Zhu Xi is the most influential Chinese thinker after Confucius himself, the philosopher of the Southern Song who gathered the diffuse currents of the Confucian revival into the coherent system known as Neo-Confucianism (the Cheng-Zhu school, or Lixue, "the learning of principle"). His synthesis rested on the metaphysical pairing of li (principle, the underlying pattern and nature of all things) and qi (the material force that gives them concrete existence): every thing possesses the one principle of the cosmos as its own nature, obscured in most people by turbid qi, and self-cultivation is the work of clarifying the qi so that the innate principle, which is moral, shines through.
To this he joined a programme of learning summed up in the "investigation of things" (gewu), the patient study of the principle in things and texts. His most consequential act was editorial: he selected and canonized the Four Books, the Analects, the Mencius, the Great Learning, and the Doctrine of the Mean, and his commentaries on them. In 1313 these, with his interpretations, became the required basis of the imperial civil-service examinations, a status they held for six centuries, making Zhu Xi's reading of Confucianism the intellectual orthodoxy of China, Korea, and Japan into the modern era.