What is oikeiosis, the Stoic idea of "appropriation"?

Oikeiosis (sometimes translated "appropriation," "affiliation," or "familiarization") is the Stoic term for the natural process by which a creature comes to recognize something as belonging to it and worth preserving, starting with its own body and constitution, and expanding outward from there. The fullest ancient accounts come from Cicero's De Finibus Bonorum et Malorum, Book III, and from Diogenes Laertius's Lives of Eminent Philosophers 7.85, which draws on Chrysippus's lost treatise On Ends. The Stoic argument begins with a simple observation: every animal, from the moment of birth, has an immediate awareness of and impulse toward its own constitution, aiming to preserve itself and avoid what harms it - Cicero calls this an inclination toward self-love in the most literal sense, not a learned behavior but a built-in orientation. What makes the theory distinctly Stoic is the next step: as a human being matures and reason develops, this same process of "making something one's own" does not stay confined to the individual body. A child naturally extends its sense of what belongs to it to include parents and siblings, and Stoic writers argued that a fully rational adult should, in principle, extend the same natural affiliation to friends, fellow citizens, and ultimately the whole human race, since reason itself is shared by all rational beings. The later Stoic Hierocles gave this idea its clearest picture: concentric circles radiating outward from the self, through immediate family, extended family, community, and nation, out to humanity as a whole, with the ethical task being to actively "draw the circles inward," treating outer circles with the same care naturally given to the inner ones. This is the philosophical root of Stoic cosmopolitanism, the idea, associated with Zeno's Republic and later expressed by Marcus Aurelius, that a person is a "citizen of the world" rather than of one city alone. Oikeiosis matters for understanding Stoic ethics because it grounds the school's demanding claims about justice and care for others not in an external rule imposed on selfish creatures, but in an extension of the same self-preserving impulse every living thing already has.