The character shu is built from two elements: ru, meaning like or as, above xin, the heart. The graph is thus readable as a small diagram of the virtue: as the heart, one heart taken as like another. The commentarial tradition made the etymology doctrinal, glossing shu as tui ji ji ren, extending oneself to reach others: one takes one's own case, one's own recoil from insult, hunger for recognition, dread of injury, and treats it as valid evidence about every other case. The method deserves notice for what it does not require. It does not require agreement on doctrine, shared ancestry or shared rank; it requires only that the other be conceded a heart like one's own, whereupon one's self-knowledge becomes, instantly, knowledge of how they may and may not be treated. This is why shu is often called the method of ren, humaneness.
Ren names the disposition of fellow-feeling; shu is its working procedure, the likening by which fellow-feeling computes its duties. The negative form, do not impose what you would not suffer, keeps the procedure honest: it forbids using oneself as the measure of what others must enjoy, only of what they must be spared, a modesty later comparative ethicists came to prize when weighing the world's reciprocity formulas against one another. In the Confucian frame the likening is also self-cultivating; each act of extension enlarges the heart doing the extending, so that the procedure and the virtue grow together over a lifetime.