Commentary Par XII 137-138

'Aelius Donatus, Roman scholar and rhetorician of cent. iv, said to have been the tutor of Jerome; he was the author of a commentary on Virgil (now lost, but often alluded to by Servius), and of another on Terence, but his most famous work was an elementary Latin grammar, Ars Grammatica in three books; part of this work, the Ars minor, or De octo partibus orationis, served as a model for subsequent similar treatises.  Owing to the popularity of this work in the Middle Ages it was one of the earliest books, being printed even before the invention of movable type -- the name of its author became a synonym for grammar, just as Euclid for geometry' (Donato).

Donatus was the 'people's grammarian' in that his Ars, unlike Priscian's (see [Inf XV 109]), supposedly kept grammar as simple as possible.  And grammar was itself the 'first art' in the sense that it was the first subject taught to children, the first of the seven liberal arts.  Thus his 'intellectual humility' may have, in Dante's mind, paralleled that of Illuminato and Augustino.  Both the Ottimo (DDP Ottimo.Par.XII.137-138) and John of Serravalle (DDP Serravalle.Par.XII.136-138) cite the incipit of the work: 'Ianua sum rudibus' (I am the doorway through which the unlettered may pass [to learning]).