Pliny, Epistles V.6
“In sum (and why should I not reveal to you what is either my keen judgment or my error?) I consider it the chief business of an author to read his own title and ask himself again and again what he has begun to write. He should know that he will not be tedious if he sticks to his material, but will become wearisome if he calls up and adds something extra. You see how many verses Homer and Vergil used to describe the battles of Aeneas and Achilles; but each of these authors was brief because he stuck to his established subject. You see how Aratus pursues inquiries concerning even the most insignificant stars, but he still stays within the bounds: this is not just some digression of his – it is the work itself.”
In summa – cur enim non aperiam tibi vel iudicium meum vel errorem? – primum ego officium scriptoris existimo, titulum suum legat atque identidem interroget se quid coeperit scribere, sciatque si materiae immoratur non esse longum, longissimum si aliquid accersit atque attrahit. Vides quot versibus Homerus, quot Vergilius arma hic Aeneae Achillis ille describat; brevis tamen uterque est quia facit quod instituit. Vides ut Aratus minutissima etiam sidera consectetur et colligat; modum tamen servat. Non enim excursus hic eius, sed opus ipsum est.