Quintilian, Inst. Orat. 8.1-3
“The order of the material which was collected in the previous five books introduces the concepts of Invention and Disposition. While it is necessary to understand these completely and deeply to obtain the highest level of learning, it is advantageous for those just beginning to be exposed to them rather briefly and directly.
Beginners are often deterred by the difficulty of such a complex and perplexing course of study or, at the very moment when their intelligences require nourishing and cultivation with some indulgence, they are exhausted by the handling of rather obscure material. They also may think they if they have learned these things enough they can consider themselves prepared for eloquence or, because they are addicted to some other fast rules of speaking, they resist any new attempt.
This is why it is the case that those who are the best writers of textbooks diverge the most from eloquence. Beginners need a path to guide them, but it should be clearly laid out, walkable, and easy to see. The talented teacher, then, should choose the best things from all sources and convey those things in the present which are effective without introducing delay by disputing contrary views.”
His fere, quae in proximos quinque libros conlata sunt, ratio inveniendi atque inventa disponendi continetur, quam ut per omnis numeros penitus cognoscere ad summam scientiae necessarium est, ita incipientibus brevius ac simplicius tradi magis convenit. Aut enim difficultate institutionis tam numerosae atque perplexae deterreri solent, aut eo tempore quo praecipue alenda ingenia atque indulgentia quadam enutrienda sunt asperiorum tractatu rerum atteruntur, aut si haec sola didicerunt satis se ad eloquentiam instructos arbitrantur, aut quasi ad certas quasdam dicendi leges alligati conatum omnem reformidant. Unde existimant accidisse ut qui diligentissimi artium scriptores extiterint ab eloquentia longissime fuerint. Via tamen opus est incipientibus, sed ea plana et cum ad ingrediendum tum ad demonstrandum expedita. Eligat itaque peritus ille praeceptor ex omnibus optima et tradat ea demum in praesentia quae placet, remota refutandi cetera mora.