This is somewhat of an odd letter and I'm not so sure I agree with parts of it.
Seneca speaks of a force that "drags us in one direction when we are aiming in another, urging us on to the exact place from which we long to withdraw."
In other words, why do we seemingly want to drift away from virtue and demonstrating an excellent soul and towards indifferents? I think it goes back to oikeosis and our animal nature of physical self-preservation. We have to fight the drift and recognize that our true nature is that of a rational being.
Seneca seems to think that some people are naturally (Nature made them that way) predisposed to more easily lead a virtuous life, while others need assistance.
how or when can we tear ourselves away from this folly? No man by himself has sufficient strength to rise above it; he needs a helping hand, and some one to extricate him.
He notes Epicurus, who mentions that some people make it on their own, without help. People who
have worked their way to the truth without any one's assistance, carving out their own passage. And he gives special praise to these, for their impulse has come from within, and they have forged to the front by themselves.
Another class of people cannot lead a virtuous life unless they have some help.
there are others who need outside help, who will not proceed unless someone leads the way, but who will follow faithfully.
A third class need a bit more focused help, but who can still succeed.
forced and driven into righteousness, who do not need a guide as much as they require someone to encourage and, as it were, to force them along.
He then compares different types of people to the kind of foundation they are built on. One building is built on solid rock (i.e. natural disposition toward virtue), the building of the edifice is easier and built more efficiently and quickly. Another identical (visually) building is not built on solid rock, but rather "soft and shifting ground" (i.e. does not have a natural disposition toward virtue), and therefore much more effort goes into stabilizing the foundation. Seneca, it seems, is more impressed with the person who fights a less solid, natural predisposition to overcome it and improve himself.
I should accordingly deem more fortunate the man who has never had any trouble with himself; but the other, I feel, has deserved better of himself, who has won a victory over the meanness of his own nature, and has not gently led himself, but has wrestled his way, to wisdom.
For those of us who need help, Seneca suggests we call for assistance. If we cannot find someone alive who can demonstrate excellence of soul, then we should "go to the ancients" for help. But if we can find someone living who can help us, then we should find the kind of people who are not hypocrites - people who practice what they preach. People who
teach us by their lives, men who tell us what we ought to do and then prove it by practice, who show us what we should avoid, and then are never caught doing that which they have ordered us to avoid.
Avoid the people who do not walk the talk.
they appear before the people for the express purpose of improving themselves and others, and do not practise their profession for the sake of self-seeking. For what is baser than philosophy courting applause?
The latter part of the letter gets into, what I think, specious ideas. Seneca thinks that you can judge a person's character by the slightest of signs such as a person's gait, hand movements, touching his head with a finger, shifting eyes, how he laughs and his general appearance and how he gives and receives praise. I think some of this might be accurate, but as a general rule of thumb, I do not think we can judge a person's character by physical movements. Perhaps they can be clues, but we should not cast full judgement until we really know the person.