Wisdom from the Early Stoics, Zeno of Citium 68

Furthermore, the term Duty is applied to that for which, when done, a reasonable defense can be adduced, e.g. harmony in the tenor of life's process, which indeed pervades the growth of plants and animals. For even in plants and animals, they hold, you may discern fitness of behavior. 

Zeno was the first to use this term καθῆκον of conduct. Etymologically it is derived from κατά τινας ἥκειν, i.e. reaching as far as, being up to, or incumbent on so and so. 

And it is an action in itself adapted to nature's arrangements. For of the acts done at the prompting of impulse some, they observe, are fit and meet, others the reverse, while there is a third class which is neither the one nor the other. 

Befitting acts are all those which reason prevails with us to do; and this is the case with honoring one's parents, brothers and country, and intercourse with friends. 

Unbefitting, or contrary to duty, are all acts that reason deprecates, e.g. to neglect one's parents, to be indifferent to one's brothers, not to agree with friends, to disregard the interests of one's country, and so forth. 

Acts which fall under neither of the foregoing classes are those which reason neither urges us to do nor forbids, such as picking up a twig, holding a style or a scraper, and the like. 

Again, some duties are incumbent unconditionally, others in certain circumstances. 

Unconditional duties are the following: to take proper care of health and one's organs of sense, and things of that sort. 

Duties imposed by circumstances are such as maiming oneself and sacrifice of property. 

And so likewise with acts which are violations of duty. 

Another division is into duties which are always incumbent and those which are not. 

To live in accordance with virtue is always a duty, whereas dialectic by question and answer or walking-exercise and the like are not at all times incumbent. 

The same may be said of the violations of duty. 

And in things intermediate also there are duties; as that boys should obey the attendants who have charge of them. 

—Diogenes Laërtius, 7.107-110 

IMAGE: Edward Leighton, Duty (1883)