"Some men shrink into dark corners, to such a degree that they see darkly by day."--Pomponius, quoted by Seneca, Letter 3.
Our personality shapes, as Pomponius's maxim suggests, how we view the world. This is why any interpretation of a layered text often reveals as much about the interpreter as it does about the text. In the third letter, in the context of discussing a discussing true friendships [verae amicitiae], Seneca discusses three kinds of human types: (i) the gullible, (ii) the suspicious, and (iii) those that trust after a considered judgment (if this were a Platonic dialogue, we'd be looking for a fourth.)
Seneca does not explain much how good judgment is attained. He does exhibit a feature of it in the start of the letter:
You have sent a letter to me through the hand of a "friend" of yours, as you call him. And in your very next sentence you warn me not to discuss with him all the matters that concern you, saying that even you yourself are not accustomed to do this; in oother words, you have in the same letter affirmed and denied that he is your friend. Now if you used this word of ours/a in the public [publico] sense, and called him "friend" in the same way in which we speak of all candidates for election as "honourable gentlemen,"...
Thus, a discerning judge pays attention to (a) the match between words and actions, and (b) does so by relying on (some) logic. More subtly, such a judge is aware that (c) the meaning of words is very context sensitive; in particular, (d) Seneca relies on a distinction between public the and private speech.* As we have seen, public speech is encountered in the market-place and politics, both the realm of uncertain uncertain and fickle popular opinion. One can have thousands of Facebook friends, but one's popularity need not imply credibility (and not all followers are steadfast).
Now, in addition to teaching good example by judgment (and correction), Seneca also continues his habit of imparting a daily maxim (with the apparent expectation that this gets mulled over). As we have seen, Seneca is committed to the idea that words can improve receptive minds. If his previous two maxims came from revered authorities (tradition and philosophy++), the present is derived from a (comic) poet. Apparently, the business of self-improvement need not be all serious.
A suspicious reader might wonder to what degree Seneca's expressed expectations are just a form of flattery. Rather than poisoning our minds by pursuing such suspicions, we can pose the issue in a more constructive fashion: through the first three letters Seneca suggests that the very same words -- including ordinary terms like 'poverty' and 'friendship' -- can mean different things to different audiences, depending on their values, circumstances, and personalities. To put the issue a bit melodramatically: there is real risk of combinatorial explosion in the number of possible of interpreted meanings. There are at least three personality types and at least two axiological spheres that may shape one's interpretations. One consequence of such interpretation proliferation is that one's intentions may be misunderstood (this is the stuff of comedy and tragedy, of course).
Beyond exhibiting economy of expression (the third letter is no more than 356 words), Seneca does not address how or if one ought to write for multiple possible audiences at once.
Seneca does present a picture of true friendship in which bold [audaciter] speech is a paramount virtue. But here, too, Seneca complicates matters by not recommending boldness as such, but rather to be as bold with one's friend as one is with oneself [tam audaciter cum illo loquere quam tecum]. This raises the spectre of a variety of true friendships depending on how one treats oneself. For example, if one treats oneself as steadfast [fidelem], one generates -- if one has discerning judgment -- loyal friends. This is not to deny that there may be an exemplary true friendship--Seneca adds that among such friends one is forthright.
Throughout the third letter there is an implied contrast between the ordinary axiology of politics (and the market) where one speaks to persuade based on one's judgment of the heterogeneous (and inconstant) desires of others, while among true friends one might as well consider onself as alone [putem solum]. To put the point anachronistically (and in the spirit of exploring Seneca as an alternative political economy): among true friends we are all a possible representative agent, but this is not appropriate among people we merely call friends in the market-place and politics. In a short, forgotten paper, the future Nobel laureate, and careful student of Adam Smith, George Stigler, made -- citing Aristotle and J.S. Mill-- pretty much the same point (recall).
*My use of 'private' may be a bit anachronistic in so far as it is appears to be the case that for Seneca the "private" (as I am using it), or the "true," is an axiological sphere inhabited by the few.
++ In his second letter Seneca made great show of appealing to Epicurus; in the third letter, he treats Theophrastus (Aristotle's successor) as authority. Presumably Lucilius (and the reader) knows that Seneca belongs to the Stoic school, but Seneca does not state it (yet).