. But it had so fixed itself upon him that pieces of his skin came away with it." (Boswell on the Grand Tour: Germany and Switzerland 1764. Edited by Frederick A. Pottle. Melbourne [u.a.]: Heinemann Ltd, S. 15-16)
Von Braunschweig reisten Boswell und Keith nach Berlin weiter, wo sie d’Argens, der gerade (u.a. zur Regelung von Erbschaftsangelegenheiten) in die Provence aufgebrochen war, jedoch nicht antrafen.