Hugh of St. Victor, "Didascalicon"
The Didascalicon of Hugh of St. Victor, A Medieval Guide to the Arts, translated from the Latin with an introduction and notes by Jerome Taylor, Records of Civilization, Sources and Studies, Number LXIV, Columbia University Press, 19611
Book Three, Chapter Five, p. 90:
When, therefore, we treat of any art — and especially in teaching it, when everything must be reduced to outline and presented for easy understanding — we should be content to set forth the matter in hand as briefly and as clearly as possible, lest by excessively piling up extraneous considerations we distract the student more than we instruct him. We must not say everything we can, lest we say with less effect such things as need saying. Seek, therefore, in every art what stands established as belonging specifically to it. Later, when you have studied the arts and come to know by disputation and comparison what the proper concern of each of them is, then, at this stage, it will be fitting for you to bring the principles of each to bear upon all the others, and, by a comparative and back-and-forth examination of the arts, to investigate the things in them which you did not well understand before. Do not strike into a lot of by-ways until you know the main roads : you will go along securely when you are not under the fear of going astray.
Hugonis de Sancto Victore Didascalicon de Studio legendi, Liber III, Caput V, 770A-770B:2 3
cum igitur de qualibet arte agimus, maxime in docendo, ubi omnia ad compendium restringenda sunt et ad facilem intelligentiam evocanda, sufficere debet id de quo agitur quantum brevius et apertius potest explanare, ne si alienas nimium rationes multiplicaverimus, magis turbemus quam aedificemus lectorem. non omnia dicenda sunt quae dicere possumus, ne minus utiliter dicantur ea quae dicere debemus. id tandem in unaquaque arte quaeras quod ad eam specialiter pertinere constiterit. deinde cum legeris artes, et quid uniuscuiusque sit proprium agnoveris disputando et conferendo, tunc demum rationes singularum invicem conferre licebit, et ex alterna consideratione vicissim quae minus prius intellexeras investigare. noli multiplicare diverticula quoadusque semitas didiceris. securus discurres cum errare non timueris.