Ren, humaneness, benevolence, co-humanity, is the central virtue of Confucian ethics. The character is written with the elements for "person" and "two", and the tradition has consistently read it as naming what is right between people, so that the virtue is relational at its root rather than a quality possessed in isolation.
Confucius is notably unwilling to give it a single definition. Asked about ren by different disciples in the Analects he answers differently each time, matching the answer to the questioner: to overcome oneself and return to ritual; to love others; to behave abroad as though receiving an important guest and to employ people as though officiating at a great sacrifice; and, in the formulation most often quoted, not to impose on others what one does not want for oneself, a negative statement of reciprocity that the tradition names shu and that Confucius elsewhere offers as the one word a person might practise for a lifetime. He also declines, repeatedly and pointedly, to call any living person ren, which sets the standard very high indeed.
The relation of ren to li is the axis of Confucian ethics. Ritual without humaneness is empty, and humaneness without ritual has no form; the two are presented as the inner and outer aspects of a single cultivation, and a great deal of the Analects consists of working out what that comes to in particular cases.
Mencius developed ren into a theory of human nature, grounding it in the sprout of compassion. His thought experiment is the tradition's most famous argument: anyone who suddenly saw a child about to fall into a well would feel alarm and distress, not in order to gain the parents' favour, or a reputation, or because they disliked the noise, and from that immediate and uncalculated response he concludes that the disposition is native to human beings and requires cultivation rather than installation. He also gave ren a political form as ren zheng, humane government, arguing that a ruler who genuinely attends to the people's welfare will prevail without needing force.
Later tradition extended it further. Han thought treated ren cosmologically. Neo-Confucianism, in Cheng Hao and Zhu Xi, identified ren with a person's forming one body with heaven, earth and the ten thousand things, and made use of the medical sense of the same word, in which the numbness of a paralysed limb is described by its negation, to argue that a person who lacks ren is insensible to others exactly as a numb hand is insensible to the rest of the body. Comparative discussion of ren alongside Western virtue ethics is now extensive, with the standard caution that the term's relational grounding does not map neatly onto a virtue understood as a property of an individual.