Seneca wrote a whole book "On the shortness of life," on which I have commented before. But he addresses the same topic also in the much shorter letter XLIX to his friend Lucilius, as part of what Liz Gloyn has called Seneca's informal curriculum for the study of Stoicism.
This letter includes another jab at those who mis-spend their life studying logic for logic's sake, which -- as we have seen -- was the major subject matter of Letter XLVIII. Seneca here is downright sarcastic, providing us with echoes of the later writings of Epictetus:
"When the spears of the foeman were quivering in our gates and the very ground was rocking with mines and subterranean passages -- I say, they would rightly think me mad if I were to sit idle, putting such pretty posers as this: 'What you have not lost, you have. But you have not lost any horns. Therefore, you have horns,' or other tricks constructed after the model of this piece of sheer silliness." (XLIX.8)
The not so subtle reference is to the sophists, as the piece of "reasoning" concerning horns cannot fail but to remind us of Plato's Euthydemus, an important dialogue in which Socrates battles it out with two sophist brothers, Euthydemus and Dionysodorus. Even though Seneca does not refer to the dialogue, it is noteworthy that it's there that Socrates advances the argument that wisdom -- or virtue -- is the only true good, because nothing else has positive value unless it is used wisely, and because wisdom cannot be used for ill, by definition. That argument was incorporated and elaborated upon by the Stoics, as we can see, for instance, in Cicero's De Finibus, book III (see here and here).
Shortly thereafter, Seneca returns to the main topic of the letter: not just the shortness of life, but the contrast between quality and quantity of life:
"The good in life does not depend upon life's length, but upon the use we make of it; also, it is possible, or rather usual, for a person who has lived long to have lived too little." (XLIX.10)
The apparent contradiction in saying that someone may have both lived long and little is, of course, resolved by the fact that Seneca is using two different measures: the sheer number of years, and the number of virtuous things one has done. There is, obviously, no inconsistency in saying that someone -- indeed, many, according to Seneca -- has lived for many years and yet managed to do little good during all that time. Needless to say, for the Stoic it is virtuous actions that matter, no sheer longevity.
Toward the end of the letter, Seneca tackles the issue of whether we are born with virtue or we learn it along the way, a question that had famously vexed Socrates himself:
"At our birth nature made us teachable, and gave us reason, not perfect, but capable of being perfected." (XLIX.11)
This is a crucial point in Stoic philosophy: we are naturally endowed with the capacity to reason. that is why to "live according to nature" just means to apply reason to the improvement of the human cosmopolis. The Stoics maintain that we are born with what we would today call prosocial instincts, which we know have been ingrained into us by natural selection, just as it has happened to other social primates.
According to Stoic developmental moral psychology -- again, in line with modern cognitive science -- ever since we are born we are naturally concerned with our own wellbeing as well as with the wellbeing of our caretakers and other familiar people in our immediate surroundings. But once we reach the age of reason -- around 7 years old, give or take -- we develop the capacity for more abstract thought, including reasoning by analogy and by extrapolation. It is this capacity to reason that soon makes it plain that we are no different from every other human being, no matter where they live on the planet. It follows, for the Stoics, that we ought to accord to all members of the human cosmopolis the same respect and treatment we accord to our immediate friends and relatives.
That is the meaning of the famous notion of oikeiosis, or "appropriation" of others' concerns, as spelled out explicitly in the second century Stoic philosopher Hierocles:
"Each of us is, as it were, circumscribed by many circles; some of which are less, but others larger, and some comprehend, but others are comprehended, according to the different and unequal habitudes with respect to each other. For the first, indeed, and most proximate circle is that which everyone describes about his own mind as a centre, in which circle the body, and whatever is assumed for the sake of the body, are comprehended. … The second from this, and which is at a greater distance from the centre, but comprehends the first circle, is that in which parents, brothers, wife, and children are arranged. The third circle from the centre is that which contains uncles and aunts, grandfathers and grandmothers, and the children of brothers and sisters. … Next to this is that which contains the common people, then that which comprehends those of the same tribe, afterwards that which contains the citizens; and then two other circles follow, one being the circle of those that dwell in the vicinity of the city, and the other, of those of the same province. But the outermost and greatest circle, and which comprehends all the other circles, is that of the whole human race. … It is the province of him who strives to conduct himself properly in each of these connections to collect, in a certain respect, the circles, as it were, to one centre, and always to endeavor earnestly to transfer himself from the comprehending circles to the several particulars which they comprehend." (How we ought to conduct ourselves towards our kindred)
Nature endows us with instinctive prosociality and an ability to reason. We can improve the latter -- by means of philosophical reflection -- and arrive at the Stoic notion of cosmopolitanism.