The Zhouyi Cantong Qi, conventionally the Seal of the Unity of the Three, is attributed to Wei Boyang in the second century and is the root text of Chinese alchemy in both its outer and inner forms. Its method is to describe the alchemical process using the trigrams, hexagrams and cyclical terms of the Yijing, so that a passage about firing times can be read simultaneously as instructions for a furnace, as a description of cosmic cycles, and as directions for an operation inside the practitioner's own body. The three of the title are generally taken as the Yijing, Taoist cosmology and alchemy, unified. The obscurity is deliberate, not incidental — it is a text designed to be unusable without a teacher, and it succeeded, generating a vast and mutually contradictory commentarial tradition including a famous commentary by the Neo-Confucian Zhu Xi, who wrote it under a pseudonym. Its coded vocabulary of lead and mercury, dragon and tiger, host and guest became the standard idiom of all subsequent internal alchemy.