Duty is rooted in self understanding. What are you able to do well, the Stoics ask? What service is required? Throw yourself into that. Each of us has undeniable talents and abilities, whether they are physical, emotional or intellectual. More controversially, your natural makeup and disposition suggest there are things you should not do — you will never do them well, and they will offer perennial frustration. Hopefully, or ideally, your natural abilities will be cultivated and deployed most effectively and fruitfully. This makes for joy.
In essence, Seneca calls for a change of focus: Instead of straining to discover your one true passion, and devote your life and soul to it, study yourself and the needs of those around you. Frankly assess what you can do, how you are best equipped to serve, and work. Also: identify the several jobs you are called to do — inside and outside the home — and do them well.
Seneca also urges Serenus to avoid pinning his hopes on perceived results; we may not see any. Too often, we throw up our hands in despair when we think our efforts have no impact. Too often, we misjudge the nature and standard of success. But human perception is prone to error, philosophers have long pointed out. We are often ill equipped to measure, much less detect the fruits of our labor. We must, Seneca says, “just act” — just do your duty, and think of little else.