 In his later work, Kihak (氣學 Learning of Vital Force), he incorporates the concept of unhwa 運化 (circling/movement and transformation/change) to describe the idea of Vital Force. Here he invokes two kinds of Vital Force of hyŏgjil (形質之氣 Vital Force of form and tangible quality) and Vital Force of unhwa (運化之氣). The earth, moon, sun, star, and all other substantial, thus, tangible, bodies are Vital Force of form and tangible quality, while the more basic phenomena such as rain, sunlight, wind, snow, coldness, hotness, dryness, humidity are Vital Force of circling/transformation. We can say that Vital Force of form and tangible quality supervenes on the aggregation of Vital Force of circling/transformation (Learning of Vital Force, 氣學, Book 1. Sec. 6, Ch’oe Han’gi Expanded Work, Volume 1).