Confucius (551 BCE) had spoken of ethics and social order but largely kept away from metaphysical questions — the structure of the universe, the origin of being. A thousand years later that gap was filled by the subtle cosmologies of Buddhism and Taoism. The Song-dynasty thinker Zhu Xi (1130–1200) rebuilt the Confucian tradition in the face of this challenge, giving it a deep metaphysical ground and forming the synthesis known as "Neo-Confucianism."

At the centre of his system are two concepts. *Li* — principle, pattern, the rational order determining the form each thing ought to take; and *qi* — material force, the energy-matter composing concrete beings. Every object, every event, arises from the union of li and qi: li gives a thing "what it is," qi its concrete, particular existence. Universal li is immanent in all reality; the principle in a flower and the principle governing the cosmos are parts of the same great order. The human moral task is to clarify this principle in one's own nature, purifying it from the turbidity of qi.

To this end Zhu Xi proposes a method: *gewu*, "the investigation of things." To grasp the principle behind things by attending carefully to the world, to texts, and to one's own mind — a discipline of learning in which knowledge and virtue join in a single effort. From among the classic texts he selected and annotated the "Four Books"; from 1313 this selection became the official curriculum of the Chinese civil-service examinations and shaped the minds of the elite who would govern the empire for six hundred years.

Its influence was not confined to China: the official teaching of the Joseon dynasty in Korea and of the Tokugawa period in Japan was Zhu Xi's interpretation. East Asia's understanding of morality, its ideal of education, and its bureaucratic elite culture followed this synthesis for centuries. As Aquinas fused Aristotle with Christianity in the West, so Zhu Xi fused metaphysics with Confucian ethics.