Lord Chesterfield, Letters to His Son (Letter 30):

Some learned men, proud of their knowledge, only speak to decide, and give judgment without appeal; the consequence of which is, that mankind, provoked by the insult, and injured by the oppression, revolt; and, in order to shake off the tyranny, even call the lawful authority in question. The more you know, the modester you should be: and (by the bye) that modesty is the surest way of gratifying your vanity. Even where you are sure, seem rather doubtful; represent, but do not pronounce, and, if you would convince others, seem open to conviction yourself.

Others, to show their learning, or often from the prejudices of a school education, where they hear of nothing else, are always talking of the ancients, as something more than men, and of the moderns, as something less. They are never without a classic or two in their pockets; they stick to the old good sense; they read none of the modern trash; and will show you, plainly, that no improvement has been made, in any one art or science, these last seventeen hundred years. I would by no means have you disown your acquaintance with the ancients: but still less would I have you brag of an exclusive intimacy with them. Speak of the moderns without contempt, and of the ancients without idolatry; judge them all by their merits, but not by their ages; and if you happen to have an Elzevir classic in your pocket neither show it nor mention it.

Some great scholars, most absurdly, draw all their maxims, both for public and private life, from what they call parallel cases in the ancient authors; without considering, that, in the first place, there never were, since the creation of the world, two cases exactly parallel; and, in the next place, that there never was a case stated, or even known, by any historian, with every one of its circumstances; which, however, ought to be known, in order to be reasoned from. Reason upon the case itself, and the several circumstances that attend it, and act accordingly; but not from the authority of ancient poets, or historians. Take into your consideration, if you please, cases seemingly analogous; but take them as helps only, not as guides. We are really so prejudiced by our education, that, as the ancients deified their heroes, we deify their madmen; of which, with all due regard for antiquity, I take Leonidas and Curtius to have been two distinguished ones. And yet a solid pedant would, in a speech in parliament, relative to a tax of two pence in the pound upon some community or other, quote those two heroes, as examples of what we ought to do and suffer for our country. I have known these absurdities carried so far by people of injudicious learning, that I should not be surprised, if some of them were to propose, while we are at war with the Gauls, that a number of geese should be kept in the Tower, upon account of the infinite advantage which Rome received IN A PARALLEL CASE, from a certain number of geese in the Capitol. This way of reasoning, and this way of speaking, will always form a poor politician, and a puerile declaimer.

There is another species of learned men, who, though less dogmatical and supercilious, are not less impertinent. These are the communicative and shining pedants, who adorn their conversation, even with women, by happy quotations of Greek and Latin; and who have contracted such a familiarity with the Greek and Roman authors, that they, call them by certain names or epithets denoting intimacy. As OLD Homer; that SLY ROGUE Horace; MARO, instead of Virgil; and Naso, Instead of Ovid. These are often imitated by coxcombs, who have no learning at all; but who have got some names and some scraps of ancient authors by heart, which they improperly and impertinently retail in all companies, in hopes of passing for scholars. If, therefore, you would avoid the accusation of pedantry on one hand, or the suspicion of ignorance on the other, abstain from learned ostentation. Speak the language of the company that you are in; speak it purely, and unlarded with any other. Never seem wiser, nor more learned, than the people you are with. Wear your learning, like your watch, in a private pocket: and do not pull it out and strike it; merely to show that you have one. If you are asked what o’clock it is, tell it; but do not proclaim it hourly and unasked, like the watchman.

Upon the whole, remember that learning (I mean Greek and Roman learning) is a most useful and necessary ornament, which it is shameful not to be master of; but, at the same time most carefully avoid those errors and abuses which I have mentioned, and which too often attend it. Remember, too, that great modern knowledge is still more necessary than ancient; and that you had better know perfectly the present, than the old state of Europe; though I would have you well acquainted with both.