Kathēkon (Greek: καθῆκον) (plural: kathēkonta Greek: καθήκοντα) is a Greek concept, forged by the founder of Stoicism, Zeno of Citium. It may be translated as "appropriate behaviour", "befitting actions", or "convenient action for nature", or also "proper function". Kathekon was translated in Latin by Cicero as officium, and by Seneca as convenentia. Kathēkonta are contrasted, in Stoic ethics, with katorthōma (κατόρθωμα; plural: katorthōmata), roughly "perfect action". According to Stoic philosophy, humans (and all living beings) must act in accordance with Nature, which is the primary sense of kathēkon.
According to Stoic philosophy, each being, whether animate or inanimate (plant, animal or human), carries on fitting actions corresponding to its own nature. They distinguished between "kathēkonta" and "katorthōmata," a perfect action derived from the "orthos logos" (reason) (also "teleion kathēkon": a perfect, achieved kathēkon). They said that the wise person, or sage, necessarily carried out katorthōmata, that is, virtuous kathēkon, and that what distinguished both was not the nature of the act, but the way it was done. Thus, in exceptional circumstances, a sage (a state of being which in Stoic philosophy is nearly impossible to achieve) could carry out a katorthōma which, according to ordinary standards, would be deemed monstruous (for example, having sexual intercourse with one's daughter, if the destiny of humanity is at stake, or mutilating oneself.)
Stoic morality is complex, and has various hierarchical levels. On the first, layman level, one must carry out the action corresponding to one's own nature. But, according to the Stoic strict moral ideas, the acts of a layperson are always misguided (ἁμαρτήματα hamartēmata  "mistakes," or peccata), while the acts of the rare sage are always katorthōmata, perfect actions. The sage acts in view of the good, while the ordinary being (layperson, animal or plant) acts only in view of its survival. However, both act according to their own nature.
Stoic philosophers distinguished another, intermediary level between kathēkonta and katorthōmata: mesa kathēkonta, or indifferent actions (which are neither appropriate, nor good). A list of kathēkonta would include: to stay in good health, to respect one's parents, etc. Para to kathēkon, or actions contrary to befitting actions, would be the reverse of this type of actions (to insult one's parents, etc.) Intermediary actions refers to "indifferent things" (ἀδιάφορα — adiaphora), which are in themselves neither good nor bad, but may be used in a convenient way or not. Such "indifferent things" include wealth, health, etc. These are not excluded from the domain of morality as one might expect: Cicero thus underlined, in De Finibus Bonorum et Malorum (About the Ends of Goods and Evils, III, 58-59), that when the wise person acts in the sphere of "indifferent things," he still acts conveniently, according to his own nature.
Intentionality is crucial in Stoic ethics: the morality of the act resides not in the act itself, but in the intentionality and the way in which it is realized, in other words, in the moral agent itself. Stobaeus defined kathēkonta as probable actions (probabilis ratio in Latin), or everything done for one reason (eulogos apologia in Greek). Cicero wrote: "quod autem ratione est, id officium appellamus; est igitur officium eius generis, quod nec in bonis ponatur nec in contrariis, in De Finibus Bonorum et Malorum, III, 58.
Another distinction between kathēkon and katorthōmata has been to say that katorthōmata were kathēkonta which "possessed all the numbers" (pantas apechon tous arithmous), a Stoic expression meaning perfection. Such a katorthōmata is done in harmony with all virtues, while the layperson may only act in accordance with one virtue, but not all of them. Stoics believe that all virtues are intertwined and that the perfect act encompasses all of them.