At the heart of this letter, Seneca is giving us all some food for thought, on how to influence society. While political action may be the swiftest way to influence society, Seneca seems to suggest there are other, wise ways to take action.
The first couple of paragraphs discuss the Stoic view of self-preservation. We do not live for the body, but we must take care of the body to ensure our survival.
I do not maintain that the body is not to be indulged at all; but I maintain that we must not be slaves to it ... We should conduct ourselves not as if we ought to live for the body, but as if we could not live without it ... Virtue is held too cheap by the man who counts his body too dear. We should cherish the body with the greatest care; but we should also be prepared, when reason, self-respect, and duty demand the sacrifice, to deliver it even to the flames.
And when it comes to harming the body, a Stoic would not needlessly open himself to significant self-harm or death, if the Stoic could prevent it. It is based on this reasoning, that the Stoic would not seek to offend the powerful. It's as if Seneca is saying "stay in the game; and as long as you're in the game, you have a chance to be useful to society."
So the wise man will never provoke the anger of those in power; nay, he will even turn his course, precisely as he would turn from a storm if he were steering a ship ... he holds his course far from that region notorious for its swirling waters. Our wise man does the same; he shuns a strong man who may be injurious to him, making a point of not seeming to avoid him, because an important part of one's safety lies in not seeking safety openly; for what one avoids, one condemns.
Similarly, a Stoic would avoid the danger of the mob.
Next, he shares the middle ground Cato took. Cato fought to stay in the game as long as he could. But when it became clear that there was going to be a tyrant regardless, he chose the wise path.
Philosophy itself, however, should be practised with calmness and moderation. "Very well, then," you retort, "do you regard the philosophy of Marcus Cato as moderate? Cato's voice strove to check a civil war. Cato parted the swords of maddened chieftains. When some fell foul of Pompey and others fell foul of Caesar, Cato defied both parties at once!" Nevertheless, one may well question whether, in those days, a wise man ought to have taken any part in public affairs, and ask: "What do you mean, Marcus Cato? It is not now a question of freedom; long since has freedom gone to rack and ruin. The question is, whether it is Caesar or Pompey who controls the State. Why, Cato, should you take sides in that dispute? It is no business of yours; a tyrant is being selected. What does it concern you who conquers? The better man may win; but the winner is bound to be the worse man." I have referred to Cato's final role. But even in previous years the wise man was not permitted to intervene in such plundering of the state; for what could Cato do but raise his voice and utter unavailing words? At one time he was "hustled" by the mob and spat upon and forcibly removed from the forum and marked for exile; at another, he was taken straight to prison from the senate-chamber.
consider those Stoics who, shut out from public life, have withdrawn into privacy for the purpose of improving men's existence and framing laws for the human race without incurring the displeasure of those in power. The wise man will not upset the customs of the people, nor will he invite the attention of the populace by any novel ways of living.
And finally, the wise man regards the reason for all his actions, but not the results. The beginning is in our own power; fortune decides the issue, but I do not allow her to pass sentence upon myself.
"He who needs riches least, enjoys riches most." ... He who craves riches feels fear on their account. ... While he puzzles over increasing his wealth, he forgets how to use it.