Seneca, Letters to Lucilius
84.1-2 (tr. Richard M. Gummere):
And reading, I hold, is indispensable—primarily, to keep me from being satisfied with myself alone, and besides, after I have learned what others have found out by their studies, to enable me to pass judgment on their discoveries and reflect upon discoveries that remain to be made. Reading nourishes the mind and refreshes it when it is wearied with study; nevertheless, this refreshment is not obtained without study. We ought not to confine ourselves either to writing or to reading; the one, continuous writing, will cast a gloom over our strength, and exhaust it; the other will make our strength flabby and watery. It is better to have recourse to them alternately, and to blend one with the other, so that the fruits of one's reading may be reduced to concrete form by the pen.
sunt [sc. lectiones] autem, ut existimo, necessariae, primum ne sim me uno contentus; deinde ut, cum ab aliis quaesita cognovero, tum et de inventis iudicem et cogitem de inveniendis. alit lectio ingenium et studio fatigatum, non sine studio tamen, reficit. nec scribere tantum nec tantum legere debemus; altera res contristabit vires et exhauriet, de stilo dico, altera solvet ac diluet. invicem hoc et illo commeandum est et alterum altero temperandum, ut quicquid lectione collectum est, stilus redigat in corpus.