The point of this letter:
we should play the part of a careful householder; we should increase what we have inherited
Seneca praises Quintus Sextius the Elder because his style of writing "fills [Seneca] with a mighty confidence before [he] closes his book" and causes him to say:
I want to challenge every hazard; I want to cry: "Why keep me waiting, Fortune? Enter the lists! Behold, I am ready for you!"
Therefore, we inheritors of philosophy ought to praise and honor our philosophical fore-fathers and then we ought to dedicate our time to solving problems and applying treatment.
I worship the discoveries of wisdom and their discoverers; to enter, as it were, into the inheritance of many predecessors is a delight. It was for me that they laid up this treasure; it was for me that they toiled. But we should play the part of a careful householder; we should increase what we have inherited.
It then becomes our task to "adding something further."
The cures for the spirit also have been discovered by the ancients; but it is our task to learn the method and the time of treatment. Our predecessors have worked much improvement, but have not worked out the problem.
More and more, we, humanity, are learning over again, hard-fought lessons our forbearers learned. While our challenges may not be all too different from theirs', how we approach and re-learn solutions may demand creativity.
Related to this are a few quotes from Vauvenargues, who Pierre Hadot cites in "Philosophy as a Way of Life" (see p. 108 and footnote 184 of the chapter):
"Every thought is new when the author expresses it in his own way"
"There are many things we do not know well enough, and that it is good to have repeated."
"A truly new and truly original book would be one which made people love old truths."