Definition
Duty, in Stoic philosophy, translates the Greek term kathekon, often rendered as appropriate action or fitting function. A duty is the action that suits a person's nature and circumstances, the response that a reasonable observer would recognize as the right thing to do here, now, in this role.
Stoic duty is not a rigid external rulebook. It is contextual: it depends on who one is, what relationships one holds, and what the situation calls for. The Roman Stoic Cicero devoted a whole work, On Duties, to spelling out how appropriate action is determined.
Why it matters
How it works
To identify a duty, the Stoic considers their roles and the relationships those roles create. Each role, child to a parent, neighbor to a community, professional in a craft, generates fitting actions. The practitioner asks what this role, in this moment, reasonably requires.
Duty then meets the dichotomy of control. A person performs their duty fully, because the action is theirs, while remaining detached from whether it is rewarded or even noticed. The Stoic does the right thing because it is fitting, not because of what follows from it. Over time, fulfilling duties well is part of how good character is built.