Erasmus’ treatise on letter-writing was published in Basle in 1522 with 410 pages.
Ironically for a humanist, Erasmus argues against the central doctrines of radical humanist letter-writing styles.
In the fifth chapter Erasmus seems nevertheless to yield to rhetorical formalism himself, dividing epistolary prose into three types introduced by Aristotle, namely the deliberative, forensic and epideictic genres or suasorium, demonstrativum and judiciale as he calls them.
He adds as a fourth category, the familiar letter, which was the type of epistolography celebrated by the humanists who so imitated Cicero’s style, ironically, since Erasmus is a critic of the elaborate Ciceronian style of the time.
I shall be corrected if wrong but I think it is this part of the work that is being referred to by my correspondent:
Iam uero consuetudinem insulsissimam quidem illam, sed aliquot iam seculis mire receptam, unum hominem multitudinis numero compellandi, non tam dedocendi sunt iuuenes quam animandi, ut ausint contemnere, certe quoties cum his agitur, unde nihil sit periculi. Mea certe aetate iam haec ineptia magna ex parte obsoleuit, paulatim subolescentibus bonis literis. Quanquam supersunt et hodie non pauci uossissatores, qui quod pueri male didicerunt, non solum ipsi mordicus tenent, uerumetiam ab aliis exigunt. Inexpiabili contumeia se putant affici, si quis unum singulari numeto salutet; ac plane rem putant iniuriarum actione dignam, ac legibus uindicandam.
[Cap 4, pp.266-8]
"As for that absurd practice, curiously in vogue now for several centuries, of addressing a single person in the plural, young men should not so much be untaught this vice as encouraged to scorn it altogether, at least in dealing with those with whom there is no risk involved. In my lifetime, at least, this folly has now largely fallen into disuse with the gradual coming of age of sound learning. Yet even today a good many users of the vos form of address, who not only hang on tenaciously to what they wrongly learnt as boys but also require it of others are still with us. They consider it an unforgivable insult if someone greets them in the singular; indeed, they deem it deserving of an action for damages and punishable by law!"
[Translation from Collected Works of Erasmus, 25-26, Toronto 1985, p.45]
This, of course, is more a reference to the excessive use of the plural address, more evident in European languages now than in English but then in use even in English by the use of "thou" (singular) and "you" (plural).
"You" being now universal in English and "thou" having died out, save in prayer and poetry, the issue is only live in other European countries where it is remains customary, Erasmus notwithstanding, to refer to familiars and servants as "thou" and others (whether or not noble) by the plural "you".
For example:
In German: "Du" and "Sie".
In French: "Tu" and "Vous".
Digressing slightly, some may know that the word "Thou" derives from the Anglo-Saxon equivalent of the German "Du", the hard "D" becoming a soft "Th" in Anglo-Saxon, written with the Anglo-Saxon letter resembling a "y".
"Sie" became "thee" and both "thou" and "thee" were later rendered "you" and "ye", as plurals, perhaps confusing the Anglo-Saxon letter for a "y" in the Greek sense.
"Die" became also "ye" and later "the" as in "Ye olde shoppe" (although one should not discount the effect of bad spelling in former times).
Even so, the plural was especially reserved for royal persons.
Use of the royal plural is not inappropriate and emphasizes the importance of the monarch at the apex of the Constitution.
It is true that Julius Caesar was addressed in the singular (as Erasmus points out) but then he was never emperor. Later Roman emperors were titled "Caesar Augustus" and addressed in the plural.
This was, in turn, as much a reflection of the Hebrew practice of addressing the great in the plural, most notably Almighty God, who, as you can see from one of the side-bar comments on this blog, was addressed in the plural as Elohim (when not addressed as Adonai).
In any event, I do not think that Erasmus objected to clerical and noble titles as such but more to their misuse.
...