The exchange is Analects 15:23. Zigong, the disciple with the merchant's mind, asks a merchant's question: is there one word which may serve as a rule of practice for all one's life? It is a request for compression, ethics at its minimum viable size, and Confucius, who elsewhere refuses such shortcuts, this time supplies one: is not shu such a word? What you do not want done to yourself, do not do to others. The choice of shu over grander candidates is itself the teaching. Asked for one word, Confucius does not name humaneness, the crown of his ethics, or rightness, or ritual; he names the operation by which an ordinary person, in an ordinary moment, can compute what humaneness requires: consult your own feelings and refuse to export what you would not import.
The formula needs no learning, no leisure and no priest, which is exactly what fits it for all one's life, a rule usable at every age, in every station, at the speed of daily decisions. Confucius was frank about the difficulty concealed in the simplicity: when Zigong on another occasion announced that he did not do to others what he did not want done to himself, the Master replied that he had not yet reached that point. The rule can be stated in a breath and practiced for sixty years without exhausting it, and the tradition treats that ratio, minimum statement to maximum demand, as the mark of its truth.