It seems there is an accusation or belief that philosophers are "stubborn and rebellious, scorners of magistrates or kings or of those who control the administration of public affairs." But Seneca contends it is the opposite; that philosophers ought to appreciate kings and public officials, who make the city and country a place where it is possible for philosophers to study and practice their theories. He states,
those who are greatly profited, as regards their purpose of right living, by the security of the State, must needs cherish as a father the author of this good; much more so, at any rate, than those restless persons who are always in the public eye.
The assumption here, from what I understand, is that kings and public officials are not tyrants and have done an adequate job at securing some degree of peace and prosperity for most of their citizens. Otherwise, I can't fathom how philosophers would or could extend gratitude to them if they were tyrants or dictators.
He does mention the ungrateful and covetous people, who perhaps are the types of people who eventually turn into tyrants.
there is no greater evil in covetousness than its ingratitude. ... That is the trouble with every sort of ambition; it does not look back. Nor is it ambition alone that is fickle, but also every sort of craving, because it always begins where it ought to end.
But the good men in public life, who know the difficulties of governing, they more fully appreciate those who remain in public life and who continue to work to secure peace and prosperity for the people.
that other man, upright and pure, who has left the senate and the bar and all affairs of state, that he may retire to nobler affairs, cherishes those who have made it possible for him to do this in security ... the sage honours these men, also, under whose guardianship he can put his good theories into practice. ... the benefits of this peace, which extends to all, are more deeply appreciated by those who make good use of it.
Seneca seems to take a practical perspective and appreciates the work public officials do. Just as the sun shines on all, as does the rain fall on all, what matters is our individual attitude. We ought to acknowledge this fact and realize it is not our individual wishes which have made these things so. Yet nonetheless, we can still show gratitude.
I owe a great debt to the sun and to the moon; and yet they do not rise for me alone. I am personally beholden to the seasons and to the god who controls them, although in no respect have they been apportioned for my benefit. ... But our philosopher considers nothing more truly his own than that which he shares in partnership with all mankind. ... the great and true goods are not divided in such a manner that each has but a slight interest; they belong in their entirety to each individual.
Seneca continues the letter with a reminder of focusing on what is up to us, which does not require division and sharing with others.
These goods, however, are indivisible, – I mean peace and liberty, – and they belong in their entirety to all men just as much as they belong to each individual.
And then he continues with his gratitude towards those who govern.
the philosopher thinks of the person who makes it possible for him to use and enjoy these things ... he gives thanks to the helmsman of his state. This is what philosophy teaches most of all, – honourably to avow the debt of benefits received, and honourably to pay them.
Next, Seneca briefly returns to the topic of virtue and excellence. Virtue is independent of time. Seneca uses the example of Zeus (Nature) as well as two wise men, one who lives briefly and one who lives longer, to make his point.
In what respect is Jupiter superior to our good man? His goodness lasts longer; but the wise man does not set a lower value upon himself, just because his virtues are limited by a briefer span. Or take two wise men; he who has died at a greater age is not happier than he whose virtue has been limited to fewer years: similarly, a god has no advantage over a wise man in point of happiness, even though he has such an advantage in point of years. That virtue is not greater which lasts longer.
The way to virtue is via knowledge and moral ethics. Seneca quotes Sextius.
"This is 'the way to the stars'; this is the way, by observing thrift, self-restraint, and courage!"
Seneca closes the letter in a way that is similar to the parable of the ground and seeds, which Jesus taught. It seems that both analogies focus on the good nature of the seed, but the different way that the seed is received is what matters if the seed grows or not. In Jesus' parable, the soil represents our unique choice as to whether the seed takes root or not. Similarly, in Seneca's analogy, the way the husbandman receives the seed determines if the roots (the good) take hold or not.
Here is the link to Jesus' parable and below is the passage from Seneca.
Divine seeds are scattered throughout our mortal bodies; if a good husbandman receives them, they spring up in the likeness of their source and of a parity with those from which they came. If, however, the husbandman be bad, like a barren or marshy soil, he kills the seeds, and causes tares to grow up instead of wheat.
To be more clear, from a Stoic perspective - each of us has divinity within ("divine seeds"), which is pneuma. What we choose to do with that divinity is up to us. We can either give in to vice or we can demonstrate excellence of character by exercising moral, ethical excellence, by rationally and actively choosing to live and act this way.