KATHEKON is ‘natural duty’. It is an ancient Greek word indicating ‘appropriate behaviour’, ‘befitting action’, ‘convenient activity for nature’. Also, another of its meanings is ‘proper function’. Marcus Tullius Cicero translates <kathekon> in Latin as <officium> (service). Instead, for Lucius Annaeus Seneca (Seneca the Younger), the Latin translation of <kathekon> is <convenientia>.
In Stoic ethics, there is a frequent comparison between <kathēkonta> (plural) and <katorthōmata> (singular <katorthōma>), that is ‘perfect actions’. According to Stoic philosophy, humans (and all the other living beings) must “behave in accordance with Nature”. Indeed, this is the primary meaning of <kathēkon>. And this natural, simple, coherent behavior is the basic stage concerning all people. Nonetheless, on a much higher level (a stage which only sages could possibly reach), there is <katorthōma> (the perfect action). However, we must underline that a <katorthōma> could well be an action which ordinary people would simply consider awful. This is because they are unable to see it in a wider perspective. For instance, killing some innocent people if the future of the entire world is at stake might well be <katorthōma>.
<Kathekon> is ‘natural duty’ and it is a simpler notion. Much simpler because it is natural. So any <kathekon> should come to us in an easy, genuine and unfeigned way. For instance, it is a ‘natural duty’ taking care of one’s sons. It is something which happens in a spontaneous and unaffected fashion. In other words, one does not think or mull over the feeding and the protection of his children. It is a natural thing.