Leonardo Bruni, Whether the Literate and the Mob Spoke the Same Way in the Times of Terence and Cicero
Let the question be then whether the common people in Rome through the times of the poet Terence and of Cicero spoke as those whom we now say speak in good, literate Latin, or whether there was one type of speech among the common people and another among the literate.
Your first and indeed strongest argument is that orators in the senate, in court, and in meetings delivered their speeches in Latin, which they would not have done if they were not understood by everyone.
Further, the comedies of Terence and Plautus were recited to the people in the very same language in which they were written, and you say that this indicates that the common people spoke in the same way. For who could they delight if they were not understood?
These seem to you to be the firmest proofs and most certain arguments for your opinion. I, however, don’t think that they are any greater proofs than the fact that the Gospel and the solemn rites of the Mass are delivered in literate Latin before a crowd of listeners.
Indeed, people understand this stuff even if they are illiterate, yet they themselves nevertheless do not speak thus, nor do they know how to speak thus (though they understand), precisely because it is far easier to understand foreign speech than to produce it.
Therefore, let us brush aside your arguments if you please.

Sit igitur quaestio utrum Romae per Terentii poetae et M. Tullii tempora vulgus ita loquebatur ut loquuntur hi quos nunc latine litterateque loqui dicimus, vel alius fuerit vulgi sermo, alius litteratorum.
Tua quidem prima ac potissima ratio est quod oratores in senatu iudiciisque et concionibus latine orabant, quod non fecissent nisi a cunctis intelligerentur.
Praeterea Terentii Plautique comoediae recitabantur ad populum ea ipsa lingua qua scriptae sunt, idque signum esse ais quod eodem modo vulgus loquebatur. Quomodo enim delectarent nisi intelligerentur?
Hae tibi firmissimae probationes videntur ac certissima argumenta opinionis tuae. Ego autem non maiora ista puto quam nunc sint Evangelia Missarumque solemnia latine ac litterate in audientium turba pronunciari.
Intelligunt enim homines, licet inlitterati sint, nec tamen ipsi ita loquuntur nec illo modo loqui scirent, licet intelligant, propterea quod longe facilius est intelligere alienum sermonem quam proferre.
Discutiamus igitur, si placet, argumenta tua, et quid valeant videamus.