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 has been translated in Latin by Cicero as officium, and by Seneca as convenentia. Kathēkonta are contrasted, in Stoic moral, with katorthōma (κατόρθωμα; plural: katorthōmata), roughly "perfect action." According to Stoic philosophy, man (and all living beings) must act in accordance with Nature, which is the primary sense of kathēkon.
Kathēkonta and katorthōmata
According to Stoic philosophy, each being, whether animate or inanimate (plant, animal or human being), 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 man 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 wise man (which state of being, 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, lay-man level, one must carry out the action corresponding to his own nature. But, according to the Stoic strict moral ideas, the acts of laymen are always insane (ἁμαρτήματα hamartēmata  "mistakes," or peccata), while the acts of the rare wise-man are always katorthōmata, perfect actions. The wise man acts in view of the good, while the ordinary being (layman, 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 man acts in the sphere of "indifferent things," he still acts conveniently, according to his own nature.
Intentionality and perfection
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 lay-man 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.
- Nova Roma, interview of A. Poliseno, "Stoicism in Ancient Rome",
- Section 2: Hellenistic and Roman Ethics
- Two Concepts of Morality: A Distinction of Adam Smith's Ethics and its Stoic Origin, extract on Jstor
- Stobaeus, in Long, A. A., Sedley, D. N. (1987). The Hellenistic Philosophers: vol. 1. translations of the principal sources with philosophical commentary, 59B. Cambridge, England: Cambridge University Press (SVF III, 494)
- Diogenes Laertius, Lives and Opinions of Eminent Philosophers, VII, 108-109 (SVF III, 495, 496; transl. in Long, A. A., Sedley, D. N. (1987), 59E)
- Review of Keimpe Algra, Jonathan Barnes, Jaap Mansfeld, Malcolm Schofield, The Cambridge History of Hellenistic Philosophy. Cambridge: Cambridge University Press, 1999. Pp. xix + 916. ISBN 0-521-25028-5.
- According to Long & Sedley, the origin of this image of containing all numbers should be researched in musical harmony, Long & Sedley, 1987, 59K
- Plutarch, On Stoic Self-Contradictions, in Moralia, 1046 E-F (SVF III, 299, 243 - see Long & Sedley, 1987, 61F)
- Long, A. A., Sedley, D. N. (1987). The Hellenistic Philosophers: vol. 1. translations of the principal sources with philosophical commentary