Stoic cosmopolitanism and oikeiōsis
How far do the demands of justice extend? To all human beings, the Stoics insist. In virtue of our shared rationality, all humans, together with Zeus, constitute the citizenry of one universal city or cosmopolis. The just agent looks beyond her immediate physical proximity and conventional political community when distributing indifferents, taking the interests of every human being — even "the most distant Mysian" — into account.
The cosmic city and universal law
One motivation for characterizing the cosmos as a city is Stoic thinking on law, which draws on Stoic physics and metaphysics and stoic-ethics-telos-and-virtue. To live in agreement with nature involves "engaging in no activity wont to be forbidden by the universal law (ho nomos ho koinos), which is the right reason pervading everything and identical to Zeus." Since "a city is a group of people living in the same place and administered by law" (Dio Chrysostom), and all humans are bound by this universal law — "diffused over everyone, consistent, everlasting" (Cicero, De Legibus) — the cosmos qualifies as a city. The providential activity of Zeus serves as universal law, setting a perfectly rational standard for human beings to follow and emulate. Marcus Aurelius returns to this idea repeatedly in the Meditations, describing his duty to treat all people — even his enemies in the Marcomannic Wars — with justice, fairness, and kindness, as fellow citizens of the universe.
Oikeiōsis: appropriation and affiliation
To support cosmopolitan justice and the claim that only virtue is good, the Stoics rely on the doctrine of oikeiôsis ("appropriation" or "affiliation" — from oikeion, "appropriate," "affiliated," "belonging"). The doctrine begins with a naturalistic observation: from birth, every creature seeks to preserve its own constitution, even when painful. Each animal is born with a capacity for self-perception (to detect what is appropriate or harmful to its constitution) and a capacity for impulse (to pursue or avoid accordingly). A turtle fallen on its back perceives the unfavorable situation by nature, without training, and forms an impulse to right itself.
Newborn humans are no different: prior to acquiring reason (around age seven), we employ a non-rational soul and pursue what is favorable to that constitution. The transition to adulthood comes when "reason supervenes as the craftsman of impulse" and we acquire a rational nature with different objects of appropriation — the perfection of reason (virtue) becomes what is genuinely oikeion to us.
A social dimension is added by observing that humans, like bees and ants, naturally cooperate, and that nature's concern for offspring makes the extension of affiliation to others natural: "it could not be consistent for nature both to desire the production of offspring and not to be concerned that offspring should be loved" (Cicero). One major source on oikeiôsis concludes with an account of the ideal agent's development, culminating in the appreciation that consistency and harmony of conduct is the only true source of benefit.
Three scholarly difficulties
Scholarly debate on oikeiôsis focuses on three unresolved questions. First, how can apparently descriptive claims about natural impulses provide normative guidance — an instance of the is-ought problem? Second, how does a natural disposition toward self-preservation underwrite the other-regarding demands of cosmopolitan justice — the bridge from self-concern to concern for all rational beings? Third, in what ways is a non-rational creature's pursuit of food and shelter analogous to the Sage's attachment to virtue as the sole good? Magrin (2018) and Klein (2016) address these with particular clarity.