In a.d. 64 the Stoic philosopher Seneca pondered friendship. The Stoics’ intellectual adversaries, the Epicureans, had claimed that a man sought friends for purely instrumental reasons, “for the purpose of having someone to come and sit beside his bed when he is ill or come to his rescue when he is hard up or thrown into chains.” But Seneca knew better. A wise man wanted friends “so that he may have someone by whose sickbed he himself may sit, or whom he may himself release when that person is held prisoner by hostile hands.” Kindness was man’s duty but also his joy: “No one can live a happy life if he turns everything to his own purposes. Live for others if you want to live for yourself.”

People need other people, not just for companionship or support in hard times but to fulfill their humanity. This theme ran through all of ancient thought but was strongest among the Stoics, who propounded a moral psychology based on oikeiôsis, the attachment of self to other. Stoics were famously self-reliant, but the self on which a Stoic relied was not singular but communal. Stoics regarded reality as governed by a Logos, a divine principle of rationality, which manifested itself as ­reason in ­every human soul. No man was an island, as John Donne wrote centuries later; all belonged to the great “community of reason” and were precious to each other for their common humanity. The world was but a “single city,” the Stoic emperor Marcus Aurelius averred, whose citizens were united by reason and “mutual affection.”

Not everyone agreed with this communalism. Epicureans ­certainly did not, describing humanity not as a unity but as an ag­glomeration of individuals, each driven by self-love and the pursuit of personal pleasure. The Stoics by contrast, while acknowledging the existence of self-love, interpreted it in non­individualist terms: each person, they argued, is born with a primary self-attachment which, as he matures into the fellowship of reason, fosters attachments to others. Aristotle had argued that friendship was self-love extended outward. The Stoics developed the idea into a concept of the self as the center point of concentric circles of oikeiôsis, of which the inner­most circles were composed of blood relatives, followed by friends and neighbors, with the circles gradually radiating outward to encompass all humanity. Whether the degree of attachment was the same at all levels was a matter of controversy. Aristotle had described affection for humankind in general as “diffuse” and “watery” and some Stoics concurred, arguing that affective bonds increased in strength the closer the connection, with parents and children experiencing the strongest attachment while goodwill to strangers tended...