In letter #124, Seneca engages the fountainhead of the ‘true good’ in human action. He concludes that virtue is the cradle of happiness, and the cradle of virtue is reason.
“And what is this good? I will tell you: It is a free and upstanding mind which subjects other things to itself and itself to nothing.
“We assert that ‘happy’ is what is in accordance with nature, and what is in accordance with nature is directly obvious, just as wholeness is obvious.
“As far as perception of good and evil is concerned both are equally mature; an infant is no more capable of the good than is a tree or some dumb animal. And why is the good not present in tree or dumb animal? Because reason is not.
“Confusion is applicable only where non-confusion can also occur, as anxiety is applicable only where serenity can obtain. No man is vicious unless he is capable of virtue.
“Pronounce yourself happy only when all your satisfactions are begotten of reason, and when, having surveyed what men struggle for, pray for, watch over, you find nothing to desire let alone prefer.
*All excerpts have been taken from The Stoic Philosophy of Seneca: Essays and Letters, W.W. Norton.