Cheng Yi, or Cheng Yi-chuan, was the most outstanding Chinese teacher of his time, a lecturer to the emperor on Confucian classics, and cofounder, with his brother Cheng Hao, of the neo-Confucian school of principle (li) that dominated Chinese thought for many centuries.

The central concept of the school is principle. The concept, negligible in ancient Confucianism, had been developed by the neo-Daoists and Buddhists, but the Cheng brothers were the first to build their philosophy primarily on it. To them, principle is self-evident and self-sufficient, extending everywhere and governing all things. It is laid before our very eyes. It cannot be augmented or diminished. It is many, but it is essentially one, for "definite principles" are but principle. "Principle is one but its manifestations are many." It is universal truth, universal order, universal law. Most important of all, it is the universal principle...