Zigong asks: 'Is there one word that can be practiced all one's life?' The Master replies: 'It is shu: do not impose on others what you yourself do not desire.' The same formula appears at 5.12 and 12.2, and the Great Learning and Doctrine of the Mean develop shu as the measuring-square principle. The negative formulation predates Hillel's negative golden rule (Shabbat 31a) by centuries and differs from the Christian positive form in Matthew 7:12.
Western commentators often call the negative form a timid version of 'do unto others'. Defenders note the negative form is more robust: it demands only restraint, which anyone can perform, rather than benevolence, which requires knowing what others want.
Shu in the classical commentaries is a geometry: use the square of your own desire to measure others, and never cut what you would not want cut. Confucius says of himself he has not achieved it (5.12), making shu the rare virtue its own inventor confessed to failing.
If one word can govern a whole life, why does every era keep losing it?