The Avatamsaka Sutra describes a cosmos in which every phenomenon reflects every other, a claim easy to state and difficult to make anyone actually feel. Fazang, called on to explain Huayan philosophy to Empress Wu Zetian, is said to have solved the problem with furniture rather than argument: he placed a golden lion statue in a room lined with mirrors on every wall, ceiling and floor, so a single glance produced an unbroken cascade of the lion reflected in its own reflections, each image containing every other at once. The short text he wrote to accompany the demonstration, the Essay on the Gold Lion, remains one of the clearest statements of interpenetration in the tradition: the gold is principle, unified and unchanging; the lion's shape is the passing phenomenal form; neither exists without the other, exactly as no single mirror's image exists without every other feeding into it.
As the primary systematizer of the Huayan school, ranked third among its patriarchs behind founder Dushun and his own teacher Zhiyan, he organized Buddhist teachings into a hierarchy of five, Huayan's vision of total mutual interpenetration at the summit, above even the Madhyamaka and Yogacara systems that had dominated Chinese Buddhist philosophy before him. He assisted the monk Siksananda's new eighty-fascicle translation of the Avatamsaka Sutra, closing the loop between the text his system rested on and the philosophy he built from it. Huayan's institutional independence faded within centuries, absorbed into Chan and Tiantai practice, but its central image, a cosmos where part and whole endlessly contain each other, is the one piece of technical Chinese Buddhist philosophy most likely encountered, sometimes without attribution, well outside Buddhist studies.