Pliny the Younger, Letter
7.9.1-3 (to Fuscus Salinator, tr. Betty Radice):
You ask me what course of study I think you should follow during your present prolonged holiday. The most useful thing, which is always being suggested, is to translate Greek into Latin and Latin into Greek. This kind of exercise develops in one a precision and richness of vocabulary, a wide range of metaphor, and power of exposition, and, moreover, imitation of the best models leads to a like aptitude for original composition. At the same time, any point which might have been overlooked by a reader cannot escape the eye of a translator. All this cultivates perception and critical sense.
Quaeris quemadmodum in secessu, quo iam diu frueris, putem te studere oportere. utile in primis, et multi praecipiunt, vel ex Graeco in Latinum vel ex Latino vertere in Graecum. quo genere exercitationis proprietas splendorque verborum, copia figurarum, vis explicandi, praeterea imitatione optimorum similia inveniendi facultas paratur; simul quae legentem fefellissent, transferentem fugere non possunt. intellegentia ex hoc et iudicium acquiritur.