The gist of this letter is found towards the end of the middle. While Seneca spends a lot of time talking about style of speech and other styles, his point is that the outward stems from the inward. A sound soul leads to a sound body and style.
take care of the soul; for from the soul issue our thoughts, from the soul our words, from the soul our dispositions, our expressions, and our very gait. When the soul is sound and strong, the style too is vigorous, energetic, manly; but if the soul lose its balance, down comes all the rest in ruins.
Early in the letter, he cites a Greek proverb and then expounds a bit more on it.
"Man's speech is just like his life." Exactly as each individual man's actions seem to speak, so people's style of speaking often reproduces the general character of the time, if the morale of the public has relaxed and has given itself over to effeminacy. Wantonness in speech is proof of public luxury, if it is popular and fashionable, and not confined to one or two individual instances. A man's ability cannot possibly be of one sort and his soul of another. If his soul be wholesome, well-ordered, serious, and restrained, his ability also is sound and sober. Conversely, when the one degenerates, the other is also contaminated.
Does this apply to all? I've seen some people who seem to not have a well-ordered gait or even fit body or style of speech, but upon talking with them, they seem to be wise and virtuous. Conversely, I've seen people who are able to speak very well and their outward appearance and style seems to be fit and ordered, but upon talking to them, they only had appearance and not substance.
Is Seneca's analysis a hard and fast rule? Perhaps not. He does have a point, that if you take care of the soul and strive to keep it ordered, logical, virtuous and resilient, then perhaps the style will follow.
Sometimes, as individuals, we have to overcome the sentiment of the time. When culture and all around us has focused on the wrong things for so long, we have to strive, even harder as individuals, to rise above the polluted air and breath the fresh, clean air.
This fault is due sometimes to the man, and sometimes to his epoch. When prosperity has spread luxury far and wide, men begin by paying closer attention to their personal appearance.
Take care of our own soul first. Appearances will follow. Don't focus on your appearance and style, rather focus on what is virtuous, what is good. Strive for courage, wisdom, and temperance.
Here are a few more quotes from the letter which stood out to me.
Just as luxurious banquets and elaborate dress are indications of disease in the state, similarly a lax style, if it be popular, shows that the mind (which is the source of the word) has lost its balance. Indeed you ought not to wonder that corrupt speech is welcomed not merely by the more squalid mob but also by our more cultured throng.
The soul is our king. If it be safe, the other functions remain on duty and serve with obedience; but the slightest lack of equilibrium in the soul causes them to waver along with it. And when the soul has yielded to pleasure, its functions and actions grow weak, and any undertaking comes from a nerveless and unsteady source.
We should be sensible, and our wants more reasonable, if each of us were to take stock of himself, and to measure his bodily needs also, and understand how little he can consume, and for how short a time! But nothing will give you so much help toward moderation as the frequent thought that life is short and uncertain here below; whatever you are doing, have regard to death.