This is another long letter, in which Seneca provides many similar ideas to make a point. Let me try to frame it briefly and then proceed with the supporting quotes from the letter.
One of the bigger questions of life is: are we simply to live or are we to live well?
Most will admit that we are to live well.
The next question becomes: how does one live well?
Seneca and other philosophers would state that the practice of philosophy informs us how to live well. In this scope, philosophy informs us of a great number of things, which I'll get into in this post. But later in the letter, Seneca notes that Posidonius argued that philosophy "discovered the arts of which life makes use in its daily round" such as the invention of homes, big buildings, farming and clothing. Seneca disagrees and notes that philosophy solely focuses on living well and wisely, but does not steer into the domain of luxurious living. Nature has already provided this; but humans - by desiring luxury - have introduced the various arts of life. In a sense, these other arts are a distraction from the important goal of living well.
And now to the specific ideas and quotes from the letter.
Seneca argues "a good life is more of a benefit than mere life." For a human to merely live would put us on the same footing as the rest of the beasts. Our unique nature - rationality - prompts us to discover the good life. He proposes the following, in the pursuit of understanding the good life.
to discover the truth about things divine and things human ... to worship that which is divine, to love that which is human.
As society evolved, people desired rulers and the first rulers were indeed wise and knew what ought to be done in the conduct of ruling.
the government was under the jurisdiction of the wise. They kept their hands under control, and protected the weaker from the stronger. They gave advice, both to do and not to do; they showed what was useful and what was useless. Their forethought provided that their subjects should lack nothing; their bravery warded off dangers; their kindness enriched and adorned their subjects.
The ruled obeyed because the rulers were wise.
But, "vice stole in" and tyrannies and laws needed to be established.
At this point of the letter, Seneca quotes Posidonius, who claims it was philosophy which lead to the arts of life.
"When men were scattered over the earth, protected by caves or by the dug-out shelter of a cliff or by the trunk of a hollow tree, it was philosophy that taught them to build houses."
Seneca disagrees with Posidonius.
But I, for my part, do not hold that philosophy devised these shrewdly-contrived dwellings of ours which rise story upon story, where city crowds against city.
Seneca goes to the root. The desire for vice and the failure of rulers to rule wisely, introduced the ideas of protecting oneself against the vice of others. If all were taught well, and knew not to steal, there would be no need for the superfluous.
Was it philosophy that taught the use of keys and bolts? Nay, what was that except giving a hint to avarice?
And to be more precise, the desire for more (indifferent) things stems from the desire for luxury.
All this sort of thing was born when luxury was being born.
The big portion of the rest of the letter is a variation on this theme. Why were all these arts of life invented? Because people desired luxury more than learning to live well.
Seneca seems to contend that the mind busied with these types of inventions is one occupied with making physical living easier, at the expense of spending time on the higher: how to live well.
the sort of men who discover such things are the sort of men who are busied with them. ... They were both invented by some man whose mind was nimble and keen, but not great or exalted; and the same holds true of any other discovery which can only be made by means of a bent body and of a mind whose gaze is upon the ground.
He argues that the wise mine is content with little possessions which support his physical support of life.
The wise man was easy-going in his way of living. And why not? Even in our own times he would prefer to be as little cumbered as possible.
Wise men will abandon a cup, when their hand will do. He will prove
to others, as well as to himself, that nature has laid upon us no stern and difficult law when she tells us that we can live without the marble-cutter and the engineer, that we can clothe ourselves without traffic in silk fabrics ... [he will be] content with what the earth has placed on its surface [and that] the care of the body a problem easy to solve ... [and] will need no skilled craftsmen.
With regard to the vice of luxury, Seneca states,
Luxury has turned her back upon nature; each day she expands herself, in all the ages she has been gathering strength, and by her wit promoting the vices. At first, luxury began to lust for what nature regarded as superfluous, then for that which was contrary to nature; and finally she made the soul a bondsman to the body, and bade it be an utter slave to the body's lusts.
He admits that people's reason was put to use for the invention of all these things, but it was not "right reason."
Reason did indeed devise all these things, but it was not right reason. It was man, but not the wise man, that discovered them. ... All this sort of thing has been devised by the lowest grade of slaves. 26. Wisdom's seat is higher; she trains not the hands, but is mistress of our minds.
Wisdom shows us how to live well.
wisdom's course is toward the state of happiness ... she delivers to us the knowledge of the whole of nature ... [she will] inquire about the soul ... [and] has turned her attention from the corporeal to the incorporeal, and has closely examined truth and the marks whereby truth is known.
Furthermore, the wise man, in pursuit of wisdom, teaches us to
not merely to know the gods, but to follow them, and to welcome the gifts of chance precisely as if they were divine commands. He has forbidden us to give heed to false opinions, and has weighed the value of each thing by a true standard of appraisement. He has condemned those pleasures with which remorse is intermingled, and has praised those goods which will always satisfy; and he has published the truth abroad that he is most happy who has no need of happiness, and that he is most powerful who has power over himself.
Seneca later seems to note that putting the genie of vice and luxury back into the bottle will be an impossible task if people constantly and relentlessly pursue more.
no enlargement of our boundaries will bring us back to the condition from which we have departed. ... Care vexes us in our purple, and routs us from our beds with the sharpest of goads; but how soft was the sleep the hard earth bestowed upon the men of that day!
After all this discussion and many examples, he wonders what is to be done. He advocates learning and training and constant practice of philosophy.
Virtue is not vouchsafed to a soul unless that soul has been trained and taught, and by unremitting practice brought to perfection.