Analects 4:15 is two sentences long and has carried a disproportionate weight of Confucian self-understanding. The Master says to Zengzi: my way has one thread running through it. Zengzi assents; the Master leaves; the other disciples ask what he meant, and Zengzi answers: the way of the Master is zhong and shu, nothing more. Everything turns on the pairing. Shu, reciprocity, is defined elsewhere in the Analects by Confucius himself: what you do not want done to yourself, do not do to others. Zhong is left undefined, and the commentarial tradition built its account from the word's texture: doing one's utmost, wholeheartedness, the state of being fully engaged and undivided in what one owes.
Zhu Xi's influential gloss makes the geometry explicit: zhong is the exhaustion of oneself, shu is the extension of oneself to others, the inward face and the outward face of a single disposition. On this reading the thread is not two virtues but one motion seen twice. A person cannot extend to others a self that is scattered, grudging or false; consideration without wholeheartedness is etiquette, and wholeheartedness without consideration is mere intensity. The thread runs from the integrity of the agent out through every relationship, which is why Zengzi could say it was the whole of the teaching: everything else in the Analects, the rites, the filial duties, the government by virtue, is this thread woven through different cloth.