18 Do not be carried away by some mad caprice and rush into authorship. Learn long and carefully what you propose to teach. Do not credit all that flatterers say to you, or, I should rather say, do not lend too ready an ear to those who mean to mock you. They will fawn upon you with fulsome praise and do their best to blind your judgment; yet if you suddenly look behind you, you will find that they are making gestures of derision with their hands, either a stork's neck or the flapping ears of a donkey or a thirsty dog's protruding tongue.
Never speak evil of anyone or suppose that you make yourself better by assailing the reputations of others. The charges we bring against them often come home to ourselves; we inveigh against faults which are as much ours as theirs; and so our eloquence ends by telling against ourselves. It is as though dumb persons were to criticize orators. When the grunter wished to speak he used to come forward at a snail's pace and to utter a word now and again with such long pauses between that he seemed less making a speech than gasping for breath. Then, when he had placed his table and arranged on it his pile of books, he used to knit his brow, to draw in his nostrils, to wrinkle his forehead and to snap his fingers, signs meant to engage the attention of his pupils. Then he would pour forth a torrent of nonsense and declaim so vehemently against every one that you would take him for a critic like Longinus or fancy him a second Cato the Censor passing judgment on Roman eloquence and excluding whom he pleased from the senate of the learned. As he had plenty of money he made himself still more popular by giving entertainments. Numbers of persons shared in his hospitality; and thus it was not surprising that when he went out he was surrounded always by a buzzing throng. At home he was a monster like Nero, abroad a paragon like Cato. Made up of different and opposing natures, as a whole he baffled description. You would say that he was formed of jarring elements like that unnatural and unheard of monster of which the poet tells us that it was 'in front a lion, behind a dragon, in the middle the goat whose name it bears.'
Source: Letters (New Advent)