Voltaire was forty-two years of age, and already one of the most famous men of the day, when, in August 1736, he received a letter from the Crown Prince of Prussia.  This letter was the first in a correspondence which was to last, with a few remarkable intervals, for a space of over forty years.  It was written by a young man of twenty-four, of whose personal qualities very little was known, and whose importance seemed to lie simply in the fact that he was heir-apparent to one of the secondary European monarchies.  Voltaire, however, was not the man to turn up his nose at royalty, in whatever form it might present itself; and it was moreover clear that the young prince had picked up at least a smattering of French culture, that he was genuinely anxious to become acquainted with the tendencies of modern thought, and, above all, that his admiration for the author of the Henriade and Zaire was unbounded.

La douceur et le support [wrote Frederick] que vous marquez pour tous ceux qui se vouent aux arts et aux sciences, me font esperer que vous ne m’exclurez pas du nombre de ceux que vous trouvez dignes de vos instructions.  Je nomme ainsi votre commerce de lettres, qui ne peut etre que profitable a tout etre pensant.  J’ose meme avancer, sans deroger au merite d’autrui, que dans l’univers entier il n’y aurait pas d’exception a faire de ceux dont vous ne pourriez etre le maitre.

The great man was accordingly delighted; he replied with all that graceful affability of which he was a master, declared that his correspondent was ‘un prince philosophe qui rendra les hommes heureux,’ and showed that he meant business by plunging at once into a discussion of the metaphysical doctrines of ‘le sieur Wolf,’ whom Frederick had commended as ‘le plus celebre philosophe de nos jours.’  For the next four years the correspondence continued on the lines thus laid down.  It was a correspondence between a master and a pupil:  Frederick, his passions divided between German philosophy and French poetry, poured out with equal copiousness disquisitions upon Free Will and la raison suffisante, odes sur la Flatterie, and epistles sur l’Humanite, while Voltaire kept the ball rolling with no less enormous philosophical replies, together with minute criticisms of His Royal Highness’s mistakes in French metre and French orthography.  Thus, though the interest of these early letters must have been intense to the young Prince, they have far too little personal flavour to be anything but extremely tedious to the reader of to-day.  Only very occasionally is it possible to detect, amid the long and careful periods, some faint signs of feeling or of character.  Voltaire’s empressement seems to take on, once or twice, the colours of something like a real enthusiasm; and one notices that, after two years, Frederick’s letters begin no longer with ‘Monsieur’ but with ‘Mon cher ami,’ which glides at last insensibly into ‘Mon cher Voltaire’; though the careful poet continues with his ‘Monseigneur’ throughout.  Then, on one occasion, Frederick makes a little avowal, which reads oddly in the light of future events.