Amazing pieces of advice that are 400 years old and can be used still today.

Don Quixote, the main hero of Cervantes' novel with the same title, suffered from a mental disorder that today could probably classified as a psychotic disorder with a reasonably well systematised set of grandiose delusions, in the single area of him becoming a knight errant. Apart from that, his personality was rather intact, and when he discussed the topics that remained outside the system of his delusions, he amazed everyone with his knowledge and wisedom.

He gave excellent pieces of advice to Sancho Panza., his squire, who was just about to become the governor of an island. The first set are the "instructions for the adornment of the mind":

"First of all, my son, thou must fear God, for in the fear of him is wisdom, and being wise thou canst not err in aught.
"Secondly, thou must keep in view what thou art, striving to know thyself, the most difficult thing to know that the mind can imagine. If thou knowest thyself, it will follow thou wilt not puff thyself up like the frog that strove to make himself as large as the ox; if thou dost, the recollection of having kept pigs in thine own country will serve as the ugly feet for the wheel of thy folly."
"That's the truth," said Sancho; "but that was when I was a boy; afterwards when I was something more of a man it was geese I kept, not pigs. But to my thinking that has nothing to do with it; for all who are governors don't come of a kingly stock."
"True," said Don Quixote, "and for that reason those who are not of noble origin should take care that the dignity of the office they hold he accompanied by a gentle suavity, which wisely managed will save them from the sneers of malice that no station escapes.
"Glory in thy humble birth, Sancho, and he not ashamed of saying thou art peasant-born; for when it is seen thou art not ashamed no one will set himself to put thee to the blush; and pride thyself rather
upon being one of lowly virtue than a lofty sinner. Countless are they who, born of mean parentage, have risen to the highest dignities, pontifical and imperial, and of the truth of this I could give thee
instances enough to weary thee.
"Remember, Sancho, if thou make virtue thy aim, and take a pride in doing virtuous actions, thou wilt have no cause to envy those who have princely and lordly ones, for blood is an inheritance, but virtue
an acquisition, and virtue has in itself alone a worth that blood does not possess.
"This being so, if perchance anyone of thy kinsfolk should come to see thee when thou art in thine island, thou art not to repel or slight him, but on the contrary to welcome him, entertain him, and
make much of him; for in so doing thou wilt be approved of heaven (which is not pleased that any should despise what it hath made), and wilt comply with the laws of well-ordered nature.
"If thou carriest thy wife with thee (and it is not well for those that administer governments to be long without their wives), teach and instruct her, and strive to smooth down her natural roughness; for all
that may be gained by a wise governor may be lost and wasted by a boorish stupid wife.
"If perchance thou art left a widower- a thing which may happen- and in virtue of thy office seekest a consort of higher degree, choose not one to serve thee for a hook, or for a fishing-rod, or for the hood of thy 'won't have it;' for verily, I tell thee, for all the judge's wife receives, the husband will be held accountable at the general calling to account; where he will have repay in death fourfold, items that in life he regarded as naught.
"Never go by arbitrary law, which is so much favoured by ignorant men who plume themselves on cleverness.
"Let the tears of the poor man find with thee more compassion, but not more justice, than the pleadings of the rich.
"Strive to lay bare the truth, as well amid the promises and presents of the rich man, as amid the sobs and entreaties of the poor.
"When equity may and should be brought into play, press not the utmost rigour of the law against the guilty; for the reputation of the stern judge stands not higher than that of the compassionate. 
"If perchance thou permittest the staff of justice to swerve, let it be not by the weight of a gift, but by that of mercy. 
"If it should happen thee to give judgement in the cause of one who is thine enemy, turn thy thoughts away from thy injury and fix them on the justice of the case.
"Let not thine own passion blind thee in another man's cause; for the errors thou wilt thus commit will be most frequently irremediable; or if not, only to be remedied at the expense of thy good name and even of thy fortune.
"If any handsome woman come to seek justice of thee, turn away thine eyes from her tears and thine ears from her lamentations, and consider deliberately the merits of her demand, if thou wouldst not have thy reason swept away by her weeping, and thy rectitude by her sighs.
"Abuse not by word him whom thou hast to punish in deed, for the pain of punishment is enough for the unfortunate without the addition of thine objurgations.
"Bear in mind that the culprit who comes under thy jurisdiction is but a miserable man subject to all the propensities of our depraved nature, and so far as may be in thy power show thyself lenient and
forbearing; for though the attributes of God are all equal, to our eyes that of mercy is brighter and loftier than that of justice.
"If thou followest these precepts and rules, Sancho, thy days will be long, thy fame eternal, thy reward abundant, thy felicity unutterable; thou wilt marry thy children as thou wouldst; they and thy grandchildren will bear titles; thou wilt live in peace and concord with all men; and, when life draws to a close, death will come to thee in calm and ripe old age, and the light and loving hands of
thy great-grandchildren will close thine eyes.
"What I have thus far addressed to thee are instructions for the adornment of thy mind; listen now to those which tend to that of the body.

And here is the second part - "instructions which tend to that of the body":

"With regard to the mode in which thou shouldst govern thy person and thy house, Sancho, the first charge I have to give thee is to be clean, and to cut thy nails, not letting them grow as some do, whose
ignorance makes them fancy that long nails are an ornament to their hands, as if those excrescences they neglect to cut were nails, and not the talons of a lizard-catching kestrel- a filthy and unnatural
abuse.
"Go not ungirt and loose, Sancho; for disordered attire is a sign of an unstable mind, unless indeed the slovenliness and slackness is to he set down to craft, as was the common opinion in the case of
Julius Caesar.
"Ascertain cautiously what thy office may be worth; and if it will allow thee to give liveries to thy servants, give them respectable and serviceable, rather than showy and gay ones, and divide them between thy servants and the poor; that is to say, if thou canst clothe six pages, clothe three and three poor men, and thus thou wilt have pages for heaven and pages for earth; the vainglorious never think of this new mode of giving liveries.
"Eat not garlic nor onions, lest they find out thy boorish origin by the smell; walk slowly and speak deliberately, but not in such a way as to make it seem thou art listening to thyself, for all
affectation is bad.
"Dine sparingly and sup more sparingly still; for the health of the whole body is forged in the workshop of the stomach.
"Be temperate in drinking, bearing in mind that wine in excess keeps neither secrets nor promises.
"Take care, Sancho, not to chew on both sides, and not to eruct in anybody's presence."
"Eruct!" said Sancho; "I don't know what that means."
"To eruct, Sancho," said Don Quixote, "means to belch, and that is one of the filthiest words in the Spanish language, though a very expressive one; and therefore nice folk have had recourse to the
Latin, and instead of belch say eruct, and instead of belches say eructations; and if some do not understand these terms it matters little, for custom will bring them into use in the course of time,
so that they will be readily understood; this is the way a language is enriched; custom and the public are all-powerful there."
"In truth, senor," said Sancho, "one of the counsels and cautions I mean to bear in mind shall be this, not to belch, for I'm constantly doing it."
"Eruct, Sancho, not belch," said Don Quixote.
"Eruct, I shall say henceforth, and I swear not to forget it," said Sancho.
"Likewise, Sancho," said Don Quixote, "thou must not mingle such a quantity of proverbs in thy discourse as thou dost; for though proverbs are short maxims, thou dost drag them in so often by the head and shoulders that they savour more of nonsense than of maxims."
"God alone can cure that," said Sancho; "for I have more proverbs in me than a book, and when I speak they come so thick together into my mouth that they fall to fighting among themselves to get out; that's why my tongue lets fly the first that come, though they may not be pat to the purpose. But I'll take care henceforward to use such as befit the dignity of my office; for 'in a house where there's plenty, supper is soon cooked,' and 'he who binds does not wrangle,' and 'the bell-ringer's in a safe berth,' and 'giving and keeping require brains.'"
"That's it, Sancho!" said Don Quixote; "pack, tack, string proverbs together; nobody is hindering thee! 'My mother beats me, and I go on with my tricks.' I am bidding thee avoid proverbs, and
here in a second thou hast shot out a whole litany of them, which have as much to do with what we are talking about as 'over the hills of Ubeda.' Mind, Sancho, I do not say that a proverb aptly brought in is objectionable; but to pile up and string together proverbs at random makes conversation dull and vulgar.
"When thou ridest on horseback, do not go lolling with thy body on the back of the saddle, nor carry thy legs stiff or sticking out from the horse's belly, nor yet sit so loosely that one would
suppose thou wert on Dapple; for the seat on a horse makes gentlemen of some and grooms of others.
"Be moderate in thy sleep; for he who does not rise early does not get the benefit of the day; and remember, Sancho, diligence is the mother of good fortune, and indolence, its opposite, never yet
attained the object of an honest ambition.
"The last counsel I will give thee now, though it does not tend to bodily improvement, I would have thee carry carefully in thy memory, for I believe it will be no less useful to thee than those I have
given thee already, and it is this- never engage in a dispute about families, at least in the way of comparing them one with another; for necessarily one of those compared will be better than the other, and thou wilt be hated by the one thou hast disparaged, and get nothing in any shape from the one thou hast exalted.
"Thy attire shall be hose of full length, a long jerkin, and a cloak a trifle longer; loose breeches by no means, for they are becoming neither for gentlemen nor for governors.
"For the present, Sancho, this is all that has occurred to me to advise thee; as time goes by and occasions arise my instructions shall follow, if thou take care to let me know how thou art circumstanced."

(DON QUIXOTE, by Miguel de Cervantes, translated by John Ormsby. Excerpts from Book II, Chapters LXII and LXIII)

I do not think that the above requires much commenting. I was quite stunned when I had read it. I have actually started expanding and explaining these pieces of guidance in the context of the contemporary world, this work can be found here.