There are a number of syllogisms and Peripatetic responses to them in this letter. This is another long-winded, somewhat meandering letter Seneca writes, in order to draw out minute distinctions to support the idea of living a simple life. I'm going to attempt to summarize this, with some supporting quotes.
I'll frame the letter with some straight-forward questions, which I think take the direct route as to the purpose of this letter.
Do you need riches in order to be good? The short answer is: no.
Can you pursue riches and still be good? The short answer is: yes; but there are perils along the way.
Are riches evil? The short answer is: no.
Can riches cause evil? The short answer is: maybe.
Riches and wealth are Stoic indifferents. They fall under the category of "not up to us." True, one may pursue riches and gain them and still be virtuous; but they can also be taken away through Fate and Fortune. Regardless if we pursue and attain them or if they fall by chance, into our lot, how we demonstrate excellence of character remains "up to us" while the acquisition or removal of wealth is "not up to us."
But, if you are going to pursue wealth, keep in mind that it does not bestow virtue (arete) on you and in the pursuit, you will encounter temptations of committing evils and may be swayed from a virtuous path.
Now to the quotes and some commentary.
"I was shipwrecked before I got aboard." ... the journey showed me this: how much we possess that is superfluous; and how easily we can make up our minds to do away with things whose loss, whenever it is necessary to part with them, we do not feel.
Seneca packed light for his journey and it made him realize how little we need.
the soul is never greater than when it has laid aside all extraneous things, and has secured peace for itself by fearing nothing, and riches by craving no riches.
The fewer things we are in need of, the closer to the gods we move; for the gods need nothing. This is what the Cynic philosophers hoped to prove. But so few people could get on board with that lifestyle, the Stoics eased the thinking back a bit and introduced the idea of preferred indifferents. Many, through the years, have demonstrated how little we need. Diogenes the Cynic and Henry David Thoreau come to mind.
I have not yet the courage openly to acknowledge my thriftiness. Even yet I am bothered by what other travellers think of me.
This is rich, coming from Seneca. Should I insert an facepalm or shaking-my-head emoji here?
after you have mentioned all these facts, he is poor. And why? He is in debt.
Not all that glitters is gold. Do not be so mystified at other people's wealth. While they may be glitzy on the outside, you better reserve judgement until you see their debts. Some people can manage debt quite well, to leverage their way to wealth. But this takes skill and discipline. Others fall into the traps of debt and then plead for a bail-out.
Next are the syllogisms.
"That which is good makes men good. For example, that which is good in the art of music makes the musician. But chance events do not make a good man; therefore, chance events are not goods."
The unique skill found entirely within the individual is the good. Through willpower, dedication and discipline, one may become good at a certain skill. Indeed, they will have to use instruments and tools, but these are incidental to the will. Under the category of incidentals are chance events.
Seneca clarifies this syllogism:
We define the good in the art of music in two ways: first, that by which the performance of the musician is assisted, and second, that by which his art is assisted. ... he is an artist even without [instruments].
"That which can fall to the lot of any man, no matter how base or despised he may be, is not a good. But wealth falls to the lot of the pander and the trainer of gladiators; therefore wealth is not a good."
In sum, chance events are indifferents and therefore not a good. The good is wholly "up to us."
It is virtue that uplifts man and places him superior to what mortals hold dear; virtue neither craves overmuch nor fears to excess that which is called good or that which is called bad. ... what it is that produces the wise man? That which produces a god. You must grant that the wise man has in an element of godliness, heavenliness, grandeur. The good does not come to every one, nor does it allow any random person to possess it.
"Good does not result from evil. But riches result from greed; therefore, riches are not a good."
Good can only result from the will; from within the soul. If you are going to pursue riches, the reason is important. If the reason is greed, then you have fallen into vice. Therefore, getting precise and specific as to the reason for the pursuit of indifferents, matters.
Things which grow correspond to their seed; and goods cannot depart from their class. As that which is honourable does not grow from that which is base, so neither does good grow from evil. For the honourable and the good are identical.
"That which, while we are desiring to attain it, involves us in many evils, is not a good. But while we are desiring to attain riches, we become involved in many evils; therefore, riches are not a good,"
The intent of this syllogism is to throw caution at anyone who is pursuing riches. Seneca later writes:
Riches injure no one; it is a man's own folly, or his neighbour's wickedness, that harms him in each case, just as a sword by itself does not slay; it is merely the weapon used by the slayer. ... Posidonius is better: he holds that riches are a cause of evil, not because, of themselves, they do any evil, but because they goad men on so that they are ready to do evil.
Going back to intent, we need to be crystal clear as to why we would pursue riches. Riches are indifferent, but it is the use of indifferents which determines excellence or mediocrity of soul.
"Things which bestow upon the soul no greatness or confidence or freedom from care are not goods. But riches and health and similar conditions do none of these things; therefore, riches and health are not goods."
"Things which bestow upon the soul no greatness or confidence or freedom from care, but on the other hand create in it arrogance, vanity, and insolence, are evils. But things which are the gift of Fortune drive us into these evil ways. Therefore these things are not goods."
It is clear, riches are not goods; they are indifferents. What is good?
A thing is not good if it contains more benefit than injury, but only if it contains nothing but benefit ... The good, however, can be predicated of the wise man alone.
In sum, it may be easier to check our desires and intent, than to try to justify specious pursuits of riches via rationalization.
it were better to support this law by our conduct and to subdue our desires by direct assault rather than to circumvent them by logic.