The letter answers the question: are virtues living things? In sum, Seneca replies they are not; the soul is a living thing, which possesses virtues. He discusses this somewhat extensively, and then ends the letter with some thoughts on bravery and discipline.
Seneca doesn't think talking about this question is a good use of time, but nonetheless answers it.
You wish me to write to you my opinion concerning this question, which has been mooted by our school – whether justice, courage, foresight, and the other virtues, are living things. By such niceties as this, my beloved Lucilius, we have made people think that we sharpen our wits on useless objects, and waste our leisure time in discussions that will be unprofitable.
He gets directly to the answer.
The soul, men are agreed, is a living thing ... But virtue is nothing else than a soul in a certain condition; therefore it is a living thing. Again, virtue is active, and no action can take place without impulse. And if a thing has impulse, it must be a living thing.
The soul is a living thing with impulses. Virtue is active and prompts us to have an impulse, which we then act on.
We may have multiple virtues (or vices) in our soul, but we are still one living soul. Just like the hydra has many heads, it is still one soul.
each separate head fighting and destroying independently. And yet there is no separate living thing to each head; it is the head of a living thing, and the hydra itself is one single living thing.
Later in the letter, Seneca repeats and succinctly states his answer to the main question.
Every living thing acts of itself; but virtue does nothing of itself; it must act in conjunction with man. All living things either are gifted with reason, like men and gods, or else are irrational, like beasts and cattle. Virtues, in any case, are rational; and yet they are neither men nor gods; therefore they are not living things.
To conclude the letter, he makes the very important point that use of knowledge is what matters; not discussing and nitpicking. He focuses on the virtue of bravery to make this point.
Teach me, not whether Bravery be a living thing, but prove that no living thing is happy without bravery, that is, unless it has grown strong to oppose hazards and has overcome all the strokes of chance by rehearsing and anticipating their attack.
What is bravery?
It is the impregnable fortress for our mortal weakness; when a man has surrounded himself therewith, he can hold out free from anxiety during life's siege.
He quotes Posidonius:
"There are never any occasions when you need think yourself safe because you wield the weapons of Fortune; fight with your own! Fortune does not furnish arms against herself; hence men equipped against their foes are unarmed against Fortune herself."
Then he focuses on Alexander the Great, who conquered nations, but could not conquer himself and ended up killing his own friends.
he, the conqueror of so many kings and nations, was laid low by anger and grief! For he had made it his aim to win control over everything except his emotions.
Seneca writes a few, excellent reminders, which we all would do well to repeat to ourselves often.
Self-Command is the greatest command of all.
I must be just without reward.
May I take pleasure in devoting myself of my own free will to uphold this noblest of virtues.
Those who wish their virtue to be advertised are not striving for virtue but for renown.