Kathēkon — literally "that which reaches [its mark]", "that which befits" (from the verb καθήκω — "to reach, to be fitting"). The technical term of Stoic ethics, introduced, by the testimony of Diogenes Laertius (VII 108), by Zeno and systematically worked out in Zeno's lost treatise Περὶ τοῦ καθήκοντος (On the Appropriate). The canonical Stoic definition, preserved by DL VII 107: τὸ πραχθὲν εὔλογόν τι ἴσχει ἀπολογισμόν — "[the action] which, when performed, admits some reasonable defence." Every action is classed as one of three:

• καθῆκον — appropriate (admits εὔλογος ἀπολογισμός — a reasonable justification);
• παρὰ τὸ καθῆκον — contrary to the appropriate;
• οὐδέτερον — neutral (neither one nor the other).

Within καθήκοντα a further distinction is drawn, important to the whole of Stoic ethics:

• κατορθώματα (katorthōmata) — "right actions," performed only by the sage (σοφός) out of perfect virtue;
• μέσα καθήκοντα — "intermediate appropriate," performed by those making progress (προκόπτοντες), formally correct but not flowing from a perfect disposition of soul.

Each kathēkon is determined not abstractly, but from a concrete σχέσις (relation) and from the nature of the agent. In Epictetus (see below) this connection is codified in the formula "appropriate actions are discovered from the names": you are a father — therefore such-and-such καθήκοντα; you are a citizen — such-and-such others; you are a brother — others again. Stoic ethics is not an ethics of rules but an ethics of actions appropriate to roles.

Zeno, the lost treatise Περὶ τοῦ καθήκοντος. The first preserved definition — DL VII 107: ἔτι δ' ὁρίζονται τὸ καθῆκον τὸ ἀκόλουθον ἐν βίῳ, ὃ πραχθὲν εὔλογόν τι ἴσχει ἀπολογισμόν — "they define the appropriate as 'what follows on in life,' what, when done, admits some reasonable defence." An expanded discussion of the three-fold classification (καθῆκον / παρὰ τὸ καθῆκον / οὐδέτερον) — DL VII 108–109.

Chrysippus in Περὶ καθηκόντων worked out the detailed casuistic application of the doctrine; from this treatise indirect testimonies survive (SVF III 491–499).

Panaetius wrote a treatise of the same name Περὶ τοῦ καθήκοντος, wholly lost, but serving as the direct basis for Cicero's De Officiis — the only complete Latin equivalent of the Stoic theory of duties. The Ciceronian officium is the standard Latin rendering of kathēkon.

Epictetus, Discourses II 10 ("Πῶς ἀπὸ τῶν ὀνομάτων τὰ καθήκοντα εὑρίσκεται" — "How appropriate actions are discovered from the names") — codifies the doctrine in the form of a practical discipline: the agent is given his "names" (father, son, friend, citizen), and from each name the corresponding καθήκοντα are derived. This Epictetan approach is the main background of Marcus's own use of the term.

The long-standing standard treatment in modern literature — Long & Sedley, The Hellenistic Philosophers, vol. 1, ch. 59 ("Appropriate actions"); fragments — vol. 2, ch. 59 (= SVF III 491–499 and others).

In Marcus the technical term καθήκοντα appears in many places, most frequently in formulae of direct self-injunction of the type "perform each task as the kathēkon of your nature." It is operationally engaged in a number of passages (Books I, III, VI):

• 01-12 — the most direct use of the term in the form "τὰ κατὰ τὰς πρὸς τοὺς συμβιοῦντας σχέσεις καθήκοντα": the canonical Epictetan link "καθήκοντα from σχέσεις," with concrete application to the close circle (συμβιοῦντες).
• 01-13 — implicitly, through the three lessons of Catulus (friend, teacher, children — three σχέσεις with corresponding καθήκοντα); in explicit vocabulary Marcus does not use the term here, but the logic of the argument is canonical.
• 03-01 — the rare phrase τοῦ καθήκοντος ἀριθμοί ("the numbers / full measure of the appropriate"): the power to reckon precisely (ἀκριβοῦν) the appropriate is named among the highest operations of reason, ones that fail in dotage before the bodily functions do. The kathēkon here is what requires a trained reason and is therefore vulnerable.
• 03-05 — implicitly, like 01-13: the stack of roles (ἄρρην, πρεσβύτης, πολιτικός, Ῥωμαῖος, ἄρχων), from which the καθήκοντα are derived — the canonical Epictetan "ethics from the names"; the term καθῆκον itself does not appear in the text.
• 06-22 — a direct use in Book VI: «τὸ ἐμαυτοῦ καθῆκον ποιῶ» ("I do my own kathēkon") — concentration on one's own function, undistracted by "the rest" (the lifeless, the irrational, the strayed).
• 06-26 — "πᾶν καθῆκον ἐξ ἀριθμῶν τινῶν συμπληροῦται" ("every kathēkon is made up of certain parts"): the same phrase, the ἀριθμοί of the appropriate, as at 03-01; the duty is done part by part, methodically and without returning anger.
• 07-05 — a verbal use: ἐὰν ἄλλως τοῦτο μὴ καθήκῃ ("unless this is otherwise my kathēkon") — the reservation in the procedure of handing work over: one may yield the work to an abler doer only when it does not belong to one's own καθῆκον. The doctrine of καθήκοντα is the structural basis of what Hadot calls the discipline of action (see Hadot, The Inner Citadel, ch. 8). All three Stoic disciplines (of assent, desire, action) in Marcus are codified through derivatives of the kathēkon lexicon: "assent" — to the appropriate φαντασία; "desire" — directed at the kathēkon outcome; "action" — the very performance of the kathēkon in the given σχέσις.

The standard Latin equivalent officium (via Cicero) determined the traditional Russian rendering "долг" (and the English "duty"). This is a weak rendering: the Russian "долг" / English "duty" carries a deontological flavour (something that must be done) absent in the Stoic kathēkon (which describes an action as appropriate, not as obligatory in the Kantian sense). Alternative renderings — "appropriate," "fitting," "befitting" in English; «надлежащее», «уместное» in Russian — are more accurate but have not become standard.