One of the fundamental principle of Stoicism is cosmopolitanism: humanity is one big city of beings capable of reason, in virtue of which we should treat everyone justly, i.e., with fairness and respect. This principle of action is derived from what the Stoics thought is a natural principle, which was incorporated into stoic philosophy from the beginning, with the writings of Zeno of Citium: oikeiôsis.
The term is translated in a variety of ways, including “appropriation,” “familiarization” and “endearment,” but what is most revealing is the Greek root: oikos means household or family, and is the same root of the modern terms ecology and economics (indeed, one of the leading journals in ecology is named Oikos). The best way to understand the notion, then, is that oikeiôsis is a process by which we come to perceive something as our own, as belonging to us.
Those who followed Zeno stated that oikeiôsis is the beginning of justice. (Porphyry, quoted in Richter, p. 75)
But what, exactly, does the natural process consist of, and why do the Stoics derive from it what amounts to a moral duty?
The most famous Stoic associated with the theory is Hierocles, who flourished in the second century (not to be confused with the fifth century Neo-Platonist Hierocles of Alexandria). We don’t know much about him, though Aulus Gellius mentions that he was a contemporary of his, and that he was a “grave and holy man.” Hierocles wrote about oikeiôsis in at least two books, Elements of Ethics and On Appropriate Acts, of which we only have fragments.
Hierocles begins his account of oikeiôsis by noting that animals are characterized from the moment of birth by an instinct of self-preservation, which extends to humans. This sense of “belonging to itself” depends on our perception of the external world as distinct from our own inner feelings. As Hierocles puts it in Fragments and Excerpts: “An animal, when it has received the first perception of itself, immediately becomes its own and familiar to itself and to its constitution.”
This self-perception is the basis of the animal’s (and the human infant’s) sense of self-preservation, understood simply as an instinct to seek things that augment one’s own wellbeing and to stay away from things that might undermine it.
As the human animal grows up, though, it becomes familiar with other people surrounding it, especially its own caregivers, like parents or other close relatives. Gradually, and naturally, oikeiôsis then leads us to extend our concern to the wellbeing of those people. When we reach the age of reason, around 7 to 8 years old, and continuously thereafter, we begin to apply our reflective thinking to further extend the process, realizing that other people, who are not related or otherwise close to us, are essentially like us, with similar wants, needs, worries, and so forth.
The wise person, extrapolating the process of oikeiôsis to its logical outer limit, would then feel “at home” not just with relatives, friends, and fellow townspeople, but with humanity at large. Here is Hierocles’ famous passage explaining the idea (see top image for a visual rendition, crucial parts in boldface):
Each of us is, as it were, circumscribed by many circles; some of which are less, but others larger, and some comprehend, but others are comprehended, according to the different and unequal habitudes with respect to each other. For the first, indeed, and most proximate circle is that which everyone describes about his own mind as a centre, in which circle the body, and whatever is assumed for the sake of the body, are comprehended … The second from this, and which is at a greater distance from the centre, but comprehends the first circle, is that in which parents, brothers, wife, and children are arranged. The third circle from the centre is that which contains uncles and aunts, grandfathers and grandmothers, and the children of brothers and sisters … Next to this is that which contains the common people, then that which comprehends those of the same tribe, afterwards that which contains the citizens; and then two other circles follow, one being the circle of those that dwell in the vicinity of the city, and the other, of those of the same province. But the outermost and greatest circle, and which comprehends all the other circles, is that of the whole human race … It is the province of him who strives to conduct himself properly in each of these connections to collect, in a certain respect, the circles, as it were, to one centre, and always to endeavour earnestly to transfer himself from the comprehending circles to the several particulars which they comprehend. It is requisite, likewise, to add a proper measure conformably to the general use of appellations, calling indeed cousins, uncles and aunts, by the name of brothers, fathers and mothers; but of other kindred, to denominate some uncles, others the children of brothers or sisters, and others cousins, according to the difference of age, for the sake of the abundant extension which there is in names. For this mode of appellation will be no obscure indication of our sedulous attention to each of these relatives; and at the same time will incite, and extend us in a greater degree, to the contraction as it were of the above mentioned circles. (Fragments, How we ought to conduct ourselves towards our kindred)
Hierocles here first describes the concentric circles, beginning with one’s self in the center (not because we are more important than others, but because we have a special relation to ourselves), followed by relatives, friends, acquaintances, other people, and so forth. The largest circle, you’ll notice, is that of the human race.
(Some modern Stoics add additional circles, to include all sentient animals, that is, all animals capable of suffering. This would be unorthodox in ancient Stoicism, since the crucial criterion was a capacity for rationality, but I agree with what Jeremy Bentham wrote in this regard: “The question is not, ‘Can they reason?’ nor, ‘Can they talk?’ but ‘Can they suffer?’)
So far the descriptive part. Hierocles then shifts into prescriptive mode: if we “strive to conduct ourselves properly” we should attempt to “collect” the external circles, bringing them closer to the internal ones. That is, we should train ourselves to care more about strangers on the other side of the world, as if they were fellow citizens; and to care more for our fellow citizens, as if they were friends; and to care more for our friends, as if they were family; and to care more for our family, as if they were us.
The final bit even provides some advice on how to do this in practice, an approach that is actually in use in a number of cultures: start referring to people you don’t know as brothers or sisters, uncles or aunts (depending, says Hierocles, on what’s appropriate to their age), to constantly remind yourself that you should treat them as if they really were relatives. This is an early example of cognitive behavioral therapy, if you will: the first step is cognitive (reflect on the issue, and realize that you should care about other people), the second one behavioral (implement strategies that will gradually habituate you to feel the way you think you should).
You might have noticed that Hierocles has seamlessly shifted from an “is” (i.e., a factual description of things) to an “ought” (i.e., an ethical prescription). This, according to many modern moral philosopher, has been a no-no ever since David Hume wrote these famous words in his A Treatise of Human Nature, back in 1739:
In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.
It is actually debatable whether Hume here meant that the is/ought gap is unbridgeable or, more modestly (and likely, in my opinion) that if we do bridge it then we ought to provide reasons for our specific approach. Be that as it may, the ancients - and particularly the Stoics - would have been puzzled by the whole notion of a sharp qualitative distinction between facts and values. Cicero writes that “all duties derive from principles of nature,” and Hierocles’ theory considers “appropriate acts” those that are in “accordance to nature.”
I hasten to say that neither Cicero not the Stoics nor any other ancient philosopher were such simpletons as to commit the fallacy of appeal to nature, i.e., equating everything natural with the good. For instance, for the Stoics to work together to improve society is “in accordance to nature,” because we are social animals. But anger is also a natural human reaction, and yet the Stoics, and Seneca in particular, wrote abundantly about why anger is bad for us.
How do we separate natural/bad from natural/good, then? By exercising the highest faculty that nature itself has given us: reason. In fact, to live according to nature, for the Stoics, simply meant to exercise reason in order to improve the human cosmopolis. And it is precisely the natural process of oikeiôsis, which begins at the level of instinct and is then expanded by the application of reason, that allows us to feel at home in the world.