The duty of a man is to be useful to his fellow-men; if possible, to be useful to many of them; failing this, to be useful to a few; failing this, to be useful to his neighbours, and, failing them, to himself: for when he helps others, he advances the general interests of mankind. Just as he who makes himself a worse man does harm not only to himself but to all those to whom he might have done good if he had made himself a better one, so he who deserves well of himself does good to others by the very fact that he is preparing what will be of service to them.”
It also tells us that the key to a serene life is the realisation that some things are under our control and others are not: under our control are our values, our judgments, and the actions we choose to perform. Everything else lies outside of our control, and we should focus our attention and efforts only on the first category.
For Seneca and the Stoics, the only life worth living is one of moral rectitude, the sort of existence we look back to at the end and can honestly say we are not ashamed of.
Seneca wrote a much longer essay on the same topic of what makes for a happy life, one that includes a set of seven ‘commandments to himself’ (from book XX ‘Of a Happy Life’). They provide a way to philosophically structure our own lives:
I) I will look upon death or upon a comedy with the same expression of countenance.
II) I will despise riches when I have them as much as when I have them not.
III) I will view all lands as though they belong to me, and my own as though they belonged to all mankind.
IV) Whatever I may possess, I will neither hoard it greedily nor squander it recklessly.
V) I will do nothing because of public opinion, but everything because of conscience.
VI) I will be agreeable with my friends, gentle and mild to my foes: I will grant pardon before I am asked for it, and will meet the wishes of honourable men half-way.
VII) Whenever either Nature demands my breath again, or reason bids me dismiss it, I will quit this life, calling all to witness that I have loved a good conscience, and good pursuits.”