Oikeiōsis (οἰκείωσις) is the doctrine of "appropriation" or "making-one's-own": every living thing perceives, from birth, its own self and its own constitution as "its own" (οἰκεῖον) and strives for self-preservation. In the rational being this primary self-attachment develops in two directions: (1) inward — toward one's own reason as what is most properly "one's own" (and so virtue is identified with agreement with one's rational nature), and (2) outward — in concentric circles: those close to us, fellow-citizens, the whole of humanity, all rational beings. From a biological drive, in this way, grows the foundation of Stoic cosmopolitanism.

The doctrine goes back to Zeno and is developed by Chrysippus (SVF III 178–189). The classical exposition is in Cic. De fin. III 16–22 and III 62–68; cf. Hierocles, Elementa Ethica, with the famous image of the concentric circles (Stob. IV 27.23 H). In Marcus the doctrine is not formulated explicitly, but pervades the book: Med. 2.1; 3.4; 05-01 (born for the common work, like others); 05-16 (keep the soul social; the good lies in social action); 05-20; 05-33; 06-07 (joy and rest in continuous social action); 06-14 (keep one's soul social and cooperate with one's kind, τῷ ὁμογενεῖ); 06-23 (treat the rational as equals, κοινωνικῶς); 06-30 (the one fruit — piety and benefit to men: "reverence the gods, save human beings"); 06-39 (love the people your lot gave you — but truly, ἀληθινῶς); 06-44 (rational-social nature; two cities — Rome and the cosmos); 06-45 (what benefits one benefits others too); 06-53 (get inside the speaker's soul — fellow-feeling); 06-54 (what is not good for the swarm is not good for the bee); 07-13 (μέλος, not μέρος — to love human beings from the heart; beneficence as one's own good); 07-31 (φίλησον τὸ ἀνθρώπινον γένος — "love the human race"); 07-55 (τὰ λογικὰ ἀλλήλων ἕνεκεν — rational beings for one another's sake); 07-65 (not to become like the inhuman — philanthropy is unconditional); 07-70 (the gods "care" (κήδονται) for the bad — a model of care); 07-74 (in benefiting another you receive benefit yourself — hence never tire); 08-08 (not to be angry at the unfeeling and ungrateful, but to care for them); 08-12 (οἰκειότερον — social action as most one's "own," not an external duty); 08-23 (refer every act ἐπ’ ἀνθρώπων εὐποιίαν — to the benefit of human beings); 08-26 (εὔνοια πρὸς τὸ ὁμόφυλον — proper to a human); 08-27 (the relation πρὸς τοὺς συμβιοῦντας — to fellow-livers); 08-34 (ἀκοινώνητόν τι πράσσων — an unsocial act cuts one off from the Whole); 08-42 (not to grieve oneself — as one never grieved another); 08-43 (the sound mind turns away from no human being); 08-59 (ἀλλήλων ἕνεκεν γεγόναμεν — born for one another: "teach or bear"); 08-61 (to enter another's ruling part and open one's own — the extreme of mutual perviousness of minds); 09-01 (rational beings "made for one another — to help by desert, not harm"; injustice = impiety); 09-03 (not to be angry at fellow-livers but to care and bear gently); 09-05 (injustice by omission — the neglect of the positive duty); 09-08 (one intelligent soul for all the rational — the physical ground of sociability); 09-09 (all that is akin strives toward the akin, from elements to stars — sociability as a physical law); 09-10 (κοινὸν καρπόν — the "common fruit" of reason, not only "one's own"); 09-11 (εὐμένεια toward the erring — "teach or bear"; care for one who errs in ignorance); 09-22 (enter the neighbour's ἡγεμονικόν — to understand the ignorance and the kinship, not to be angry); 09-23 (refer every act to the κοινωνικὸν τέλος; an act without the end is mutiny against the common); 09-27 (toward detractors — εὐνοεῖν: "by nature they are friends," φύσει φίλοι); 10-02 (the ladder of natures along which the appropriation of "one's own" unfolds: the rational is straightway also the social); 10-04 (διδάσκειν εὐμενῶς — teach the one who stumbles kindly, do not punish him); 10-06 (ἔχω πως οἰκείως πρὸς τὰ ὁμογενῆ μέρη — the second premise of the inference, on the οἰκει- root itself); 10-30 (ἄφελε αὐτοῦ τὸ βιαζόμενον — neither forgive nor endure but remove the compulsion: re-teach his judgement about the good); 10-36 (διαλύομαι ὡς ἀπὸ οἰκείων — at death people are called "his own" on the same οἰκει- root, and the parting is described as the undoing of what nature had joined); 11-08 (δῶρον τοῦ συστησαμένου τὴν κοινωνίαν Διός — the power to grow back together (συμφῦναι) is called a gift of Zeus and the privilege of the rational; with the proviso that repeated breaks leave what detached itself δυσένωτον); 11-09 (let the obstructors not knock the εὐμένεια toward them out of you: the hinderer remains φύσει συγγενὴς καὶ φίλος); 11-18 (the first of the nine grounds: ἀλλήλων ἕνεκεν γεγόναμεν, and with the image of the herd-leader — κριὸς ποίμνης ἢ ταῦρος ἀγέλης, not of a shepherd); 12.30. See also Sen. Ep. 95.52. In Book XII — 12-26 (συγγένεια with the whole human race — not by blood but by mind); 12-30 (the physical ground: mind of itself stretches toward its kind, τείνεται καὶ συνίσταται, whereas the lifeless is held only by ἕξις and by weight). In Books I–IV: 04-03 (τὰ λογικὰ ζῷα ἀλλήλων ἕνεκεν γέγονε — rational beings came into being for one another), 04-12 (ἐπ’ ὠφελείᾳ ἀνθρώπων), 04-24 (πολιτικὸν ζῷον).

Passage 02-01 is the ethical consequence drawn from oikeiōsis: human beings are TERMkindred to one another not by blood but by reason, and so irritation toward one's neighbour contradicts the structure of "what is one's own." Linked to unity-of-cosmos (the ontological ground of kinship) and to TERMsynergia (its active expression). See also the exercise view-from-above, which unfolds the concentric circles in visual form.

Passage 03-02 gives a rare cosmic-aesthetic extension of oikeiōsis: the incidental beauty of natural processes (the cracks of baked bread, over-ripe olives, old age) is seen "only by one who has become genuinely made akin to nature and her works" (γνησίως ᾠκειωμένῳ). Here the appropriation is directed neither at the self nor at the human community, but at TERMφύσις herself as a whole; the perception of natural beauty turns out to be a function of moral intimacy with her.

In 03-09 the very word οἰκείωσις appears (one of the few instances in Marcus): "affection toward human beings" (ἡ πρὸς ἀνθρώπους οἰκείωσις) is named as one of the three fruits of the guarded faculty of judgement — alongside ἀπροπτωσία (assent) and ἀκολουθία toward the gods (desire). Here oikeiōsis is the social discipline (action), flowing from a correct judgement about what others are.